[GRASS-user] [GRASSLIST:1178] how to use cloud cover with r.sun?

[Dylan Beaudette]

On Monday 21 August 2006 22:00, Vishal Mehta wrote:
> Hello good people,
>
> Can anyone suggest how I could use cloud cover (percentage) with r.sun, to
> generate global radiation maps which account for cloud cover?
>
> I dont believe I can get the data to calibrate the a and b coefficients as
> mentioned in the manual. This is why I am hoping there is a simple equation
> that allows me to use cloud cover percentage directly..I have the cloud
> cover maps.
>
> FYI, I am using Grass 6.0.1 on Ubuntu 5.1.
>
> And I am attempting to generate monthly Potential Evapotranspiration maps
> from 1960 to 2002 for the Indian subcontinent.
>
> cheers!
> vishal

Interesting questions. I have found that for cases in which cloud cover data 
is missing, a best-estimate of the Linke Turbidity Factor can make large 
improvements in the modeled daily summed radiation. see attached comparison 
of radiation as measured by a WX station.

From the manual it looks like estimations of the _real_ atmospheric condition 
must be described by two values: coefbh  and coefdh :

Besides clear-sky radiations, user can compute a real-sky radiation (beam, 
diffuse) using coefbh and coefdh input raster maps defining the fraction of 
the respective clear-sky radiations reduced by atmospheric factors (e.g. 
cloudiness). The value is between 0-1. Usually these coefficients can be 
obtained from a long-terms meteorological measurements.


Good luck!

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