[GRASS-user] Re: [GRASS-dev] Changes to r.sun made Feb 2006 [resolved]

H_B · December 1, 2006, 6:13am

Dylan Beaudette wrote:

PS: with even better results from r.sun:
http://169.237.35.250/~dylan/temp/11-yr_variation_vs_modeled.png

when using longer term averages, the R^2 goes to 0.95 ! (right hand
sub-figure), but still working on optimal Linke turbidity values.

Did you extract the Linke turbidity values from the app. on the EU web
site, derive it from measured in situ values, or are you tweaking that
variable as a calibration to get r.sun to match your measured radiation
values?

I see you have a bit of sawtooth noise in your plots. FWIW to avoid
introducing this monthly jump I wrote a little app some time ago which
interpolated daily Linke value from the monthly Linke averages (for use
in r.sun bash script loop) [assume monthly averages are for day 15 of
each month and interpolate for the days in between]. I'd have to dig it
out, & it's probably a mix of Matlab+C. Maybe something for the wiki
add-ons.

How to deal with cloud cover? (coefbh= and coefdh=) Use real clear-day
measured data (find a day with a clean curve) to estimate the cloud
attenuation factors for neighboring cloudy days (theoretical curve vs
what actually happened), then generalize the cloudiness factor over your
decade long dataset? e.g. find max (midday) light for each year's day
90, then take the max. maybe include +/-1 day. then run a smoothing
filter on the result. ?? probably want the coeffs to vary throughout
the year.

Another idea I've been playing with is getting cloud cover data from
recorded PV voltage in one of our remote solar-powered weather stations
(it lacks a real light meter). There's a pretty obvious pattern in the
voltage signal to play with. Just a matter of calibrating it..

Hamish

Dylan_Beaudette1 · December 1, 2006, 6:58pm

Hi Hamish,
replies are in-line:

On Thursday 30 November 2006 22:13, Hamish wrote:

Dylan Beaudette wrote:
> PS: with even better results from r.sun:
> http://169.237.35.250/~dylan/temp/11-yr_variation_vs_modeled.png
>
> when using longer term averages, the R^2 goes to 0.95 ! (right hand
> sub-figure), but still working on optimal Linke turbidity values.

Did you extract the Linke turbidity values from the app. on the EU web
site, derive it from measured in situ values, or are you tweaking that
variable as a calibration to get r.sun to match your measured radiation
values?

I used the 'rural' values from the r.sun manual page as a starting point, and
then modified according to my observations of the local climate + tweaking to
make r.sun match the weather station: i.e. i need a more robust approach!

I see you have a bit of sawtooth noise in your plots. FWIW to avoid
introducing this monthly jump I wrote a little app some time ago which
interpolated daily Linke value from the monthly Linke averages (for use
in r.sun bash script loop) [assume monthly averages are for day 15 of
each month and interpolate for the days in between]. I'd have to dig it
out, & it's probably a mix of Matlab+C. Maybe something for the wiki
add-ons.

Indeed! I am using a constant value for the entire month. this approach is a
step in the right direction, but introduces those sawtooth artifacts. Since I
am working in R most of the time (for numerical operations at least), I think
that i might implement it there, and then save the results for use in a bash
script - which invokes r.sun.

Did your code look something like this;
# define monthly averages
l <- c(5.9,5.8,5.6,5.6,5.3,5.0,4.8,5.0,5.0,5.2,5.4,5.6)
# use periodic spline interpolation to fill in the 365 days of the year
l.s <- spline(l, method="periodic", n=365)
# plot results:
plot(l, xlab="month", ylab="Linke Turbidity")
lines(l.s)

note that this is not quite correct as it interpolates from the *start* of the
month, as opposed to the middle. Doing that would require knowing the day of
the middle of the month- i'll cook up that example shortly.

How to deal with cloud cover? (coefbh= and coefdh=) Use real clear-day
measured data (find a day with a clean curve) to estimate the cloud
attenuation factors for neighboring cloudy days (theoretical curve vs
what actually happened), then generalize the cloudiness factor over your
decade long dataset? e.g. find max (midday) light for each year's day
90, then take the max. maybe include +/-1 day. then run a smoothing
filter on the result. ?? probably want the coeffs to vary throughout
the year.

Nice idea. I will hang onto this for future reference. Since we are interested
relatively long time scales (effects of clouds goes to some constant when
viewed by glacial vs. interglacial episodes) estimating the cloud cover might
be overkill. However, in other regions where the orographic effect is more
pronounced it might be a good idea.

Another idea I've been playing with is getting cloud cover data from
recorded PV voltage in one of our remote solar-powered weather stations
(it lacks a real light meter). There's a pretty obvious pattern in the
voltage signal to play with. Just a matter of calibrating it..

interesting. i will ask my colleagues about their solar cell voltage records.

Thanks for all the great ideas!

Cheers,

--
Dylan Beaudette
Soils and Biogeochemistry Graduate Group
University of California at Davis
530.754.7341

H_B · December 4, 2006, 6:46am

Dylan Beaudette wrote:

I can think of a couple of things to be added to the manual page: much
of which I would be happy to write.

may I suggest we prepare the draft in a Wiki page. A 28 page long man
page isn't useful (too much info to quickly find what you need). A
concise man page with a link to "the r.sun users' guide" (pdf/html)
would be much better IMO.

H:

> Did you extract the Linke turbidity values from the app. on the EU
> web site, derive it from measured in situ values, or are you
> tweaking that variable as a calibration to get r.sun to match your
> measured radiation values?

D:

I used the 'rural' values from the r.sun manual page as a starting
point, and then modified according to my observations of the local
climate + tweaking to make r.sun match the weather station: i.e. i
need a more robust approach!

I thought the SoDa page used to be linked from the r.sun page, but no
longer I guess as the link has changed.

Get local Linke values from the SoDa Solar radiation Databases
http://www.soda-is.com/

--
Java applet: (old link, no longer works)
  http://prime.jrc.it/SoDa/SoDa.html
  SoDa Prototype -> Climatological Data & Derived -> Climatological Data
      -> Monthly values of the Linke turbidity factor [double-click]
--

Now you extract local values with a PHP script:
http://www.soda-is.com/eng/services/climat_eng.html
info: (+ links to worldwide monthly maps)
http://stratus.meteotest.ch/soda_tl/soda_tl.html

the worldwide database was available for purchase, I'm not sure of the
current state of that.

other useful links:
Jaro's talk at the 2002 GRASS conference:
http://www.ing.unitn.it/~grass/conferences/GRASS2002/proceedings/proceedings/pdfs/Hofierka_Jaroslav.pdf
Video of the presentation:
http://www.jus.unitn.it/services/arc/2002/0911/7.ram

> I see you have a bit of sawtooth noise in your plots. FWIW to avoid
> introducing this monthly jump I wrote a little app some time ago
> which interpolated daily Linke value from the monthly Linke averages
> (for use in r.sun bash script loop) [assume monthly averages are for
> day 15 of each month and interpolate for the days in between]. I'd
> have to dig it out, & it's probably a mix of Matlab+C. Maybe
> something for the wiki add-ons.

Indeed! I am using a constant value for the entire month. this
approach is a step in the right direction, but introduces those
sawtooth artifacts. Since I am working in R most of the time (for
numerical operations at least), I think that i might implement it
there, and then save the results for use in a bash script - which
invokes r.sun.

Did your code look something like this;
# define monthly averages
l <- c(5.9,5.8,5.6,5.6,5.3,5.0,4.8,5.0,5.0,5.2,5.4,5.6)
# use periodic spline interpolation to fill in the 365 days of the
# year
l.s <- spline(l, method="periodic", n=365)
# plot results:
plot(l, xlab="month", ylab="Linke Turbidity")
lines(l.s)

note that this is not quite correct as it interpolates from the
*start* of the month, as opposed to the middle. Doing that would
require knowing the day of the middle of the month- i'll cook up that
example shortly.

something like that (I don't understand much R syntax). I didn't bother
to figure out if the middle of the month was day 14|14.5|15|15.5, I just
figured day 15 was close enough -- we shouldn't fool ourselves into
thinking the interpolated values will be high precision data.

I am not familiar with "linke_monthly[1]=5.9" in bash. Is that legal??

Hamish

H_B · December 4, 2006, 7:55am

here we go, worldwide Linke data for r.sun:
http://www.helioclim.net/linke/linke_helioserve.html

extract data for one location:
http://www.soda-is.com/eng/services/climat_eng.html
http://stratus.meteotest.ch/soda_tl/soda_tl.html

Hamish

Dylan_Beaudette1 · December 5, 2006, 11:12pm

Hi Hamish:

On Sunday 03 December 2006 22:46, Hamish wrote:

Dylan Beaudette wrote:
> I can think of a couple of things to be added to the manual page: much
> of which I would be happy to write.

may I suggest we prepare the draft in a Wiki page. A 28 page long man
page isn't useful (too much info to quickly find what you need). A
concise man page with a link to "the r.sun users' guide" (pdf/html)
would be much better IMO.

Good point. Lets try and do it this way.

H:
> > Did you extract the Linke turbidity values from the app. on the EU
> > web site, derive it from measured in situ values, or are you
> > tweaking that variable as a calibration to get r.sun to match your
> > measured radiation values?

D:
> I used the 'rural' values from the r.sun manual page as a starting
> point, and then modified according to my observations of the local
> climate + tweaking to make r.sun match the weather station: i.e. i
> need a more robust approach!

I thought the SoDa page used to be linked from the r.sun page, but no
longer I guess as the link has changed.

Looks like it.

Get local Linke values from the SoDa Solar radiation Databases
http://www.soda-is.com/

Tried this, and got unrealistic values for my site.

--
Java applet: (old link, no longer works)
  http://prime.jrc.it/SoDa/SoDa.html
  SoDa Prototype -> Climatological Data & Derived -> Climatological Data
      -> Monthly values of the Linke turbidity factor [double-click]
--

Now you extract local values with a PHP script:
  http://www.soda-is.com/eng/services/climat_eng.html
info: (+ links to worldwide monthly maps)
  http://stratus.meteotest.ch/soda_tl/soda_tl.html

the worldwide database was available for purchase, I'm not sure of the
current state of that.

other useful links:
Jaro's talk at the 2002 GRASS conference:
http://www.ing.unitn.it/~grass/conferences/GRASS2002/proceedings/proceeding
s/pdfs/Hofierka_Jaroslav.pdf Video of the presentation:
http://www.jus.unitn.it/services/arc/2002/0911/7.ram

thanks. had this on file, and forgot about it.

> > I see you have a bit of sawtooth noise in your plots. FWIW to avoid
> > introducing this monthly jump I wrote a little app some time ago
> > which interpolated daily Linke value from the monthly Linke averages
> > (for use in r.sun bash script loop) [assume monthly averages are for
> > day 15 of each month and interpolate for the days in between]. I'd
> > have to dig it out, & it's probably a mix of Matlab+C. Maybe
> > something for the wiki add-ons.
>
> Indeed! I am using a constant value for the entire month. this
> approach is a step in the right direction, but introduces those
> sawtooth artifacts. Since I am working in R most of the time (for
> numerical operations at least), I think that i might implement it
> there, and then save the results for use in a bash script - which
> invokes r.sun.
>
> Did your code look something like this;
> # define monthly averages
> l <- c(5.9,5.8,5.6,5.6,5.3,5.0,4.8,5.0,5.0,5.2,5.4,5.6)
> # use periodic spline interpolation to fill in the 365 days of the
> # year
> l.s <- spline(l, method="periodic", n=365)
> # plot results:
> plot(l, xlab="month", ylab="Linke Turbidity")
> lines(l.s)
>
> note that this is not quite correct as it interpolates from the
> *start* of the month, as opposed to the middle. Doing that would
> require knowing the day of the middle of the month- i'll cook up that
> example shortly.

something like that (I don't understand much R syntax). I didn't bother
to figure out if the middle of the month was day 14|14.5|15|15.5, I just
figured day 15 was close enough -- we shouldn't fool ourselves into
thinking the interpolated values will be high precision data.

Ok. Here is some more sample code, with annotation- this time associating the
mean monthly values with the center of each month:

# same monthly estimates
ld <- c(5.9,5.8,5.6,5.6,5.3,5.0,4.8,5.0,5.0,5.2,5.4,5.6)

# approximate month centers: pretend months are always 30.41 days long
d <- seq(15,365, by=30.41)

# combine into matrix, by column
ldd <- cbind(d,ld)

# spline interpolation;
ldd.s <- spline(ldd, method="periodic", n=365)

# plot
plot(ldd, xlab="Day of the Year", ylab="Linke Turbidity")
lines(ldd.s)

# dump daily interpolated values in a script-friendly format:
write.table(paste('linke[', 1:365, ']=', round(ldd.s$y,3), sep=''),
row.names=F, quote=F)

I am not familiar with "linke_monthly[1]=5.9" in bash. Is that legal??

I am not sure if it is legal, but it seems to work for me.

Thanks for the tips,

--
Dylan Beaudette
Soils and Biogeochemistry Graduate Group
University of California at Davis
530.754.7341

H_B · December 6, 2006, 7:42am

Dylan Beaudette wrote:

> Get local Linke values from the SoDa Solar radiation Databases
> http://www.soda-is.com/

Tried this, and got unrealistic values for my site.

how so? Have you calculated good Linke values from your light meters?
(formula for that, please?)

beware some LICOR meters return PAR (400-700nm) and some full spectrum.
The PAR measurements should be adjusted before comparison with r.sun
results to compensate for the missing wavelengths (but fortunately
evolution has calibrated plants (and eyes) pretty well to match the
sun's dominant energies).

Hamish

Dylan_Beaudette1 · December 6, 2006, 5:28pm

On 12/5/06, Hamish <hamish_nospam@yahoo.com> wrote:

Dylan Beaudette wrote:
>
> > Get local Linke values from the SoDa Solar radiation Databases
> > http://www.soda-is.com/
>
> Tried this, and got unrealistic values for my site.

how so? Have you calculated good Linke values from your light meters?
(formula for that, please?)

The soda-is values ranged from 2.8-3.8 - however through
experimentation, and the finally a calculation (reference in a minute)
the real values should be closer to the range 2-6 : smallest values in
the middle of summer and fall, highest values in the winter and
spring.

I have calculated Linke turbidity for my site with the equation
presented in [1]. an image of the equation is attached to this
message. If you would like I can send a snippet of the article to you
off-list. Unfortunately, there are some aspects to this equation which
I do not understand: namely the two constants present. I am currently
working with an atmospheric scientist to track down the sources of
error in this equation - apparantly there are some issues.

Hopefully, by the end of the week I will have a more appropriate
equation for connecting r.sun (mode 1) + local pyranometer data to
calculate Linke turbidity.

1. Becker, S. Caclulation of Direct Solar And Diffuse Radiation in
Israel International Journal of Climatology, 2001, 21, 1561 - 1576

beware some LICOR meters return PAR (400-700nm) and some full spectrum.
The PAR measurements should be adjusted before comparison with r.sun
results to compensate for the missing wavelengths (but fortunately
evolution has calibrated plants (and eyes) pretty well to match the
sun's dominant energies).

Good point. Looking at the literature associated with this sensor
(LI-200SZ) the pyranometer is integrating over energy from 400nm -
1000nm . This is a subset of the entire solar spectrum, but covers a
bt more than the PAR subset.

Mostly, I am looking for a relatively simple approach to calculate
local linke values. Since these will be estimates, based on other
estimates, I am certainly not expecting perfection.

Cheers,

Dylan

(attachments)

Dylan_Beaudette1 · December 7, 2006, 12:29am

On Tuesday 05 December 2006 23:42, Hamish wrote:

Dylan Beaudette wrote:
> > Get local Linke values from the SoDa Solar radiation Databases
> > http://www.soda-is.com/
>
> Tried this, and got unrealistic values for my site.

how so? Have you calculated good Linke values from your light meters?
(formula for that, please?)

beware some LICOR meters return PAR (400-700nm) and some full spectrum.
The PAR measurements should be adjusted before comparison with r.sun
results to compensate for the missing wavelengths (but fortunately
evolution has calibrated plants (and eyes) pretty well to match the
sun's dominant energies).

Hamish

Hi Hamish

An update:

Attached is a plot of estimated Linke turbidity values, compared to the SoDa
derived data. Details on this calculation soon to follow in a new thread.

cheers,

--
Dylan Beaudette
Soils and Biogeochemistry Graduate Group
University of California at Davis
530.754.7341

(attachments)

H_B · December 8, 2006, 2:48am

Dylan Beaudette wrote:

> > > Get local Linke values from the SoDa Solar radiation Databases
> > > http://www.soda-is.com/
> >
> > Tried this, and got unrealistic values for my site.
>
> how so? Have you calculated good Linke values from your light
> meters? (formula for that, please?)

..

Attached is a plot of estimated Linke turbidity values, compared to
the SoDa derived data. Details on this calculation soon to follow in
a new thread.

I assume the physical basis for the Linke turbidity factor is the amount
of scattering dust, ash, water vapour, turbulence etc. in either the
column of air directly above the sampling site or in the ray between the
site and the Sun (which will pass through more atm in winter due to the
lower angle). Maybe the paper sheds some light on that. So e.g. LA
county would have very high turbidity numbers.

I would figure in the winter the air to be crisper/clearer due to less
water vapour, less turbulence, and less upwelling of aerosol laiden warm
air from the Earth's surface. So lower turbidity numbers in the winter
as the SoDa data shows. BUT if you live in a place which is prone to
inversion layers in winter and many locals heat their homes with wood or
coal, winter air quality may turn out to be much more turbid.

I have calculated Linke turbidity for my site with the equation

T_linke = ln( G / ( I_0 * sin(h) * 0.84) ) * ( sin(h) / -0.027 )

Unfortunately, there are some aspects to this equation which
I do not understand: namely the two constants present.

perhaps they are empirically derived?

I take it I_0 is pure-clear air intentensity?; h is latitude?; and is G
measured intensity, or ..?

1. Becker, S. Caclulation of Direct Solar And Diffuse Radiation in
Israel International Journal of Climatology, 2001, 21, 1561 - 1576

I will have to track down a copy of that. Holy cow, for once our library
is actually subscribed to the e-journal. Downloading the PDF now..

What software did you use to make that plot? Looks nice.

Hamish

Dylan_Beaudette1 · December 8, 2006, 6:13am

On 12/7/06, Hamish <hamish_nospam@yahoo.com> wrote:

Dylan Beaudette wrote:
> > > > Get local Linke values from the SoDa Solar radiation Databases
> > > > http://www.soda-is.com/
> > >
> > > Tried this, and got unrealistic values for my site.
> >
> > how so? Have you calculated good Linke values from your light
> > meters? (formula for that, please?)
..
> Attached is a plot of estimated Linke turbidity values, compared to
> the SoDa derived data. Details on this calculation soon to follow in
> a new thread.

Hi Hamish,

Looks like I sent an update this morning with an attachment that was too large!

I'll Include it at the bottom of my replies to you last message.

I assume the physical basis for the Linke turbidity factor is the amount
of scattering dust, ash, water vapour, turbulence etc. in either the
column of air directly above the sampling site or in the ray between the
site and the Sun (which will pass through more atm in winter due to the
lower angle). Maybe the paper sheds some light on that. So e.g. LA
county would have very high turbidity numbers.

I would figure in the winter the air to be crisper/clearer due to less
water vapour, less turbulence, and less upwelling of aerosol laiden warm
air from the Earth's surface. So lower turbidity numbers in the winter
as the SoDa data shows. BUT if you live in a place which is prone to
inversion layers in winter and many locals heat their homes with wood or
coal, winter air quality may turn out to be much more turbid.

Actually have a couple good papers on the defintion for the Linke
turbidity values:

Louche, A.; Peri, G. & Iqbal, M. An analysis of linke turbidity factor
Solar Energy, 1986, 37, 393-396

Rapti, A. S. Atmospheric transparency, atmospheric turbidity and
climatic parameters Solar Energy, 2000, 69, 99-111

Kasten, F. The linke turbidity factor based on improved values of the
integral Rayleigh optical thickness Solar Energy, 1996, 56, 239-244

My site is in a mediterranean, semi-coastal / semi-arid setting where
winter time moisture and summer time dryness are the usual weather
patterns. Summer through fall are usually cloud free, and quite warm.
Also- this site is rather remote, so urban ag-related dust influences
are minimal. These are some of the reasons that I was having second
thoughts about the SoDa values.

> I have calculated Linke turbidity for my site with the equation

T_linke = ln( G / ( I_0 * sin(h) * 0.84) ) * ( sin(h) / -0.027 )

> Unfortunately, there are some aspects to this equation which
> I do not understand: namely the two constants present.

perhaps they are empirically derived?

Ok- big change in direction. I posted an update to the use of the
above equation earlier today but it was rejected due to a large
attachment. Details below....

I take it I_0 is pure-clear air intentensity?; h is latitude?; and is G
measured intensity, or ..?

G = global radiance measured on the ground (beam+diffuse)
I_o = extraterrestrial radiance
h = solar elevation (angle)

> 1. Becker, S. Caclulation of Direct Solar And Diffuse Radiation in
> Israel International Journal of Climatology, 2001, 21, 1561 - 1576

I will have to track down a copy of that. Holy cow, for once our library
is actually subscribed to the e-journal. Downloading the PDF now..

... i know how that can be sometimes!

What software did you use to make that plot? Looks nice.

Thanks. It was made in R, a fun command-line based environment for
plotting, analysis, etc. I can post the code used after a small
cleanup.

Ok, here is the text from the message rejected this morning:

Some updates on my findings with respect to obtaining optimal Linke
turbidity values via local pyranometer measurements.

Chatting with a local atmospheric scientist, I learned that it is
possible (with a loss of some precision of course) to dissaggregate
beam radiance from global radiance (as measured by pyranometer), for
use in an equation [1] for the linke turbidity factor. Compared to a
previous paper I mentioned on grass-dev [2], the equation presented in
[1] is a much more appropriate approach to calculating Linke turbidity values.

The key points to using the equation from [1] are:

- historical pyranometer data from a weather station, with coordinates!
- r.sun mode 1 to calculate solar elevation at this station, for each
day at some time. extraterrestrial radiance is also calculated in this
step
- dissagregation of the beam component from global radiance with the
assumption that diffuse radiance is approx 10%-20% that of the beam
radiance
- conversion of dissaggregated beam radiance to beam radiance on the
normal (B/sin alpha)

here is a link to a bit of my thesis on this topic: (rather incomplete!)
http://169.237.35.250/~dylan/temp/linke-cucumo2000.png

Here is a link of linke turbidity values, as calculated by the method
described above, along with a comparison to the SoDa-derived values.
There is considerable difference between the two sources!

http://169.237.35.250/~dylan/temp/cucumo-estimated_daily_linke_values.png

With the equation from [1] it is possible to estimate the linke
turbidity value for each day (using the 12pm calculation as
representative for an entire day), and use these estimates in r.sun
mode 2. here is a summary of the incorporation of estimated linke
values into r.sun - as compared to the weather station data:

http://169.237.35.250/~dylan/temp/11-yr_variation_vs_modeled.png

I'll post some updates as they come. If I have anything wrong please
don't hesitate to point it out!

Cheers,

Dylan

1. Cucumo, M.; Kaliakatsos, D. & Marinelli, V. A calculation method
for the estimation of the Linke turbidity factor Renewable Energy,
2000, 19, 249-258

2. Becker, S. Caclulation of Direct Solar And Diffuse Radiation in
Israel International Journal of Climatology, 2001, 21, 1561 - 1576

A_Ruiz_Arias · December 8, 2006, 12:09pm

Hi all,

firstly, I really hope not to interfere in this conversation. I assist
expectant to the thread cause is very interesting for me.

It seems you want to get diffuse component from global radiation. If I
have understood well, you estimate it as the 10%-20% of the global one.
Just a little suggestion that I hope not to be very simplistic. Have you
think about a correlation between the clearness-index and the proportion
of diffuse? There exist several correlations around the world such as the
studies of Orgill, Reindl, Erbs, Holland or Duffie, for example. They
usually use hourly values. You can easily obtain a estimation for the
hourly diffuse component (and, therefore, for the hourly direct beam one)
from the hourly global irradiation measured with a pyranometer as the
LICOR. If you are interesting in this approach just let me know and I will
be delighted of sending you more references. Now I can't be more explicit
because I don't have access to my personal data-base cause I am not at the
office.

Kind regards,

José A.

On 12/7/06, Hamish <hamish_nospam@yahoo.com> wrote:

Dylan Beaudette wrote:
> > > > Get local Linke values from the SoDa Solar radiation Databases
> > > > http://www.soda-is.com/
> > >
> > > Tried this, and got unrealistic values for my site.
> >
> > how so? Have you calculated good Linke values from your light
> > meters? (formula for that, please?)
..
> Attached is a plot of estimated Linke turbidity values, compared to
> the SoDa derived data. Details on this calculation soon to follow in
> a new thread.

Hi Hamish,

Looks like I sent an update this morning with an attachment that was too
large!

I'll Include it at the bottom of my replies to you last message.

I assume the physical basis for the Linke turbidity factor is the amount
of scattering dust, ash, water vapour, turbulence etc. in either the
column of air directly above the sampling site or in the ray between the
site and the Sun (which will pass through more atm in winter due to the
lower angle). Maybe the paper sheds some light on that. So e.g. LA
county would have very high turbidity numbers.

I would figure in the winter the air to be crisper/clearer due to less
water vapour, less turbulence, and less upwelling of aerosol laiden warm
air from the Earth's surface. So lower turbidity numbers in the winter
as the SoDa data shows. BUT if you live in a place which is prone to
inversion layers in winter and many locals heat their homes with wood or
coal, winter air quality may turn out to be much more turbid.

Actually have a couple good papers on the defintion for the Linke
turbidity values:

Louche, A.; Peri, G. & Iqbal, M. An analysis of linke turbidity factor
Solar Energy, 1986, 37, 393-396

Rapti, A. S. Atmospheric transparency, atmospheric turbidity and
climatic parameters Solar Energy, 2000, 69, 99-111

Kasten, F. The linke turbidity factor based on improved values of the
integral Rayleigh optical thickness Solar Energy, 1996, 56, 239-244

My site is in a mediterranean, semi-coastal / semi-arid setting where
winter time moisture and summer time dryness are the usual weather
patterns. Summer through fall are usually cloud free, and quite warm.
Also- this site is rather remote, so urban ag-related dust influences
are minimal. These are some of the reasons that I was having second
thoughts about the SoDa values.

> I have calculated Linke turbidity for my site with the equation

T_linke = ln( G / ( I_0 * sin(h) * 0.84) ) * ( sin(h) / -0.027 )

> Unfortunately, there are some aspects to this equation which
> I do not understand: namely the two constants present.

perhaps they are empirically derived?

Ok- big change in direction. I posted an update to the use of the
above equation earlier today but it was rejected due to a large
attachment. Details below....

I take it I_0 is pure-clear air intentensity?; h is latitude?; and is G
measured intensity, or ..?

G = global radiance measured on the ground (beam+diffuse)
I_o = extraterrestrial radiance
h = solar elevation (angle)

> 1. Becker, S. Caclulation of Direct Solar And Diffuse Radiation in
> Israel International Journal of Climatology, 2001, 21, 1561 - 1576

I will have to track down a copy of that. Holy cow, for once our library
is actually subscribed to the e-journal. Downloading the PDF now..

... i know how that can be sometimes!

What software did you use to make that plot? Looks nice.

Thanks. It was made in R, a fun command-line based environment for
plotting, analysis, etc. I can post the code used after a small
cleanup.

Ok, here is the text from the message rejected this morning:

Some updates on my findings with respect to obtaining optimal Linke
turbidity values via local pyranometer measurements.

Chatting with a local atmospheric scientist, I learned that it is
possible (with a loss of some precision of course) to dissaggregate
beam radiance from global radiance (as measured by pyranometer), for
use in an equation [1] for the linke turbidity factor. Compared to a
previous paper I mentioned on grass-dev [2], the equation presented in
[1] is a much more appropriate approach to calculating Linke turbidity
values.

The key points to using the equation from [1] are:

- historical pyranometer data from a weather station, with coordinates!
- r.sun mode 1 to calculate solar elevation at this station, for each
day at some time. extraterrestrial radiance is also calculated in this
step
- dissagregation of the beam component from global radiance with the
assumption that diffuse radiance is approx 10%-20% that of the beam
radiance
- conversion of dissaggregated beam radiance to beam radiance on the
normal (B/sin alpha)

here is a link to a bit of my thesis on this topic: (rather incomplete!)
http://169.237.35.250/~dylan/temp/linke-cucumo2000.png

Here is a link of linke turbidity values, as calculated by the method
described above, along with a comparison to the SoDa-derived values.
There is considerable difference between the two sources!

http://169.237.35.250/~dylan/temp/cucumo-estimated_daily_linke_values.png

With the equation from [1] it is possible to estimate the linke
turbidity value for each day (using the 12pm calculation as
representative for an entire day), and use these estimates in r.sun
mode 2. here is a summary of the incorporation of estimated linke
values into r.sun - as compared to the weather station data:

http://169.237.35.250/~dylan/temp/11-yr_variation_vs_modeled.png

I'll post some updates as they come. If I have anything wrong please
don't hesitate to point it out!

Cheers,

Dylan

1. Cucumo, M.; Kaliakatsos, D. & Marinelli, V. A calculation method
for the estimation of the Linke turbidity factor Renewable Energy,
2000, 19, 249-258

2. Becker, S. Caclulation of Direct Solar And Diffuse Radiation in
Israel International Journal of Climatology, 2001, 21, 1561 - 1576

_______________________________________________
grassuser mailing list
grassuser@grass.itc.it
http://grass.itc.it/mailman/listinfo/grassuser

Dylan_Beaudette1 · December 8, 2006, 5:15pm

Hi Jose,

On 12/8/06, jararias@ujaen.es <jararias@ujaen.es> wrote:

Hi all,

firstly, I really hope not to interfere in this conversation. I assist
expectant to the thread cause is very interesting for me.

no problem! Much of this thread was the result of me not knowing
enough about r.sun and the pyranometer in question. All of this great
input from people who are specialists is extremely helpful.

It seems you want to get diffuse component from global radiation. If I
have understood well, you estimate it as the 10%-20% of the global one.
Just a little suggestion that I hope not to be very simplistic. Have you
think about a correlation between the clearness-index and the proportion
of diffuse? There exist several correlations around the world such as the
studies of Orgill, Reindl, Erbs, Holland or Duffie, for example. They
usually use hourly values. You can easily obtain a estimation for the
hourly diffuse component (and, therefore, for the hourly direct beam one)
from the hourly global irradiation measured with a pyranometer as the
LICOR. If you are interesting in this approach just let me know and I will
be delighted of sending you more references. Now I can't be more explicit
because I don't have access to my personal data-base cause I am not at the
office.

Kind regards,

José A.

This sounds like a much more robust approach to the estimation of
Linke turbidity values from a pyranometer. While I am very interested
in the papers you are suggesting, I am not sure if i will be able to
integrate a much more complex component into this project. However, in
further work these methods would add considerable value (i think) to
any such estimates. Feel free to post the references when you have as
chance.

Also-- much of these resources can be summarized into a min-manual for r.sun.

Cheers,

Dylan

> On 12/7/06, Hamish <hamish_nospam@yahoo.com> wrote:
>> Dylan Beaudette wrote:
>> > > > > Get local Linke values from the SoDa Solar radiation Databases
>> > > > > http://www.soda-is.com/
>> > > >
>> > > > Tried this, and got unrealistic values for my site.
>> > >
>> > > how so? Have you calculated good Linke values from your light
>> > > meters? (formula for that, please?)
>> ..
>> > Attached is a plot of estimated Linke turbidity values, compared to
>> > the SoDa derived data. Details on this calculation soon to follow in
>> > a new thread.
>
> Hi Hamish,
>
> Looks like I sent an update this morning with an attachment that was too
> large!
>
> I'll Include it at the bottom of my replies to you last message.
>
>> I assume the physical basis for the Linke turbidity factor is the amount
>> of scattering dust, ash, water vapour, turbulence etc. in either the
>> column of air directly above the sampling site or in the ray between the
>> site and the Sun (which will pass through more atm in winter due to the
>> lower angle). Maybe the paper sheds some light on that. So e.g. LA
>> county would have very high turbidity numbers.
>>
>> I would figure in the winter the air to be crisper/clearer due to less
>> water vapour, less turbulence, and less upwelling of aerosol laiden warm
>> air from the Earth's surface. So lower turbidity numbers in the winter
>> as the SoDa data shows. BUT if you live in a place which is prone to
>> inversion layers in winter and many locals heat their homes with wood or
>> coal, winter air quality may turn out to be much more turbid.
>>
>
> Actually have a couple good papers on the defintion for the Linke
> turbidity values:
>
> Louche, A.; Peri, G. & Iqbal, M. An analysis of linke turbidity factor
> Solar Energy, 1986, 37, 393-396
>
> Rapti, A. S. Atmospheric transparency, atmospheric turbidity and
> climatic parameters Solar Energy, 2000, 69, 99-111
>
> Kasten, F. The linke turbidity factor based on improved values of the
> integral Rayleigh optical thickness Solar Energy, 1996, 56, 239-244
>
> My site is in a mediterranean, semi-coastal / semi-arid setting where
> winter time moisture and summer time dryness are the usual weather
> patterns. Summer through fall are usually cloud free, and quite warm.
> Also- this site is rather remote, so urban ag-related dust influences
> are minimal. These are some of the reasons that I was having second
> thoughts about the SoDa values.
>
>>
>> > I have calculated Linke turbidity for my site with the equation
>>
>> T_linke = ln( G / ( I_0 * sin(h) * 0.84) ) * ( sin(h) / -0.027 )
>>
>> > Unfortunately, there are some aspects to this equation which
>> > I do not understand: namely the two constants present.
>>
>> perhaps they are empirically derived?
>
> Ok- big change in direction. I posted an update to the use of the
> above equation earlier today but it was rejected due to a large
> attachment. Details below....
>
>> I take it I_0 is pure-clear air intentensity?; h is latitude?; and is G
>> measured intensity, or ..?
>
> G = global radiance measured on the ground (beam+diffuse)
> I_o = extraterrestrial radiance
> h = solar elevation (angle)
>
>> > 1. Becker, S. Caclulation of Direct Solar And Diffuse Radiation in
>> > Israel International Journal of Climatology, 2001, 21, 1561 - 1576
>>
>> I will have to track down a copy of that. Holy cow, for once our library
>> is actually subscribed to the e-journal. Downloading the PDF now..
>>
>
> ... i know how that can be sometimes!
>
>> What software did you use to make that plot? Looks nice.
>>
>
> Thanks. It was made in R, a fun command-line based environment for
> plotting, analysis, etc. I can post the code used after a small
> cleanup.
>
> Ok, here is the text from the message rejected this morning:
>
> Some updates on my findings with respect to obtaining optimal Linke
> turbidity values via local pyranometer measurements.
>
> Chatting with a local atmospheric scientist, I learned that it is
> possible (with a loss of some precision of course) to dissaggregate
> beam radiance from global radiance (as measured by pyranometer), for
> use in an equation [1] for the linke turbidity factor. Compared to a
> previous paper I mentioned on grass-dev [2], the equation presented in
> [1] is a much more appropriate approach to calculating Linke turbidity
> values.
>
> The key points to using the equation from [1] are:
>
> - historical pyranometer data from a weather station, with coordinates!
> - r.sun mode 1 to calculate solar elevation at this station, for each
> day at some time. extraterrestrial radiance is also calculated in this
> step
> - dissagregation of the beam component from global radiance with the
> assumption that diffuse radiance is approx 10%-20% that of the beam
> radiance
> - conversion of dissaggregated beam radiance to beam radiance on the
> normal (B/sin alpha)
>
> here is a link to a bit of my thesis on this topic: (rather incomplete!)
> http://169.237.35.250/~dylan/temp/linke-cucumo2000.png
>
> Here is a link of linke turbidity values, as calculated by the method
> described above, along with a comparison to the SoDa-derived values.
> There is considerable difference between the two sources!
>
> http://169.237.35.250/~dylan/temp/cucumo-estimated_daily_linke_values.png
>
> With the equation from [1] it is possible to estimate the linke
> turbidity value for each day (using the 12pm calculation as
> representative for an entire day), and use these estimates in r.sun
> mode 2. here is a summary of the incorporation of estimated linke
> values into r.sun - as compared to the weather station data:
>
> http://169.237.35.250/~dylan/temp/11-yr_variation_vs_modeled.png
>
> I'll post some updates as they come. If I have anything wrong please
> don't hesitate to point it out!
>
> Cheers,
>
> Dylan
>
> 1. Cucumo, M.; Kaliakatsos, D. & Marinelli, V. A calculation method
> for the estimation of the Linke turbidity factor Renewable Energy,
> 2000, 19, 249-258
>
> 2. Becker, S. Caclulation of Direct Solar And Diffuse Radiation in
> Israel International Journal of Climatology, 2001, 21, 1561 - 1576
>
> _______________________________________________
> grassuser mailing list
> grassuser@grass.itc.it
> http://grass.itc.it/mailman/listinfo/grassuser
>

A_Ruiz_Arias · December 12, 2006, 11:59am

Hi all,

as I promised you (Dylan), here you have a useful reference to derive the
direct beam component from the global radiation.

Jacovides, C.P., Tymvios, F.S., Assimakopoulos, V.A, and Kaltsounides N.A.
(2006): ' Comparative study of various correlations in estimating hourly
diffuse fraction of global solar radiation' Renewable Energy, 31, 2492-2504

This paper offers a comparative analysis of different correlations between
diffuse and global radiation. The references [2], [3], [4] and [9] on it, I
think are especially interesting. I mention only this paper because on it
you can find a summary of the most important correlations.

Reference [3] is especially simple and easy to read although it only uses
measurements of Toronto. Also very interesting for you, perhaps, is the
reference [4] since it is based on five USA locations. For example, this
author proposes the correlation

kd = 1.45-1.67kt for 0.3<kt<0.78
kd = 1.02-0.248kt for kt<=0.3
kd = 0.147 for kt>=0.78.

where kt is the ratio of global to extraterrestrial hourly radiation and kd
is the ratio of diffuse to global hourly radiation. Therefore, to calculate
kt only need the measured global radiation and the extraterrestrial
radiation, theoretically calculated for a given location and time (see for
example, Iqbal, Muhammad (1983). An Introduction to Solar Radiation.
Academic Press, New York). Then you get kd from the correlation, and

Direct Beam = Global*(1-kd).

So, if you dispose of global measurements, you can get the direct beam
radiation directly applying the former expression.

By the way, I would like to know what you are trying to do, improving r.sun?
or just working in a personal project? I have used r.sun a little bit and I
would like to collaborate with r.sun. I could work with the model and/or
even developing some snippets of code.

Cheers,

Jose A.

-----Mensaje original-----
De: grassuser-bounces@grass.itc.it [mailto:grassuser-bounces@grass.itc.it]
En nombre de jararias@ujaen.es
Enviado el: viernes, 08 de diciembre de 2006 13:09
Para: grassuser@grass.itc.it
Asunto: Re: [GRASS-user] Re: [GRASS-dev] Changes to r.sun made Feb 2006
[resolved]

Hi all,

firstly, I really hope not to interfere in this conversation. I assist
expectant to the thread cause is very interesting for me.

It seems you want to get diffuse component from global radiation. If I
have understood well, you estimate it as the 10%-20% of the global one.
Just a little suggestion that I hope not to be very simplistic. Have you
think about a correlation between the clearness-index and the proportion
of diffuse? There exist several correlations around the world such as the
studies of Orgill, Reindl, Erbs, Holland or Duffie, for example. They
usually use hourly values. You can easily obtain a estimation for the
hourly diffuse component (and, therefore, for the hourly direct beam one)
from the hourly global irradiation measured with a pyranometer as the
LICOR. If you are interesting in this approach just let me know and I will
be delighted of sending you more references. Now I can't be more explicit
because I don't have access to my personal data-base cause I am not at the
office.

Kind regards,

José A.

> On 12/7/06, Hamish <hamish_nospam@yahoo.com> wrote:
>> Dylan Beaudette wrote:
>> > > > > Get local Linke values from the SoDa Solar radiation Databases
>> > > > > http://www.soda-is.com/
>> > > >
>> > > > Tried this, and got unrealistic values for my site.
>> > >
>> > > how so? Have you calculated good Linke values from your light
>> > > meters? (formula for that, please?)
>> ..
>> > Attached is a plot of estimated Linke turbidity values, compared to
>> > the SoDa derived data. Details on this calculation soon to follow in
>> > a new thread.
>
> Hi Hamish,
>
> Looks like I sent an update this morning with an attachment that was too
> large!
>
> I'll Include it at the bottom of my replies to you last message.
>
>
>> I assume the physical basis for the Linke turbidity factor is the
amount
>> of scattering dust, ash, water vapour, turbulence etc. in either the
>> column of air directly above the sampling site or in the ray between
the
>> site and the Sun (which will pass through more atm in winter due to the
>> lower angle). Maybe the paper sheds some light on that. So e.g. LA
>> county would have very high turbidity numbers.
>>
>> I would figure in the winter the air to be crisper/clearer due to less
>> water vapour, less turbulence, and less upwelling of aerosol laiden
warm
>> air from the Earth's surface. So lower turbidity numbers in the winter
>> as the SoDa data shows. BUT if you live in a place which is prone to
>> inversion layers in winter and many locals heat their homes with wood
or
>> coal, winter air quality may turn out to be much more turbid.
>>
>
> Actually have a couple good papers on the defintion for the Linke
> turbidity values:
>
> Louche, A.; Peri, G. & Iqbal, M. An analysis of linke turbidity factor
> Solar Energy, 1986, 37, 393-396
>
> Rapti, A. S. Atmospheric transparency, atmospheric turbidity and
> climatic parameters Solar Energy, 2000, 69, 99-111
>
> Kasten, F. The linke turbidity factor based on improved values of the
> integral Rayleigh optical thickness Solar Energy, 1996, 56, 239-244
>
> My site is in a mediterranean, semi-coastal / semi-arid setting where
> winter time moisture and summer time dryness are the usual weather
> patterns. Summer through fall are usually cloud free, and quite warm.
> Also- this site is rather remote, so urban ag-related dust influences
> are minimal. These are some of the reasons that I was having second
> thoughts about the SoDa values.
>
>>
>> > I have calculated Linke turbidity for my site with the equation
>>
>> T_linke = ln( G / ( I_0 * sin(h) * 0.84) ) * ( sin(h) / -0.027 )
>>
>> > Unfortunately, there are some aspects to this equation which
>> > I do not understand: namely the two constants present.
>>
>> perhaps they are empirically derived?
>
> Ok- big change in direction. I posted an update to the use of the
> above equation earlier today but it was rejected due to a large
> attachment. Details below....
>
>> I take it I_0 is pure-clear air intentensity?; h is latitude?; and is G
>> measured intensity, or ..?
>
> G = global radiance measured on the ground (beam+diffuse)
> I_o = extraterrestrial radiance
> h = solar elevation (angle)
>
>> > 1. Becker, S. Caclulation of Direct Solar And Diffuse Radiation in
>> > Israel International Journal of Climatology, 2001, 21, 1561 - 1576
>>
>> I will have to track down a copy of that. Holy cow, for once our
library
>> is actually subscribed to the e-journal. Downloading the PDF now..
>>
>
> ... i know how that can be sometimes!
>
>> What software did you use to make that plot? Looks nice.
>>
>
> Thanks. It was made in R, a fun command-line based environment for
> plotting, analysis, etc. I can post the code used after a small
> cleanup.
>
> Ok, here is the text from the message rejected this morning:
>
>
> Some updates on my findings with respect to obtaining optimal Linke
> turbidity values via local pyranometer measurements.
>
> Chatting with a local atmospheric scientist, I learned that it is
> possible (with a loss of some precision of course) to dissaggregate
> beam radiance from global radiance (as measured by pyranometer), for
> use in an equation [1] for the linke turbidity factor. Compared to a
> previous paper I mentioned on grass-dev [2], the equation presented in
> [1] is a much more appropriate approach to calculating Linke turbidity
> values.
>
> The key points to using the equation from [1] are:
>
> - historical pyranometer data from a weather station, with coordinates!
> - r.sun mode 1 to calculate solar elevation at this station, for each
> day at some time. extraterrestrial radiance is also calculated in this
> step
> - dissagregation of the beam component from global radiance with the
> assumption that diffuse radiance is approx 10%-20% that of the beam
> radiance
> - conversion of dissaggregated beam radiance to beam radiance on the
> normal (B/sin alpha)
>
> here is a link to a bit of my thesis on this topic: (rather incomplete!)
> http://169.237.35.250/~dylan/temp/linke-cucumo2000.png
>
> Here is a link of linke turbidity values, as calculated by the method
> described above, along with a comparison to the SoDa-derived values.
> There is considerable difference between the two sources!
>
> http://169.237.35.250/~dylan/temp/cucumo-
estimated_daily_linke_values.png
>
> With the equation from [1] it is possible to estimate the linke
> turbidity value for each day (using the 12pm calculation as
> representative for an entire day), and use these estimates in r.sun
> mode 2. here is a summary of the incorporation of estimated linke
> values into r.sun - as compared to the weather station data:
>
> http://169.237.35.250/~dylan/temp/11-yr_variation_vs_modeled.png
>
>
> I'll post some updates as they come. If I have anything wrong please
> don't hesitate to point it out!
>
> Cheers,
>
> Dylan
>
>
> 1. Cucumo, M.; Kaliakatsos, D. & Marinelli, V. A calculation method
> for the estimation of the Linke turbidity factor Renewable Energy,
> 2000, 19, 249-258
>
> 2. Becker, S. Caclulation of Direct Solar And Diffuse Radiation in
> Israel International Journal of Climatology, 2001, 21, 1561 - 1576
>
> _______________________________________________
> grassuser mailing list
> grassuser@grass.itc.it
> http://grass.itc.it/mailman/listinfo/grassuser
>

_______________________________________________
grassuser mailing list
grassuser@grass.itc.it
http://grass.itc.it/mailman/listinfo/grassuser

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Este mensaje ha sido analizado con NOD32 antivirus system
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Dylan_Beaudette1 · December 12, 2006, 6:58pm

Hi Jose,

Thank you for the great references, replies below:

On Tuesday 12 December 2006 03:59, José Antonio Ruiz Arias wrote:

Hi all,

as I promised you (Dylan), here you have a useful reference to derive the
direct beam component from the global radiation.

just printed the paper....

Jacovides, C.P., Tymvios, F.S., Assimakopoulos, V.A, and Kaltsounides N.A.
(2006): ' Comparative study of various correlations in estimating hourly
diffuse fraction of global solar radiation' Renewable Energy, 31, 2492-2504

This paper offers a comparative analysis of different correlations between
diffuse and global radiation. The references [2], [3], [4] and [9] on it, I
think are especially interesting. I mention only this paper because on it
you can find a summary of the most important correlations.

I wil look into these references as well.

Reference [3] is especially simple and easy to read although it only uses
measurements of Toronto. Also very interesting for you, perhaps, is the
reference [4] since it is based on five USA locations. For example, this
author proposes the correlation

kd = 1.45-1.67kt for 0.3<kt<0.78
kd = 1.02-0.248kt for kt<=0.3
kd = 0.147 for kt>=0.78.

This looks *similar* to the method proposed by
Cucumo, M.; Kaliakatsos, D. & Marinelli, V. A calculation method for the
estimation of the Linke turbidity factor Renewable Energy, 2000, 19, 249-258

... the various coeficients are different, suggesting the local emperical
nature of these relationships...

where kt is the ratio of global to extraterrestri
al hourly radiation and kd
is the ratio of diffuse to global hourly radiation. Therefore, to calculate
kt only need the measured global radiation and the extraterrestrial
radiation, theoretically calculated for a given location and time (see for
example, Iqbal, Muhammad (1983). An Introduction to Solar Radiation.
Academic Press, New York). Then you get kd from the correlation, and

Direct Beam = Global*(1-kd).

So, if you dispose of global measurements, you can get the direct beam
radiation directly applying the former expression.

... need to read up on this...

By the way, I would like to know what you are trying to do, improving
r.sun? or just working in a personal project? I have used r.sun a little
bit and I would like to collaborate with r.sun. I could work with the model
and/or even developing some snippets of code.

This thread started with my wanted a better set of estimated linke turbidity
factors for my study area- to best parameterize r.sun. Since then, it has
become apparent that others might be interested in a small document for
r.sun: i.e. establishing local linke turbidity values based on pyranometer
data for use in r.sun.

Cheers,

Jose A.

Thanks Jose.

Cheers,

Dylan

> -----Mensaje original-----
> De: grassuser-bounces@grass.itc.it
> [mailto:grassuser-bounces@grass.itc.it] En nombre de jararias@ujaen.es
> Enviado el: viernes, 08 de diciembre de 2006 13:09
> Para: grassuser@grass.itc.it
> Asunto: Re: [GRASS-user] Re: [GRASS-dev] Changes to r.sun made Feb 2006
> [resolved]
>
> Hi all,
>
> firstly, I really hope not to interfere in this conversation. I assist
> expectant to the thread cause is very interesting for me.
>
> It seems you want to get diffuse component from global radiation. If I
> have understood well, you estimate it as the 10%-20% of the global one.
> Just a little suggestion that I hope not to be very simplistic. Have you
> think about a correlation between the clearness-index and the proportion
> of diffuse? There exist several correlations around the world such as the
> studies of Orgill, Reindl, Erbs, Holland or Duffie, for example. They
> usually use hourly values. You can easily obtain a estimation for the
> hourly diffuse component (and, therefore, for the hourly direct beam one)
> from the hourly global irradiation measured with a pyranometer as the
> LICOR. If you are interesting in this approach just let me know and I
> will be delighted of sending you more references. Now I can't be more
> explicit because I don't have access to my personal data-base cause I am
> not at the office.
>
> Kind regards,
>
> José A.
>
> > On 12/7/06, Hamish <hamish_nospam@yahoo.com> wrote:
> >> Dylan Beaudette wrote:
> >> > > > > Get local Linke values from the SoDa Solar radiation Databases
> >> > > > > http://www.soda-is.com/
> >> > > >
> >> > > > Tried this, and got unrealistic values for my site.
> >> > >
> >> > > how so? Have you calculated good Linke values from your light
> >> > > meters? (formula for that, please?)
> >>
> >> ..
> >>
> >> > Attached is a plot of estimated Linke turbidity values, compared to
> >> > the SoDa derived data. Details on this calculation soon to follow
> >> > in a new thread.
> >
> > Hi Hamish,
> >
> > Looks like I sent an update this morning with an attachment that was
> > too large!
> >
> > I'll Include it at the bottom of my replies to you last message.
> >
> >> I assume the physical basis for the Linke turbidity factor is the
>
> amount
>
> >> of scattering dust, ash, water vapour, turbulence etc. in either the
> >> column of air directly above the sampling site or in the ray between
>
> the
>
> >> site and the Sun (which will pass through more atm in winter due to
> >> the lower angle). Maybe the paper sheds some light on that. So e.g. LA
> >> county would have very high turbidity numbers.
> >>
> >> I would figure in the winter the air to be crisper/clearer due to less
> >> water vapour, less turbulence, and less upwelling of aerosol laiden
>
> warm
>
> >> air from the Earth's surface. So lower turbidity numbers in the winter
> >> as the SoDa data shows. BUT if you live in a place which is prone to
> >> inversion layers in winter and many locals heat their homes with wood
>
> or
>
> >> coal, winter air quality may turn out to be much more turbid.
> >
> > Actually have a couple good papers on the defintion for the Linke
> > turbidity values:
> >
> > Louche, A.; Peri, G. & Iqbal, M. An analysis of linke turbidity factor
> > Solar Energy, 1986, 37, 393-396
> >
> > Rapti, A. S. Atmospheric transparency, atmospheric turbidity and
> > climatic parameters Solar Energy, 2000, 69, 99-111
> >
> > Kasten, F. The linke turbidity factor based on improved values of the
> > integral Rayleigh optical thickness Solar Energy, 1996, 56, 239-244
> >
> > My site is in a mediterranean, semi-coastal / semi-arid setting where
> > winter time moisture and summer time dryness are the usual weather
> > patterns. Summer through fall are usually cloud free, and quite warm.
> > Also- this site is rather remote, so urban ag-related dust influences
> > are minimal. These are some of the reasons that I was having second
> > thoughts about the SoDa values.
> >
> >> > I have calculated Linke turbidity for my site with the equation
> >>
> >> T_linke = ln( G / ( I_0 * sin(h) * 0.84) ) * ( sin(h) / -0.027 )
> >>
> >> > Unfortunately, there are some aspects to this equation which
> >> > I do not understand: namely the two constants present.
> >>
> >> perhaps they are empirically derived?
> >
> > Ok- big change in direction. I posted an update to the use of the
> > above equation earlier today but it was rejected due to a large
> > attachment. Details below....
> >
> >> I take it I_0 is pure-clear air intentensity?; h is latitude?; and is
> >> G measured intensity, or ..?
> >
> > G = global radiance measured on the ground (beam+diffuse)
> > I_o = extraterrestrial radiance
> > h = solar elevation (angle)
> >
> >> > 1. Becker, S. Caclulation of Direct Solar And Diffuse Radiation in
> >> > Israel International Journal of Climatology, 2001, 21, 1561 - 1576
> >>
> >> I will have to track down a copy of that. Holy cow, for once our
>
> library
>
> >> is actually subscribed to the e-journal. Downloading the PDF now..
> >
> > ... i know how that can be sometimes!
> >
> >> What software did you use to make that plot? Looks nice.
> >
> > Thanks. It was made in R, a fun command-line based environment for
> > plotting, analysis, etc. I can post the code used after a small
> > cleanup.
> >
> > Ok, here is the text from the message rejected this morning:
> >
> >
> > Some updates on my findings with respect to obtaining optimal Linke
> > turbidity values via local pyranometer measurements.
> >
> > Chatting with a local atmospheric scientist, I learned that it is
> > possible (with a loss of some precision of course) to dissaggregate
> > beam radiance from global radiance (as measured by pyranometer), for
> > use in an equation [1] for the linke turbidity factor. Compared to a
> > previous paper I mentioned on grass-dev [2], the equation presented in
> > [1] is a much more appropriate approach to calculating Linke turbidity
> > values.
> >
> > The key points to using the equation from [1] are:
> >
> > - historical pyranometer data from a weather station, with coordinates!
> > - r.sun mode 1 to calculate solar elevation at this station, for each
> > day at some time. extraterrestrial radiance is also calculated in this
> > step
> > - dissagregation of the beam component from global radiance with the
> > assumption that diffuse radiance is approx 10%-20% that of the beam
> > radiance
> > - conversion of dissaggregated beam radiance to beam radiance on the
> > normal (B/sin alpha)
> >
> > here is a link to a bit of my thesis on this topic: (rather
> > incomplete!) http://169.237.35.250/~dylan/temp/linke-cucumo2000.png
> >
> > Here is a link of linke turbidity values, as calculated by the method
> > described above, along with a comparison to the SoDa-derived values.
> > There is considerable difference between the two sources!
> >
> > http://169.237.35.250/~dylan/temp/cucumo-
>
> estimated_daily_linke_values.png
>
> > With the equation from [1] it is possible to estimate the linke
> > turbidity value for each day (using the 12pm calculation as
> > representative for an entire day), and use these estimates in r.sun
> > mode 2. here is a summary of the incorporation of estimated linke
> > values into r.sun - as compared to the weather station data:
> >
> > http://169.237.35.250/~dylan/temp/11-yr_variation_vs_modeled.png
> >
> >
> > I'll post some updates as they come. If I have anything wrong please
> > don't hesitate to point it out!
> >
> > Cheers,
> >
> > Dylan
> >
> >
> > 1. Cucumo, M.; Kaliakatsos, D. & Marinelli, V. A calculation method
> > for the estimation of the Linke turbidity factor Renewable Energy,
> > 2000, 19, 249-258
> >
> > 2. Becker, S. Caclulation of Direct Solar And Diffuse Radiation in
> > Israel International Journal of Climatology, 2001, 21, 1561 - 1576
> >
> > _______________________________________________
> > grassuser mailing list
> > grassuser@grass.itc.it
> > http://grass.itc.it/mailman/listinfo/grassuser
>
> _______________________________________________
> grassuser mailing list
> grassuser@grass.itc.it
> http://grass.itc.it/mailman/listinfo/grassuser
>
> __________ Información de NOD32 1914 (20061211) __________
>
> Este mensaje ha sido analizado con NOD32 antivirus system
> http://www.nod32.com

_______________________________________________
grassuser mailing list
grassuser@grass.itc.it
http://grass.itc.it/mailman/listinfo/grassuser

--
Dylan Beaudette
Soils and Biogeochemistry Graduate Group
University of California at Davis
530.754.7341

Dylan_Beaudette1 · December 12, 2006, 10:27pm

Hi Jose, some updates:

On Tuesday 12 December 2006 03:59, José Antonio Ruiz Arias wrote:

Hi all,

as I promised you (Dylan), here you have a useful reference to derive the
direct beam component from the global radiation.

Jacovides, C.P., Tymvios, F.S., Assimakopoulos, V.A, and Kaltsounides N.A.
(2006): ' Comparative study of various correlations in estimating hourly
diffuse fraction of global solar radiation' Renewable Energy, 31, 2492-2504

This paper offers a comparative analysis of different correlations between
diffuse and global radiation. The references [2], [3], [4] and [9] on it, I
think are especially interesting. I mention only this paper because on it
you can find a summary of the most important correlations.

Reference [3] is especially simple and easy to read although it only uses
measurements of Toronto. Also very interesting for you, perhaps, is the
reference [4] since it is based on five USA locations. For example, this
author proposes the correlation

kd = 1.45-1.67kt for 0.3<kt<0.78
kd = 1.02-0.248kt for kt<=0.3
kd = 0.147 for kt>=0.78.

Ok. I found these papers and have read over them. I think that if I were to
get this approach working* I will use equation 7 from Jacovides et al.
2006.... details below:

where kt is the ratio of global to extraterrestrial hourly radiation and kd
is the ratio of diffuse to global hourly radiation. Therefore, to calculate
kt only need the measured global radiation and the extraterrestrial
radiation, theoretically calculated for a given location and time (see for
example, Iqbal, Muhammad (1983). An Introduction to Solar Radiation.
Academic Press, New York). Then you get kd from the correlation, and

Direct Beam = Global*(1-kd).

So, if you dispose of global measurements, you can get the direct beam
radiation directly applying the former expression.

Ok. this is the expression I used (after calculating kt and kd):

kt = global radiance at 12pm / extraterrestrial radiance
kd = (from eqation 2 or 9)

unfortunately, when i compute a normal beam radiance from the kd values:

B = G(1 - kd)
Bn = B/sin(alpha)

and then compute linke turbidity values (using the equation from Cucumo et al
2000) : I end up with very high Linke values in the winter months:

http://169.237.35.250/~dylan/temp/reindl_linke_bad-estimates.png

This may be a result of not having a *single* clear day for my winter months
(I am using the max radiance at 12pm, over the course of 11 years to get as
close as possible to clear sky conditions).

Note that linke turbidity values in the above figure track those that I had
been estimating in the summer months: with the diffuse = 10-20% of beam
radiance assumption.

Quite possibly I am doing something incorrectly

Cheers,

Dylan

By the way, I would like to know what you are trying to do, improving
r.sun? or just working in a personal project? I have used r.sun a little
bit and I would like to collaborate with r.sun. I could work with the model
and/or even developing some snippets of code.

Cheers,

Jose A.

> -----Mensaje original-----
> De: grassuser-bounces@grass.itc.it
> [mailto:grassuser-bounces@grass.itc.it] En nombre de jararias@ujaen.es
> Enviado el: viernes, 08 de diciembre de 2006 13:09
> Para: grassuser@grass.itc.it
> Asunto: Re: [GRASS-user] Re: [GRASS-dev] Changes to r.sun made Feb 2006
> [resolved]
>
> Hi all,
>
> firstly, I really hope not to interfere in this conversation. I assist
> expectant to the thread cause is very interesting for me.
>
> It seems you want to get diffuse component from global radiation. If I
> have understood well, you estimate it as the 10%-20% of the global one.
> Just a little suggestion that I hope not to be very simplistic. Have you
> think about a correlation between the clearness-index and the proportion
> of diffuse? There exist several correlations around the world such as the
> studies of Orgill, Reindl, Erbs, Holland or Duffie, for example. They
> usually use hourly values. You can easily obtain a estimation for the
> hourly diffuse component (and, therefore, for the hourly direct beam one)
> from the hourly global irradiation measured with a pyranometer as the
> LICOR. If you are interesting in this approach just let me know and I
> will be delighted of sending you more references. Now I can't be more
> explicit because I don't have access to my personal data-base cause I am
> not at the office.
>
> Kind regards,
>
> José A.
>
> > On 12/7/06, Hamish <hamish_nospam@yahoo.com> wrote:
> >> Dylan Beaudette wrote:
> >> > > > > Get local Linke values from the SoDa Solar radiation Databases
> >> > > > > http://www.soda-is.com/
> >> > > >
> >> > > > Tried this, and got unrealistic values for my site.
> >> > >
> >> > > how so? Have you calculated good Linke values from your light
> >> > > meters? (formula for that, please?)
> >>
> >> ..
> >>
> >> > Attached is a plot of estimated Linke turbidity values, compared to
> >> > the SoDa derived data. Details on this calculation soon to follow
> >> > in a new thread.
> >
> > Hi Hamish,
> >
> > Looks like I sent an update this morning with an attachment that was
> > too large!
> >
> > I'll Include it at the bottom of my replies to you last message.
> >
> >> I assume the physical basis for the Linke turbidity factor is the
>
> amount
>
> >> of scattering dust, ash, water vapour, turbulence etc. in either the
> >> column of air directly above the sampling site or in the ray between
>
> the
>
> >> site and the Sun (which will pass through more atm in winter due to
> >> the lower angle). Maybe the paper sheds some light on that. So e.g. LA
> >> county would have very high turbidity numbers.
> >>
> >> I would figure in the winter the air to be crisper/clearer due to less
> >> water vapour, less turbulence, and less upwelling of aerosol laiden
>
> warm
>
> >> air from the Earth's surface. So lower turbidity numbers in the winter
> >> as the SoDa data shows. BUT if you live in a place which is prone to
> >> inversion layers in winter and many locals heat their homes with wood
>
> or
>
> >> coal, winter air quality may turn out to be much more turbid.
> >
> > Actually have a couple good papers on the defintion for the Linke
> > turbidity values:
> >
> > Louche, A.; Peri, G. & Iqbal, M. An analysis of linke turbidity factor
> > Solar Energy, 1986, 37, 393-396
> >
> > Rapti, A. S. Atmospheric transparency, atmospheric turbidity and
> > climatic parameters Solar Energy, 2000, 69, 99-111
> >
> > Kasten, F. The linke turbidity factor based on improved values of the
> > integral Rayleigh optical thickness Solar Energy, 1996, 56, 239-244
> >
> > My site is in a mediterranean, semi-coastal / semi-arid setting where
> > winter time moisture and summer time dryness are the usual weather
> > patterns. Summer through fall are usually cloud free, and quite warm.
> > Also- this site is rather remote, so urban ag-related dust influences
> > are minimal. These are some of the reasons that I was having second
> > thoughts about the SoDa values.
> >
> >> > I have calculated Linke turbidity for my site with the equation
> >>
> >> T_linke = ln( G / ( I_0 * sin(h) * 0.84) ) * ( sin(h) / -0.027 )
> >>
> >> > Unfortunately, there are some aspects to this equation which
> >> > I do not understand: namely the two constants present.
> >>
> >> perhaps they are empirically derived?
> >
> > Ok- big change in direction. I posted an update to the use of the
> > above equation earlier today but it was rejected due to a large
> > attachment. Details below....
> >
> >> I take it I_0 is pure-clear air intentensity?; h is latitude?; and is
> >> G measured intensity, or ..?
> >
> > G = global radiance measured on the ground (beam+diffuse)
> > I_o = extraterrestrial radiance
> > h = solar elevation (angle)
> >
> >> > 1. Becker, S. Caclulation of Direct Solar And Diffuse Radiation in
> >> > Israel International Journal of Climatology, 2001, 21, 1561 - 1576
> >>
> >> I will have to track down a copy of that. Holy cow, for once our
>
> library
>
> >> is actually subscribed to the e-journal. Downloading the PDF now..
> >
> > ... i know how that can be sometimes!
> >
> >> What software did you use to make that plot? Looks nice.
> >
> > Thanks. It was made in R, a fun command-line based environment for
> > plotting, analysis, etc. I can post the code used after a small
> > cleanup.
> >
> > Ok, here is the text from the message rejected this morning:
> >
> >
> > Some updates on my findings with respect to obtaining optimal Linke
> > turbidity values via local pyranometer measurements.
> >
> > Chatting with a local atmospheric scientist, I learned that it is
> > possible (with a loss of some precision of course) to dissaggregate
> > beam radiance from global radiance (as measured by pyranometer), for
> > use in an equation [1] for the linke turbidity factor. Compared to a
> > previous paper I mentioned on grass-dev [2], the equation presented in
> > [1] is a much more appropriate approach to calculating Linke turbidity
> > values.
> >
> > The key points to using the equation from [1] are:
> >
> > - historical pyranometer data from a weather station, with coordinates!
> > - r.sun mode 1 to calculate solar elevation at this station, for each
> > day at some time. extraterrestrial radiance is also calculated in this
> > step
> > - dissagregation of the beam component from global radiance with the
> > assumption that diffuse radiance is approx 10%-20% that of the beam
> > radiance
> > - conversion of dissaggregated beam radiance to beam radiance on the
> > normal (B/sin alpha)
> >
> > here is a link to a bit of my thesis on this topic: (rather
> > incomplete!) http://169.237.35.250/~dylan/temp/linke-cucumo2000.png
> >
> > Here is a link of linke turbidity values, as calculated by the method
> > described above, along with a comparison to the SoDa-derived values.
> > There is considerable difference between the two sources!
> >
> > http://169.237.35.250/~dylan/temp/cucumo-
>
> estimated_daily_linke_values.png
>
> > With the equation from [1] it is possible to estimate the linke
> > turbidity value for each day (using the 12pm calculation as
> > representative for an entire day), and use these estimates in r.sun
> > mode 2. here is a summary of the incorporation of estimated linke
> > values into r.sun - as compared to the weather station data:
> >
> > http://169.237.35.250/~dylan/temp/11-yr_variation_vs_modeled.png
> >
> >
> > I'll post some updates as they come. If I have anything wrong please
> > don't hesitate to point it out!
> >
> > Cheers,
> >
> > Dylan
> >
> >
> > 1. Cucumo, M.; Kaliakatsos, D. & Marinelli, V. A calculation method
> > for the estimation of the Linke turbidity factor Renewable Energy,
> > 2000, 19, 249-258
> >
> > 2. Becker, S. Caclulation of Direct Solar And Diffuse Radiation in
> > Israel International Journal of Climatology, 2001, 21, 1561 - 1576
> >
> > _______________________________________________
> > grassuser mailing list
> > grassuser@grass.itc.it
> > http://grass.itc.it/mailman/listinfo/grassuser
>
> _______________________________________________
> grassuser mailing list
> grassuser@grass.itc.it
> http://grass.itc.it/mailman/listinfo/grassuser
>
> __________ Información de NOD32 1914 (20061211) __________
>
> Este mensaje ha sido analizado con NOD32 antivirus system
> http://www.nod32.com

_______________________________________________
grassuser mailing list
grassuser@grass.itc.it
http://grass.itc.it/mailman/listinfo/grassuser

--
Dylan Beaudette
Soils and Biogeochemistry Graduate Group
University of California at Davis
530.754.7341

A_Ruiz_Arias · December 13, 2006, 11:04am

Hi Dylan,

I agree with you, probably is a better choice the correlation of Erbs
(equation [9] in Jacovides el al.).

It seems that you are calculating the Linke turbidity for all the days. As
you can read in the Cucumo paper, this parameter is used to estimate beam
and diffuse solar radiation in a determinated location for clear days. What
happens under overcast conditions (normal situation in winter) is that kt is
smaller than in a clear day, so kd is greater and you are underestimating
the beam component. For this reason, according to the expression [10] in
Cucumo et al., the Linke turbidity is overestimated.

That means that probably you should use some kind of filtering on your data
to rule out the cloudy days. For example, basing on the daily clearness
index, you could take only those days with daily kt greater then 0.6 more or
less.

Try this filtering. Probably you will improve the results

Cheers,

________________________________

José A. Ruiz Arias
Departamento de Física
Escuela Politécnica Superior
Edificio A-3, Campus Lagunillas
Universidad de Jaén
23071 Jaén Spain
Tlf. +34 953212474
Email: jararias@ujaen.es
_____________________________________

-----Mensaje original-----
De: Dylan Beaudette [mailto:dylan.beaudette@gmail.com]
Enviado el: martes, 12 de diciembre de 2006 23:27
Para: grassuser@grass.itc.it
CC: José Antonio Ruiz Arias
Asunto: Re: [GRASS-user] Re: [GRASS-dev] Changes to r.sun made Feb 2006
[resolved]

Hi Jose, some updates:

On Tuesday 12 December 2006 03:59, José Antonio Ruiz Arias wrote:
> Hi all,
>
> as I promised you (Dylan), here you have a useful reference to derive
the
> direct beam component from the global radiation.
>
> Jacovides, C.P., Tymvios, F.S., Assimakopoulos, V.A, and Kaltsounides
N.A.
> (2006): ' Comparative study of various correlations in estimating hourly
> diffuse fraction of global solar radiation' Renewable Energy, 31, 2492-
2504
>
> This paper offers a comparative analysis of different correlations
between
> diffuse and global radiation. The references [2], [3], [4] and [9] on
it, I
> think are especially interesting. I mention only this paper because on
it
> you can find a summary of the most important correlations.
>
> Reference [3] is especially simple and easy to read although it only
uses
> measurements of Toronto. Also very interesting for you, perhaps, is the
> reference [4] since it is based on five USA locations. For example, this
> author proposes the correlation
>
> kd = 1.45-1.67kt for 0.3<kt<0.78
> kd = 1.02-0.248kt for kt<=0.3
> kd = 0.147 for kt>=0.78.

Ok. I found these papers and have read over them. I think that if I were
to
get this approach working* I will use equation 7 from Jacovides et al.
2006.... details below:

> where kt is the ratio of global to extraterrestrial hourly radiation and
kd
> is the ratio of diffuse to global hourly radiation. Therefore, to
calculate
> kt only need the measured global radiation and the extraterrestrial
> radiation, theoretically calculated for a given location and time (see
for
> example, Iqbal, Muhammad (1983). An Introduction to Solar Radiation.
> Academic Press, New York). Then you get kd from the correlation, and
>
> Direct Beam = Global*(1-kd).
>
> So, if you dispose of global measurements, you can get the direct beam
> radiation directly applying the former expression.

Ok. this is the expression I used (after calculating kt and kd):

kt = global radiance at 12pm / extraterrestrial radiance
kd = (from eqation 2 or 9)

unfortunately, when i compute a normal beam radiance from the kd values:

B = G(1 - kd)
Bn = B/sin(alpha)

and then compute linke turbidity values (using the equation from Cucumo et
al
2000) : I end up with very high Linke values in the winter months:

http://169.237.35.250/~dylan/temp/reindl_linke_bad-estimates.png

This may be a result of not having a *single* clear day for my winter
months
(I am using the max radiance at 12pm, over the course of 11 years to get
as
close as possible to clear sky conditions).

Note that linke turbidity values in the above figure track those that I
had
been estimating in the summer months: with the diffuse = 10-20% of beam
radiance assumption.

Quite possibly I am doing something incorrectly

Cheers,

Dylan

> By the way, I would like to know what you are trying to do, improving
> r.sun? or just working in a personal project? I have used r.sun a little
> bit and I would like to collaborate with r.sun. I could work with the
model
> and/or even developing some snippets of code.
>
> Cheers,
>
> Jose A.
>
> > -----Mensaje original-----
> > De: grassuser-bounces@grass.itc.it
> > [mailto:grassuser-bounces@grass.itc.it] En nombre de jararias@ujaen.es
> > Enviado el: viernes, 08 de diciembre de 2006 13:09
> > Para: grassuser@grass.itc.it
> > Asunto: Re: [GRASS-user] Re: [GRASS-dev] Changes to r.sun made Feb
2006
> > [resolved]
> >
> > Hi all,
> >
> > firstly, I really hope not to interfere in this conversation. I assist
> > expectant to the thread cause is very interesting for me.
> >
> > It seems you want to get diffuse component from global radiation. If I
> > have understood well, you estimate it as the 10%-20% of the global
one.
> > Just a little suggestion that I hope not to be very simplistic. Have
you
> > think about a correlation between the clearness-index and the
proportion
> > of diffuse? There exist several correlations around the world such as
the
> > studies of Orgill, Reindl, Erbs, Holland or Duffie, for example. They
> > usually use hourly values. You can easily obtain a estimation for the
> > hourly diffuse component (and, therefore, for the hourly direct beam
one)
> > from the hourly global irradiation measured with a pyranometer as the
> > LICOR. If you are interesting in this approach just let me know and I
> > will be delighted of sending you more references. Now I can't be more
> > explicit because I don't have access to my personal data-base cause I
am
> > not at the office.
> >
> > Kind regards,
> >
> > José A.
> >
> > > On 12/7/06, Hamish <hamish_nospam@yahoo.com> wrote:
> > >> Dylan Beaudette wrote:
> > >> > > > > Get local Linke values from the SoDa Solar radiation
Databases
> > >> > > > > http://www.soda-is.com/
> > >> > > >
> > >> > > > Tried this, and got unrealistic values for my site.
> > >> > >
> > >> > > how so? Have you calculated good Linke values from your light
> > >> > > meters? (formula for that, please?)
> > >>
> > >> ..
> > >>
> > >> > Attached is a plot of estimated Linke turbidity values, compared
to
> > >> > the SoDa derived data. Details on this calculation soon to
follow
> > >> > in a new thread.
> > >
> > > Hi Hamish,
> > >
> > > Looks like I sent an update this morning with an attachment that was
> > > too large!
> > >
> > > I'll Include it at the bottom of my replies to you last message.
> > >
> > >> I assume the physical basis for the Linke turbidity factor is the
> >
> > amount
> >
> > >> of scattering dust, ash, water vapour, turbulence etc. in either
the
> > >> column of air directly above the sampling site or in the ray
between
> >
> > the
> >
> > >> site and the Sun (which will pass through more atm in winter due to
> > >> the lower angle). Maybe the paper sheds some light on that. So e.g.
LA
> > >> county would have very high turbidity numbers.
> > >>
> > >> I would figure in the winter the air to be crisper/clearer due to
less
> > >> water vapour, less turbulence, and less upwelling of aerosol laiden
> >
> > warm
> >
> > >> air from the Earth's surface. So lower turbidity numbers in the
winter
> > >> as the SoDa data shows. BUT if you live in a place which is prone
to
> > >> inversion layers in winter and many locals heat their homes with
wood
> >
> > or
> >
> > >> coal, winter air quality may turn out to be much more turbid.
> > >
> > > Actually have a couple good papers on the defintion for the Linke
> > > turbidity values:
> > >
> > > Louche, A.; Peri, G. & Iqbal, M. An analysis of linke turbidity
factor
> > > Solar Energy, 1986, 37, 393-396
> > >
> > > Rapti, A. S. Atmospheric transparency, atmospheric turbidity and
> > > climatic parameters Solar Energy, 2000, 69, 99-111
> > >
> > > Kasten, F. The linke turbidity factor based on improved values of
the
> > > integral Rayleigh optical thickness Solar Energy, 1996, 56, 239-244
> > >
> > > My site is in a mediterranean, semi-coastal / semi-arid setting
where
> > > winter time moisture and summer time dryness are the usual weather
> > > patterns. Summer through fall are usually cloud free, and quite
warm.
> > > Also- this site is rather remote, so urban ag-related dust
influences
> > > are minimal. These are some of the reasons that I was having second
> > > thoughts about the SoDa values.
> > >
> > >> > I have calculated Linke turbidity for my site with the equation
> > >>
> > >> T_linke = ln( G / ( I_0 * sin(h) * 0.84) ) * ( sin(h) / -0.027 )
> > >>
> > >> > Unfortunately, there are some aspects to this equation which
> > >> > I do not understand: namely the two constants present.
> > >>
> > >> perhaps they are empirically derived?
> > >
> > > Ok- big change in direction. I posted an update to the use of the
> > > above equation earlier today but it was rejected due to a large
> > > attachment. Details below....
> > >
> > >> I take it I_0 is pure-clear air intentensity?; h is latitude?; and
is
> > >> G measured intensity, or ..?
> > >
> > > G = global radiance measured on the ground (beam+diffuse)
> > > I_o = extraterrestrial radiance
> > > h = solar elevation (angle)
> > >
> > >> > 1. Becker, S. Caclulation of Direct Solar And Diffuse Radiation
in
> > >> > Israel International Journal of Climatology, 2001, 21, 1561 -
1576
> > >>
> > >> I will have to track down a copy of that. Holy cow, for once our
> >
> > library
> >
> > >> is actually subscribed to the e-journal. Downloading the PDF now..
> > >
> > > ... i know how that can be sometimes!
> > >
> > >> What software did you use to make that plot? Looks nice.
> > >
> > > Thanks. It was made in R, a fun command-line based environment for
> > > plotting, analysis, etc. I can post the code used after a small
> > > cleanup.
> > >
> > > Ok, here is the text from the message rejected this morning:
> > >
> > >
> > > Some updates on my findings with respect to obtaining optimal Linke
> > > turbidity values via local pyranometer measurements.
> > >
> > > Chatting with a local atmospheric scientist, I learned that it is
> > > possible (with a loss of some precision of course) to dissaggregate
> > > beam radiance from global radiance (as measured by pyranometer), for
> > > use in an equation [1] for the linke turbidity factor. Compared to a
> > > previous paper I mentioned on grass-dev [2], the equation presented
in
> > > [1] is a much more appropriate approach to calculating Linke
turbidity
> > > values.
> > >
> > > The key points to using the equation from [1] are:
> > >
> > > - historical pyranometer data from a weather station, with
coordinates!
> > > - r.sun mode 1 to calculate solar elevation at this station, for
each
> > > day at some time. extraterrestrial radiance is also calculated in
this
> > > step
> > > - dissagregation of the beam component from global radiance with the
> > > assumption that diffuse radiance is approx 10%-20% that of the beam
> > > radiance
> > > - conversion of dissaggregated beam radiance to beam radiance on the
> > > normal (B/sin alpha)
> > >
> > > here is a link to a bit of my thesis on this topic: (rather
> > > incomplete!) http://169.237.35.250/~dylan/temp/linke-cucumo2000.png
> > >
> > > Here is a link of linke turbidity values, as calculated by the
method
> > > described above, along with a comparison to the SoDa-derived values.
> > > There is considerable difference between the two sources!
> > >
> > > http://169.237.35.250/~dylan/temp/cucumo-
> >
> > estimated_daily_linke_values.png
> >
> > > With the equation from [1] it is possible to estimate the linke
> > > turbidity value for each day (using the 12pm calculation as
> > > representative for an entire day), and use these estimates in r.sun
> > > mode 2. here is a summary of the incorporation of estimated linke
> > > values into r.sun - as compared to the weather station data:
> > >
> > > http://169.237.35.250/~dylan/temp/11-yr_variation_vs_modeled.png
> > >
> > >
> > > I'll post some updates as they come. If I have anything wrong please
> > > don't hesitate to point it out!
> > >
> > > Cheers,
> > >
> > > Dylan
> > >
> > >
> > > 1. Cucumo, M.; Kaliakatsos, D. & Marinelli, V. A calculation method
> > > for the estimation of the Linke turbidity factor Renewable Energy,
> > > 2000, 19, 249-258
> > >
> > > 2. Becker, S. Caclulation of Direct Solar And Diffuse Radiation in
> > > Israel International Journal of Climatology, 2001, 21, 1561 - 1576
> > >
> > > _______________________________________________
> > > grassuser mailing list
> > > grassuser@grass.itc.it
> > > http://grass.itc.it/mailman/listinfo/grassuser
> >
> > _______________________________________________
> > grassuser mailing list
> > grassuser@grass.itc.it
> > http://grass.itc.it/mailman/listinfo/grassuser
> >
> > __________ Información de NOD32 1914 (20061211) __________
> >
> > Este mensaje ha sido analizado con NOD32 antivirus system
> > http://www.nod32.com
>
> _______________________________________________
> grassuser mailing list
> grassuser@grass.itc.it
> http://grass.itc.it/mailman/listinfo/grassuser

--
Dylan Beaudette
Soils and Biogeochemistry Graduate Group
University of California at Davis
530.754.7341

__________ Información de NOD32 1918 (20061212) __________

Este mensaje ha sido analizado con NOD32 antivirus system
http://www.nod32.com

Dylan_Beaudette1 · December 13, 2006, 8:37pm

On Wednesday 13 December 2006 03:04, José Antonio Ruiz Arias wrote:

Hi Dylan,

I agree with you, probably is a better choice the correlation of Erbs
(equation [9] in Jacovides el al.).

Hi Jose, thanks for the quick reply. I have had nearly equal results with the
Erbs equation and (Jacovides et al., 2006) equation 2: (Reindl et al.,
1990).

It seems that you are calculating the Linke turbidity for all the days. As
you can read in the Cucumo paper, this parameter is used to estimate beam
and diffuse solar radiation in a determinated location for clear days. What
happens under overcast conditions (normal situation in winter) is that kt
is smaller than in a clear day, so kd is greater and you are
underestimating the beam component. For this reason, according to the
expression [10] in Cucumo et al., the Linke turbidity is overestimated.

Yes. I am calculating Linke values for each day of the year. In an *attempt*
to get *only* clear days I have used the *maximum* global radiance observed
at the weather station over an 11 year period. Perhaps I need more years of
data to get the true clear sky radiance.

That means that probably you should use some kind of filtering on your data
to rule out the cloudy days. For example, basing on the daily clearness
index, you could take only those days with daily kt greater then 0.6 more
or less.

Try this filtering. Probably you will improve the results

ok. i will look into this. I was hoping for daily values. But if I can get
*good* monthly or weekly values I will interpolate accordingly.

I'll post the results back later today.

Cheers,

Dylan

Cheers,

________________________________

José A. Ruiz Arias
Departamento de Física
Escuela Politécnica Superior
Edificio A-3, Campus Lagunillas
Universidad de Jaén
23071 Jaén Spain
Tlf. +34 953212474
Email: jararias@ujaen.es
_____________________________________

> -----Mensaje original-----
> De: Dylan Beaudette [mailto:dylan.beaudette@gmail.com]
> Enviado el: martes, 12 de diciembre de 2006 23:27
> Para: grassuser@grass.itc.it
> CC: José Antonio Ruiz Arias
> Asunto: Re: [GRASS-user] Re: [GRASS-dev] Changes to r.sun made Feb 2006
> [resolved]
>
> Hi Jose, some updates:
>
> On Tuesday 12 December 2006 03:59, José Antonio Ruiz Arias wrote:
> > Hi all,
> >
> > as I promised you (Dylan), here you have a useful reference to derive
>
> the
>
> > direct beam component from the global radiation.
> >
> > Jacovides, C.P., Tymvios, F.S., Assimakopoulos, V.A, and Kaltsounides
>
> N.A.
>
> > (2006): ' Comparative study of various correlations in estimating
> > hourly diffuse fraction of global solar radiation' Renewable Energy,
> > 31, 2492-
>
> 2504
>
> > This paper offers a comparative analysis of different correlations
>
> between
>
> > diffuse and global radiation. The references [2], [3], [4] and [9] on
>
> it, I
>
> > think are especially interesting. I mention only this paper because on
>
> it
>
> > you can find a summary of the most important correlations.
> >
> > Reference [3] is especially simple and easy to read although it only
>
> uses
>
> > measurements of Toronto. Also very interesting for you, perhaps, is the
> > reference [4] since it is based on five USA locations. For example,
> > this author proposes the correlation
> >
> > kd = 1.45-1.67kt for 0.3<kt<0.78
> > kd = 1.02-0.248kt for kt<=0.3
> > kd = 0.147 for kt>=0.78.
>
> Ok. I found these papers and have read over them. I think that if I were
> to
> get this approach working* I will use equation 7 from Jacovides et al.
>
> 2006.... details below:
> > where kt is the ratio of global to extraterrestrial hourly radiation
> > and
>
> kd
>
> > is the ratio of diffuse to global hourly radiation. Therefore, to
>
> calculate
>
> > kt only need the measured global radiation and the extraterrestrial
> > radiation, theoretically calculated for a given location and time (see
>
> for
>
> > example, Iqbal, Muhammad (1983). An Introduction to Solar Radiation.
> > Academic Press, New York). Then you get kd from the correlation, and
> >
> > Direct Beam = Global*(1-kd).
> >
> > So, if you dispose of global measurements, you can get the direct beam
> > radiation directly applying the former expression.
>
> Ok. this is the expression I used (after calculating kt and kd):
>
> kt = global radiance at 12pm / extraterrestrial radiance
> kd = (from eqation 2 or 9)
>
> unfortunately, when i compute a normal beam radiance from the kd values:
>
> B = G(1 - kd)
> Bn = B/sin(alpha)
>
> and then compute linke turbidity values (using the equation from Cucumo
> et al
> 2000) : I end up with very high Linke values in the winter months:
>
> http://169.237.35.250/~dylan/temp/reindl_linke_bad-estimates.png
>
> This may be a result of not having a *single* clear day for my winter
> months
> (I am using the max radiance at 12pm, over the course of 11 years to get
> as
> close as possible to clear sky conditions).
>
> Note that linke turbidity values in the above figure track those that I
> had
> been estimating in the summer months: with the diffuse = 10-20% of beam
> radiance assumption.
>
> Quite possibly I am doing something incorrectly
>
> Cheers,
>
> Dylan
>
> > By the way, I would like to know what you are trying to do, improving
> > r.sun? or just working in a personal project? I have used r.sun a
> > little bit and I would like to collaborate with r.sun. I could work
> > with the
>
> model
>
> > and/or even developing some snippets of code.
> >
> > Cheers,
> >
> > Jose A.
> >
> > > -----Mensaje original-----
> > > De: grassuser-bounces@grass.itc.it
> > > [mailto:grassuser-bounces@grass.itc.it] En nombre de
> > > jararias@ujaen.es Enviado el: viernes, 08 de diciembre de 2006 13:09
> > > Para: grassuser@grass.itc.it
> > > Asunto: Re: [GRASS-user] Re: [GRASS-dev] Changes to r.sun made Feb
>
> 2006
>
> > > [resolved]
> > >
> > > Hi all,
> > >
> > > firstly, I really hope not to interfere in this conversation. I
> > > assist expectant to the thread cause is very interesting for me.
> > >
> > > It seems you want to get diffuse component from global radiation. If
> > > I have understood well, you estimate it as the 10%-20% of the global
>
> one.
>
> > > Just a little suggestion that I hope not to be very simplistic. Have
>
> you
>
> > > think about a correlation between the clearness-index and the
>
> proportion
>
> > > of diffuse? There exist several correlations around the world such as
>
> the
>
> > > studies of Orgill, Reindl, Erbs, Holland or Duffie, for example. They
> > > usually use hourly values. You can easily obtain a estimation for the
> > > hourly diffuse component (and, therefore, for the hourly direct beam
>
> one)
>
> > > from the hourly global irradiation measured with a pyranometer as the
> > > LICOR. If you are interesting in this approach just let me know and I
> > > will be delighted of sending you more references. Now I can't be more
> > > explicit because I don't have access to my personal data-base cause I
>
> am
>
> > > not at the office.
> > >
> > > Kind regards,
> > >
> > > José A.
> > >
> > > > On 12/7/06, Hamish <hamish_nospam@yahoo.com> wrote:
> > > >> Dylan Beaudette wrote:
> > > >> > > > > Get local Linke values from the SoDa Solar radiation
>
> Databases
>
> > > >> > > > > http://www.soda-is.com/
> > > >> > > >
> > > >> > > > Tried this, and got unrealistic values for my site.
> > > >> > >
> > > >> > > how so? Have you calculated good Linke values from your light
> > > >> > > meters? (formula for that, please?)
> > > >>
> > > >> ..
> > > >>
> > > >> > Attached is a plot of estimated Linke turbidity values, compared
>
> to
>
> > > >> > the SoDa derived data. Details on this calculation soon to
>
> follow
>
> > > >> > in a new thread.
> > > >
> > > > Hi Hamish,
> > > >
> > > > Looks like I sent an update this morning with an attachment that
> > > > was too large!
> > > >
> > > > I'll Include it at the bottom of my replies to you last message.
> > > >
> > > >> I assume the physical basis for the Linke turbidity factor is the
> > >
> > > amount
> > >
> > > >> of scattering dust, ash, water vapour, turbulence etc. in either
>
> the
>
> > > >> column of air directly above the sampling site or in the ray
>
> between
>
> > > the
> > >
> > > >> site and the Sun (which will pass through more atm in winter due
> > > >> to the lower angle). Maybe the paper sheds some light on that. So
> > > >> e.g.
>
> LA
>
> > > >> county would have very high turbidity numbers.
> > > >>
> > > >> I would figure in the winter the air to be crisper/clearer due to
>
> less
>
> > > >> water vapour, less turbulence, and less upwelling of aerosol
> > > >> laiden
> > >
> > > warm
> > >
> > > >> air from the Earth's surface. So lower turbidity numbers in the
>
> winter
>
> > > >> as the SoDa data shows. BUT if you live in a place which is prone
>
> to
>
> > > >> inversion layers in winter and many locals heat their homes with
>
> wood
>
> > > or
> > >
> > > >> coal, winter air quality may turn out to be much more turbid.
> > > >
> > > > Actually have a couple good papers on the defintion for the Linke
> > > > turbidity values:
> > > >
> > > > Louche, A.; Peri, G. & Iqbal, M. An analysis of linke turbidity
>
> factor
>
> > > > Solar Energy, 1986, 37, 393-396
> > > >
> > > > Rapti, A. S. Atmospheric transparency, atmospheric turbidity and
> > > > climatic parameters Solar Energy, 2000, 69, 99-111
> > > >
> > > > Kasten, F. The linke turbidity factor based on improved values of
>
> the
>
> > > > integral Rayleigh optical thickness Solar Energy, 1996, 56, 239-244
> > > >
> > > > My site is in a mediterranean, semi-coastal / semi-arid setting
>
> where
>
> > > > winter time moisture and summer time dryness are the usual weather
> > > > patterns. Summer through fall are usually cloud free, and quite
>
> warm.
>
> > > > Also- this site is rather remote, so urban ag-related dust
>
> influences
>
> > > > are minimal. These are some of the reasons that I was having second
> > > > thoughts about the SoDa values.
> > > >
> > > >> > I have calculated Linke turbidity for my site with the equation
> > > >>
> > > >> T_linke = ln( G / ( I_0 * sin(h) * 0.84) ) * ( sin(h) / -0.027 )
> > > >>
> > > >> > Unfortunately, there are some aspects to this equation which
> > > >> > I do not understand: namely the two constants present.
> > > >>
> > > >> perhaps they are empirically derived?
> > > >
> > > > Ok- big change in direction. I posted an update to the use of the
> > > > above equation earlier today but it was rejected due to a large
> > > > attachment. Details below....
> > > >
> > > >> I take it I_0 is pure-clear air intentensity?; h is latitude?; and
>
> is
>
> > > >> G measured intensity, or ..?
> > > >
> > > > G = global radiance measured on the ground (beam+diffuse)
> > > > I_o = extraterrestrial radiance
> > > > h = solar elevation (angle)
> > > >
> > > >> > 1. Becker, S. Caclulation of Direct Solar And Diffuse Radiation
>
> in
>
> > > >> > Israel International Journal of Climatology, 2001, 21, 1561 -
>
> 1576
>
> > > >> I will have to track down a copy of that. Holy cow, for once our
> > >
> > > library
> > >
> > > >> is actually subscribed to the e-journal. Downloading the PDF now..
> > > >
> > > > ... i know how that can be sometimes!
> > > >
> > > >> What software did you use to make that plot? Looks nice.
> > > >
> > > > Thanks. It was made in R, a fun command-line based environment for
> > > > plotting, analysis, etc. I can post the code used after a small
> > > > cleanup.
> > > >
> > > > Ok, here is the text from the message rejected this morning:
> > > >
> > > >
> > > > Some updates on my findings with respect to obtaining optimal Linke
> > > > turbidity values via local pyranometer measurements.
> > > >
> > > > Chatting with a local atmospheric scientist, I learned that it is
> > > > possible (with a loss of some precision of course) to dissaggregate
> > > > beam radiance from global radiance (as measured by pyranometer),
> > > > for use in an equation [1] for the linke turbidity factor. Compared
> > > > to a previous paper I mentioned on grass-dev [2], the equation
> > > > presented
>
> in
>
> > > > [1] is a much more appropriate approach to calculating Linke
>
> turbidity
>
> > > > values.
> > > >
> > > > The key points to using the equation from [1] are:
> > > >
> > > > - historical pyranometer data from a weather station, with
>
> coordinates!
>
> > > > - r.sun mode 1 to calculate solar elevation at this station, for
>
> each
>
> > > > day at some time. extraterrestrial radiance is also calculated in
>
> this
>
> > > > step
> > > > - dissagregation of the beam component from global radiance with
> > > > the assumption that diffuse radiance is approx 10%-20% that of the
> > > > beam radiance
> > > > - conversion of dissaggregated beam radiance to beam radiance on
> > > > the normal (B/sin alpha)
> > > >
> > > > here is a link to a bit of my thesis on this topic: (rather
> > > > incomplete!) http://169.237.35.250/~dylan/temp/linke-cucumo2000.png
> > > >
> > > > Here is a link of linke turbidity values, as calculated by the
>
> method
>
> > > > described above, along with a comparison to the SoDa-derived
> > > > values. There is considerable difference between the two sources!
> > > >
> > > > http://169.237.35.250/~dylan/temp/cucumo-
> > >
> > > estimated_daily_linke_values.png
> > >
> > > > With the equation from [1] it is possible to estimate the linke
> > > > turbidity value for each day (using the 12pm calculation as
> > > > representative for an entire day), and use these estimates in r.sun
> > > > mode 2. here is a summary of the incorporation of estimated linke
> > > > values into r.sun - as compared to the weather station data:
> > > >
> > > > http://169.237.35.250/~dylan/temp/11-yr_variation_vs_modeled.png
> > > >
> > > >
> > > > I'll post some updates as they come. If I have anything wrong
> > > > please don't hesitate to point it out!
> > > >
> > > > Cheers,
> > > >
> > > > Dylan
> > > >
> > > >
> > > > 1. Cucumo, M.; Kaliakatsos, D. & Marinelli, V. A calculation method
> > > > for the estimation of the Linke turbidity factor Renewable Energy,
> > > > 2000, 19, 249-258
> > > >
> > > > 2. Becker, S. Caclulation of Direct Solar And Diffuse Radiation in
> > > > Israel International Journal of Climatology, 2001, 21, 1561 - 1576
> > > >
> > > > _______________________________________________
> > > > grassuser mailing list
> > > > grassuser@grass.itc.it
> > > > http://grass.itc.it/mailman/listinfo/grassuser
> > >
> > > _______________________________________________
> > > grassuser mailing list
> > > grassuser@grass.itc.it
> > > http://grass.itc.it/mailman/listinfo/grassuser
> > >
> > > __________ Información de NOD32 1914 (20061211) __________
> > >
> > > Este mensaje ha sido analizado con NOD32 antivirus system
> > > http://www.nod32.com
> >
> > _______________________________________________
> > grassuser mailing list
> > grassuser@grass.itc.it
> > http://grass.itc.it/mailman/listinfo/grassuser
>
> --
> Dylan Beaudette
> Soils and Biogeochemistry Graduate Group
> University of California at Davis
> 530.754.7341
>
> __________ Información de NOD32 1918 (20061212) __________
>
> Este mensaje ha sido analizado con NOD32 antivirus system
> http://www.nod32.com

--
Dylan Beaudette
Soils and Biogeochemistry Graduate Group
University of California at Davis
530.754.7341

A_Ruiz_Arias · December 14, 2006, 12:15pm

Ok, Dylan, let hope better results.

The overcast conditions are the problem with this parameter. That means you
will sample it better during summer than in winter and, inevitably, you will
have to interpolate in some cases.

________________________________

José A. Ruiz Arias
Departamento de Física
Escuela Politécnica Superior
Edificio A-3, Campus Lagunillas
Universidad de Jaén
23071 Jaén Spain
Tlf. +34 953212474
Email: jararias@ujaen.es
_____________________________________

-----Mensaje original-----
De: Dylan Beaudette [mailto:dylan.beaudette@gmail.com]
Enviado el: miércoles, 13 de diciembre de 2006 21:37
Para: José Antonio Ruiz Arias
CC: grassuser@grass.itc.it
Asunto: Re: [GRASS-user] Re: [GRASS-dev] Changes to r.sun made Feb 2006
[resolved]

On Wednesday 13 December 2006 03:04, José Antonio Ruiz Arias wrote:
> Hi Dylan,
>
> I agree with you, probably is a better choice the correlation of Erbs
> (equation [9] in Jacovides el al.).

Hi Jose, thanks for the quick reply. I have had nearly equal results with
the
Erbs equation and (Jacovides et al., 2006) equation 2: (Reindl et al.,
1990).

> It seems that you are calculating the Linke turbidity for all the days.
As
> you can read in the Cucumo paper, this parameter is used to estimate
beam
> and diffuse solar radiation in a determinated location for clear days.
What
> happens under overcast conditions (normal situation in winter) is that
kt
> is smaller than in a clear day, so kd is greater and you are
> underestimating the beam component. For this reason, according to the
> expression [10] in Cucumo et al., the Linke turbidity is overestimated.

Yes. I am calculating Linke values for each day of the year. In an
*attempt*
to get *only* clear days I have used the *maximum* global radiance
observed
at the weather station over an 11 year period. Perhaps I need more years
of
data to get the true clear sky radiance.

> That means that probably you should use some kind of filtering on your
data
> to rule out the cloudy days. For example, basing on the daily clearness
> index, you could take only those days with daily kt greater then 0.6
more
> or less.
>
> Try this filtering. Probably you will improve the results

ok. i will look into this. I was hoping for daily values. But if I can get
*good* monthly or weekly values I will interpolate accordingly.

I'll post the results back later today.

Cheers,

Dylan

> Cheers,
>
> ________________________________
>
> José A. Ruiz Arias
> Departamento de Física
> Escuela Politécnica Superior
> Edificio A-3, Campus Lagunillas
> Universidad de Jaén
> 23071 Jaén Spain
> Tlf. +34 953212474
> Email: jararias@ujaen.es
> _____________________________________
>
> > -----Mensaje original-----
> > De: Dylan Beaudette [mailto:dylan.beaudette@gmail.com]
> > Enviado el: martes, 12 de diciembre de 2006 23:27
> > Para: grassuser@grass.itc.it
> > CC: José Antonio Ruiz Arias
> > Asunto: Re: [GRASS-user] Re: [GRASS-dev] Changes to r.sun made Feb
2006
> > [resolved]
> >
> > Hi Jose, some updates:
> >
> > On Tuesday 12 December 2006 03:59, José Antonio Ruiz Arias wrote:
> > > Hi all,
> > >
> > > as I promised you (Dylan), here you have a useful reference to
derive
> >
> > the
> >
> > > direct beam component from the global radiation.
> > >
> > > Jacovides, C.P., Tymvios, F.S., Assimakopoulos, V.A, and
Kaltsounides
> >
> > N.A.
> >
> > > (2006): ' Comparative study of various correlations in estimating
> > > hourly diffuse fraction of global solar radiation' Renewable Energy,
> > > 31, 2492-
> >
> > 2504
> >
> > > This paper offers a comparative analysis of different correlations
> >
> > between
> >
> > > diffuse and global radiation. The references [2], [3], [4] and [9]
on
> >
> > it, I
> >
> > > think are especially interesting. I mention only this paper because
on
> >
> > it
> >
> > > you can find a summary of the most important correlations.
> > >
> > > Reference [3] is especially simple and easy to read although it only
> >
> > uses
> >
> > > measurements of Toronto. Also very interesting for you, perhaps, is
the
> > > reference [4] since it is based on five USA locations. For example,
> > > this author proposes the correlation
> > >
> > > kd = 1.45-1.67kt for 0.3<kt<0.78
> > > kd = 1.02-0.248kt for kt<=0.3
> > > kd = 0.147 for kt>=0.78.
> >
> > Ok. I found these papers and have read over them. I think that if I
were
> > to
> > get this approach working* I will use equation 7 from Jacovides et al.
> >
> > 2006.... details below:
> > > where kt is the ratio of global to extraterrestrial hourly radiation
> > > and
> >
> > kd
> >
> > > is the ratio of diffuse to global hourly radiation. Therefore, to
> >
> > calculate
> >
> > > kt only need the measured global radiation and the extraterrestrial
> > > radiation, theoretically calculated for a given location and time
(see
> >
> > for
> >
> > > example, Iqbal, Muhammad (1983). An Introduction to Solar Radiation.
> > > Academic Press, New York). Then you get kd from the correlation, and
> > >
> > > Direct Beam = Global*(1-kd).
> > >
> > > So, if you dispose of global measurements, you can get the direct
beam
> > > radiation directly applying the former expression.
> >
> > Ok. this is the expression I used (after calculating kt and kd):
> >
> > kt = global radiance at 12pm / extraterrestrial radiance
> > kd = (from eqation 2 or 9)
> >
> > unfortunately, when i compute a normal beam radiance from the kd
values:
> >
> > B = G(1 - kd)
> > Bn = B/sin(alpha)
> >
> > and then compute linke turbidity values (using the equation from
Cucumo
> > et al
> > 2000) : I end up with very high Linke values in the winter months:
> >
> > http://169.237.35.250/~dylan/temp/reindl_linke_bad-estimates.png
> >
> > This may be a result of not having a *single* clear day for my winter
> > months
> > (I am using the max radiance at 12pm, over the course of 11 years to
get
> > as
> > close as possible to clear sky conditions).
> >
> > Note that linke turbidity values in the above figure track those that
I
> > had
> > been estimating in the summer months: with the diffuse = 10-20% of
beam
> > radiance assumption.
> >
> > Quite possibly I am doing something incorrectly
> >
> > Cheers,
> >
> > Dylan
> >
> > > By the way, I would like to know what you are trying to do,
improving
> > > r.sun? or just working in a personal project? I have used r.sun a
> > > little bit and I would like to collaborate with r.sun. I could work
> > > with the
> >
> > model
> >
> > > and/or even developing some snippets of code.
> > >
> > > Cheers,
> > >
> > > Jose A.
> > >
> > > > -----Mensaje original-----
> > > > De: grassuser-bounces@grass.itc.it
> > > > [mailto:grassuser-bounces@grass.itc.it] En nombre de
> > > > jararias@ujaen.es Enviado el: viernes, 08 de diciembre de 2006
13:09
> > > > Para: grassuser@grass.itc.it
> > > > Asunto: Re: [GRASS-user] Re: [GRASS-dev] Changes to r.sun made Feb
> >
> > 2006
> >
> > > > [resolved]
> > > >
> > > > Hi all,
> > > >
> > > > firstly, I really hope not to interfere in this conversation. I
> > > > assist expectant to the thread cause is very interesting for me.
> > > >
> > > > It seems you want to get diffuse component from global radiation.
If
> > > > I have understood well, you estimate it as the 10%-20% of the
global
> >
> > one.
> >
> > > > Just a little suggestion that I hope not to be very simplistic.
Have
> >
> > you
> >
> > > > think about a correlation between the clearness-index and the
> >
> > proportion
> >
> > > > of diffuse? There exist several correlations around the world such
as
> >
> > the
> >
> > > > studies of Orgill, Reindl, Erbs, Holland or Duffie, for example.
They
> > > > usually use hourly values. You can easily obtain a estimation for
the
> > > > hourly diffuse component (and, therefore, for the hourly direct
beam
> >
> > one)
> >
> > > > from the hourly global irradiation measured with a pyranometer as
the
> > > > LICOR. If you are interesting in this approach just let me know
and I
> > > > will be delighted of sending you more references. Now I can't be
more
> > > > explicit because I don't have access to my personal data-base
cause I
> >
> > am
> >
> > > > not at the office.
> > > >
> > > > Kind regards,
> > > >
> > > > José A.
> > > >
> > > > > On 12/7/06, Hamish <hamish_nospam@yahoo.com> wrote:
> > > > >> Dylan Beaudette wrote:
> > > > >> > > > > Get local Linke values from the SoDa Solar radiation
> >
> > Databases
> >
> > > > >> > > > > http://www.soda-is.com/
> > > > >> > > >
> > > > >> > > > Tried this, and got unrealistic values for my site.
> > > > >> > >
> > > > >> > > how so? Have you calculated good Linke values from your
light
> > > > >> > > meters? (formula for that, please?)
> > > > >>
> > > > >> ..
> > > > >>
> > > > >> > Attached is a plot of estimated Linke turbidity values,
compared
> >
> > to
> >
> > > > >> > the SoDa derived data. Details on this calculation soon to
> >
> > follow
> >
> > > > >> > in a new thread.
> > > > >
> > > > > Hi Hamish,
> > > > >
> > > > > Looks like I sent an update this morning with an attachment that
> > > > > was too large!
> > > > >
> > > > > I'll Include it at the bottom of my replies to you last message.
> > > > >
> > > > >> I assume the physical basis for the Linke turbidity factor is
the
> > > >
> > > > amount
> > > >
> > > > >> of scattering dust, ash, water vapour, turbulence etc. in
either
> >
> > the
> >
> > > > >> column of air directly above the sampling site or in the ray
> >
> > between
> >
> > > > the
> > > >
> > > > >> site and the Sun (which will pass through more atm in winter
due
> > > > >> to the lower angle). Maybe the paper sheds some light on that.
So
> > > > >> e.g.
> >
> > LA
> >
> > > > >> county would have very high turbidity numbers.
> > > > >>
> > > > >> I would figure in the winter the air to be crisper/clearer due
to
> >
> > less
> >
> > > > >> water vapour, less turbulence, and less upwelling of aerosol
> > > > >> laiden
> > > >
> > > > warm
> > > >
> > > > >> air from the Earth's surface. So lower turbidity numbers in the
> >
> > winter
> >
> > > > >> as the SoDa data shows. BUT if you live in a place which is
prone
> >
> > to
> >
> > > > >> inversion layers in winter and many locals heat their homes
with
> >
> > wood
> >
> > > > or
> > > >
> > > > >> coal, winter air quality may turn out to be much more turbid.
> > > > >
> > > > > Actually have a couple good papers on the defintion for the
Linke
> > > > > turbidity values:
> > > > >
> > > > > Louche, A.; Peri, G. & Iqbal, M. An analysis of linke turbidity
> >
> > factor
> >
> > > > > Solar Energy, 1986, 37, 393-396
> > > > >
> > > > > Rapti, A. S. Atmospheric transparency, atmospheric turbidity and
> > > > > climatic parameters Solar Energy, 2000, 69, 99-111
> > > > >
> > > > > Kasten, F. The linke turbidity factor based on improved values
of
> >
> > the
> >
> > > > > integral Rayleigh optical thickness Solar Energy, 1996, 56, 239-
244
> > > > >
> > > > > My site is in a mediterranean, semi-coastal / semi-arid setting
> >
> > where
> >
> > > > > winter time moisture and summer time dryness are the usual
weather
> > > > > patterns. Summer through fall are usually cloud free, and quite
> >
> > warm.
> >
> > > > > Also- this site is rather remote, so urban ag-related dust
> >
> > influences
> >
> > > > > are minimal. These are some of the reasons that I was having
second
> > > > > thoughts about the SoDa values.
> > > > >
> > > > >> > I have calculated Linke turbidity for my site with the
equation
> > > > >>
> > > > >> T_linke = ln( G / ( I_0 * sin(h) * 0.84) ) * ( sin(h) / -0.027
)
> > > > >>
> > > > >> > Unfortunately, there are some aspects to this equation which
> > > > >> > I do not understand: namely the two constants present.
> > > > >>
> > > > >> perhaps they are empirically derived?
> > > > >
> > > > > Ok- big change in direction. I posted an update to the use of
the
> > > > > above equation earlier today but it was rejected due to a large
> > > > > attachment. Details below....
> > > > >
> > > > >> I take it I_0 is pure-clear air intentensity?; h is latitude?;
and
> >
> > is
> >
> > > > >> G measured intensity, or ..?
> > > > >
> > > > > G = global radiance measured on the ground (beam+diffuse)
> > > > > I_o = extraterrestrial radiance
> > > > > h = solar elevation (angle)
> > > > >
> > > > >> > 1. Becker, S. Caclulation of Direct Solar And Diffuse
Radiation
> >
> > in
> >
> > > > >> > Israel International Journal of Climatology, 2001, 21, 1561 -
> >
> > 1576
> >
> > > > >> I will have to track down a copy of that. Holy cow, for once
our
> > > >
> > > > library
> > > >
> > > > >> is actually subscribed to the e-journal. Downloading the PDF
now..
> > > > >
> > > > > ... i know how that can be sometimes!
> > > > >
> > > > >> What software did you use to make that plot? Looks nice.
> > > > >
> > > > > Thanks. It was made in R, a fun command-line based environment
for
> > > > > plotting, analysis, etc. I can post the code used after a small
> > > > > cleanup.
> > > > >
> > > > > Ok, here is the text from the message rejected this morning:
> > > > >
> > > > >
> > > > > Some updates on my findings with respect to obtaining optimal
Linke
> > > > > turbidity values via local pyranometer measurements.
> > > > >
> > > > > Chatting with a local atmospheric scientist, I learned that it
is
> > > > > possible (with a loss of some precision of course) to
dissaggregate
> > > > > beam radiance from global radiance (as measured by pyranometer),
> > > > > for use in an equation [1] for the linke turbidity factor.
Compared
> > > > > to a previous paper I mentioned on grass-dev [2], the equation
> > > > > presented
> >
> > in
> >
> > > > > [1] is a much more appropriate approach to calculating Linke
> >
> > turbidity
> >
> > > > > values.
> > > > >
> > > > > The key points to using the equation from [1] are:
> > > > >
> > > > > - historical pyranometer data from a weather station, with
> >
> > coordinates!
> >
> > > > > - r.sun mode 1 to calculate solar elevation at this station, for
> >
> > each
> >
> > > > > day at some time. extraterrestrial radiance is also calculated
in
> >
> > this
> >
> > > > > step
> > > > > - dissagregation of the beam component from global radiance with
> > > > > the assumption that diffuse radiance is approx 10%-20% that of
the
> > > > > beam radiance
> > > > > - conversion of dissaggregated beam radiance to beam radiance on
> > > > > the normal (B/sin alpha)
> > > > >
> > > > > here is a link to a bit of my thesis on this topic: (rather
> > > > > incomplete!) http://169.237.35.250/~dylan/temp/linke-
cucumo2000.png
> > > > >
> > > > > Here is a link of linke turbidity values, as calculated by the
> >
> > method
> >
> > > > > described above, along with a comparison to the SoDa-derived
> > > > > values. There is considerable difference between the two
sources!
> > > > >
> > > > > http://169.237.35.250/~dylan/temp/cucumo-
> > > >
> > > > estimated_daily_linke_values.png
> > > >
> > > > > With the equation from [1] it is possible to estimate the linke
> > > > > turbidity value for each day (using the 12pm calculation as
> > > > > representative for an entire day), and use these estimates in
r.sun
> > > > > mode 2. here is a summary of the incorporation of estimated
linke
> > > > > values into r.sun - as compared to the weather station data:
> > > > >
> > > > > http://169.237.35.250/~dylan/temp/11-yr_variation_vs_modeled.png
> > > > >
> > > > >
> > > > > I'll post some updates as they come. If I have anything wrong
> > > > > please don't hesitate to point it out!
> > > > >
> > > > > Cheers,
> > > > >
> > > > > Dylan
> > > > >
> > > > >
> > > > > 1. Cucumo, M.; Kaliakatsos, D. & Marinelli, V. A calculation
method
> > > > > for the estimation of the Linke turbidity factor Renewable
Energy,
> > > > > 2000, 19, 249-258
> > > > >
> > > > > 2. Becker, S. Caclulation of Direct Solar And Diffuse Radiation
in
> > > > > Israel International Journal of Climatology, 2001, 21, 1561 -
1576
> > > > >
> > > > > _______________________________________________
> > > > > grassuser mailing list
> > > > > grassuser@grass.itc.it
> > > > > http://grass.itc.it/mailman/listinfo/grassuser
> > > >
> > > > _______________________________________________
> > > > grassuser mailing list
> > > > grassuser@grass.itc.it
> > > > http://grass.itc.it/mailman/listinfo/grassuser
> > > >
> > > > __________ Información de NOD32 1914 (20061211) __________
> > > >
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> >
> > --
> > Dylan Beaudette
> > Soils and Biogeochemistry Graduate Group
> > University of California at Davis
> > 530.754.7341
> >
> > __________ Información de NOD32 1918 (20061212) __________
> >
> > Este mensaje ha sido analizado con NOD32 antivirus system
> > http://www.nod32.com

--
Dylan Beaudette
Soils and Biogeochemistry Graduate Group
University of California at Davis
530.754.7341

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