On Tuesday 26 June 2007 01:18, José Antonio Ruiz Arias wrote:

> -----Mensaje original-----

> De: Dylan Beaudette [mailto:dylan.beaudette@gmail.com]

> Enviado el: lunes, 25 de junio de 2007 22:37

> Para: José Antonio Ruiz Arias

> CC: grassuser@grass.itc.it; grass-dev@grass.itc.it

> Asunto: Re: [GRASS-user] Use of coefbh and coefdh in r.sun

>

> On Monday 25 June 2007 12:27, José Antonio Ruiz Arias wrote:

> > Hi Dylan,

>

> Hi, thanks for the comments!

>

> > I will expose what I modestly think about.

> >

> > 1. does (1-coefbh)=coefdh? No.

> > I think you are confusing the clear-sky indices for the direct and

> > diffuse components with the direct and diffuse fractions. These

>

> latter

>

> > are both referred to the total global radiation coming into the

>

> ground

>

> > so that, considering the reflected component negligible, the addition

> > of the direct and diffuse fractions are always the unity.

>

> ok- this makes sense.

>

> > In turn, the clear-sky index

> > for the direct horizontal component (coefbh) is the fraction of

>

> direct

>

> > component attenuated by the clouds, that is, the ratio of direct beam

> > horizontal radiation under overcast-skies to the direct beam

> > horizontal radiation under clear-skies. The same is applied to the

>

> diffuse component.

>

> It sounds like in order to use 'real-sky' estimates I would need more

> than a

> pyranometer- as I would need to measure diffuse and beam components on

> both clear AND cloudy days... ?

Thanks for the reply,

Depending on the type of radiation you need (global, direct or diffuse), if

you want a 'real-sky' estimation with r.sun you will have to measure that

radiation for one (or a few sites - better) in your study area. That will

be your "ground-truth" to tune up the r.sun results and you can - hopefully

- expect to be representative for the whole study area (raster).

Indeed. This makes sense now. I do not think that I will be able to estimate

real-sky values for a couple reasons- mainly the short historical record, and

only a single weather station with pyranometer.

> > 2. does coefdh correspond to the notion of the 'diffuse fraction'

> > which is commonly calculated via something called the 'clearness

> > index' ? I think I have answered above.

>

> Ok, it sounds like my proposed method was in error.

>

>

> In this case, Kt (see named attachment) and the derived Kd (see

> attachment) - are describing the diffuse fraction of total radiance?

> Values which cannot be used for the coefdh and coefbh inputs to r.sun.

Yes. Maybe the definition help you to see the subtle difference among these

indices:

Global_rad=Direct_rad + Diffuse_rad (If reflected rad is negligible)

Kt=Global_rad/Extraterrestrial_rad

Kb=Direct_rad/Global_rad and Kd=Diffuse_rad/Global_rad => Kb+Kd=1

Ok, this makes sense and I understand this now.

regardless the sky conditions. However, coefdh and coefbh are defined as:

coefbh=horizontal direct radiation under any-sky (to be

measured)/horizontal direct radiation under clear-skies (r.sun)

coefdh=horizontal diffuse radiation under any-sky (to be

measured)/horizontal diffuse radiation under clear-skies (r.sun)

which distinguishes among sky conditions. Thus, you multiply your clear-sky

estimation raster map (r.sun) for the corresponding coefficient coefbh or

coefdh calculated on your ground station (measured/r.sun).

Got it.

> Would it be possible to disaggregate my pyranometer data in this way-

> giving my the real irradiance on the ground, compute the clear sky

> radiance at the same location with r.sun, and then compute the ratio of

> the two in order to get coefdh / coefbh ?

Yes, you could use one of the regression equations to get Kd and Kb from

the global radiation provided by your pyranometer, then compute the

clear-sky raster maps with r.sun for your study area and compute the ratio

of the two. However, take always in account that these equations provide

you a statistical approach to your problem having a inherent statistical

error that depends on the regression chosen. In the bibliography you could

look for one in your site area.

Indeed, this was my initial reason for not doing so. It appears as if I am

back where I started estimating clear sky conditions from an estimated Linke

value.

> > My personal opinion is that the best approach consists on using a

>

> more

>

> > or less approximated turbidity coefficient of Linke (a climatological

> > value could be the easiest choice) to get the clear-sky estimation

> > (r.sun) and then use the ground global radiation measurement

> > (supposing you have it,

> > obviously) to correct the clear-sky estimation provided by r.sun.

>

> This

>

> > approach can be also applied to both, the diffuse and direct

> > components of the radiation. The problem with the components is the

> > seldom availability of measurements.

>

> This was my initial goal, and seemed to work fairly well. I was able to

> compute T_L values accurately during the summer based on my pyranometer

> data, but there were not enough clear days in the 11yr history to

> compute realistic T_L values in winter months. Using a 15% diffuse

> fraction estimate for the entire year seemed to yield data which

> followed the 11yr mean values quite well.... However, I was hoping to

> avoid a static diffuse fraction- and make better use of the pyranometer

> data for the computation of 'real-sky'

> conditions. This may not be possible...?

I think, rather than use a static 15% of diffuse fraction you should use a

regression equation that will give you slightly better estimations (remind

that if you have the global radiation, you have the clearness index, and

then, the diffuse fraction and the direct fraction through a regression

equation). The problem of estimating the sky turbidity is that you need

spectral measurements of the radiation and usually we do not have it.

However, there exist broadband methods to calculate it from direct beam

measurements. But this approach unavoidably needs of direct beam

measurements and, in the case of the TL, also needs the humidity (usually

precipitable water).

I had originally tried this- but my 11 years of data did not have enough clear

sky winter days, and therefore the computed T_L values were between 7 - 11 ;

perhaps a little unrealistic.

Thanks again,

dylan

> > I hope to have cleared I little the question José A.

>

> Yes, thanks for the tips

>

> Cheers,

>

> Dylan

>

> > > -----Mensaje original-----

> > > De: grassuser-bounces@grass.itc.it [mailto:grassuser-

> > > bounces@grass.itc.it] En nombre de Dylan Beaudette Enviado el:

> > > lunes, 25 de junio de 2007 19:25

> > > Para: grassuser@grass.itc.it

> > > CC: grass-dev@grass.itc.it

> > > Asunto: [GRASS-user] Use of coefbh and coefdh in r.sun

> > >

> > > Hi,

> > >

> > > After consulting with an atmospheric scientist, I have decided that

> > > it might be simpler to estimate 'real-sky' radiance as opposed to

> > > trying to estimate the Linke turbidity for the estimation of

> > > 'clear-sky' conditions.

> > >

> > > It looks like r.sun can use input maps: coefbh and coefdh to

>

> compute

>

> > > real-sky radiance values. I would like to be sure that I am

> > > interpreting these inputs

> > > correctly:

> > >

> > > 1. does (1- coefbh) = coefdh ?

> > > 2. does coefdh correspond to the notion of the 'diffuse fraction'

> > > which is commonly calculated via something called the 'clearness

> > > index' ?

> > >

> > >

> > > If this is the case, then it is not all that difficult to compute

> > > coefdh from pyranometer data and the output from r.sun (mode 1)

> > >

> > > I will update the man page with my approach, if these assumptions

> > > are correct.

> > >

> > > cheers,

> > >

> > > PS:

> > > a relevent references is:

> > >

> > > Jacovides, C.; Tymvios, F.; Assimakopoulos, V. & Kaltsounides, N.

> > > Comparative

> > > study of various correlations in estimating hourly diffuse fraction

> > > of global solar radiation. Renewable Energy, 2006, 31, 2492 - 2504

> > >

> > >

> > > --

> > > Dylan Beaudette

> > > Soils and Biogeochemistry Graduate Group University of California

>

> at

>

> > > Davis

> > > 530.754.7341

> > >

> > > _______________________________________________

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> > > grassuser@grass.itc.it

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> > >

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> --

> Dylan Beaudette

> Soils and Biogeochemistry Graduate Group University of California at

> Davis

> 530.754.7341

>

>

> __________ Información de NOD32 2354 (20070626) __________

>

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--

Dylan Beaudette

Soils and Biogeochemistry Graduate Group

University of California at Davis

530.754.7341