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

[Massimiliano Cannata]

Hi,
You can use clear-sky index map and multiply it by your r.sun output.
I usually calculate clear-sky index as the ratio between the 'clear sky 
condition radiation (the r.sun)' and the 'observed radiation'
then I interpolate the point to retrive the map.

Actually I don't really know what is the influence of cloud cover on 
radiation (linear function between cloud cover and energy filtered????), 
probably you could calculate the reölation from observations, and then 
apply this with r.mapcalc.

Hope it helps,
Massimiliano




Dylan Beaudette wrote:
> 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!
>
>   
>
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-- 

Dr. Eng. Massimiliano Cannata
Scuola Universitaria Professionale della Svizzera Italiana
Istituto Scienze della Terra
Via Trevano, c.p. 72
CH-6952 Canobbio-Lugano
Tel: +41 (0)58 666 62 14
Fax +41 (0)58 666 62 09