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

[José Antonio Ruiz Arias]

Hi Dylan,

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. 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.

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.

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.

I hope to have cleared I little the question

José A.

> -----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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