[GRASS-user] [GRASSLIST:1178] how to use cloud cover with r.sun?
Dylan Beaudette
dylan.beaudette at gmail.com
Tue Aug 22 13:24:50 EDT 2006
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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