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

Tue Jun 26 04:18:38 EDT 2007

> -----Mensaje original-----
> De: Dylan Beaudette [mailto:dylan.beaudette at gmail.com]
> Enviado el: lunes, 25 de junio de 2007 22:37
> Para: José Antonio Ruiz Arias
> CC: grassuser at grass.itc.it; grass-dev at 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... ?

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).

> > 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

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).

> 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.

> > 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 hope to have cleared I little the question José A.
> 
> Yes, thanks for the tips
> 
> Cheers,
> 
> Dylan
> 
> 
> > > -----Mensaje original-----
> > > De: grassuser-bounces at grass.itc.it [mailto:grassuser-
> > > bounces at grass.itc.it] En nombre de Dylan Beaudette Enviado el:
> > > lunes, 25 de junio de 2007 19:25
> > > Para: grassuser at grass.itc.it
> > > CC: grass-dev at 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 at grass.itc.it
> > > http://grass.itc.it/mailman/listinfo/grassuser
> > >
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> --
> Dylan Beaudette
> Soils and Biogeochemistry Graduate Group University of California at
> Davis
> 530.754.7341
> 
> 
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