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

[Dylan Beaudette]

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... ?

> 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. 

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 ?
 


> 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 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
> >
> > _______________________________________________
> > grassuser mailing list
> > 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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