[GRASS-user] use r.sun with monthly cloud cover data

[Hamish]

Simone wrote:
> I  would like to use the r.sun module to produce a map
> of montly solar
> radiation for a certain area. I would like to incorporate
> cloud cover
> data into the calculations, as well as information on local
> topography
> (i.e. aspect and slope).

slope and aspect are automatically taken into
account even if you don't use the slope and aspect
input map options.

actually due to a bug you shouldn't use them:
 https://trac.osgeo.org/grass/ticket/498#comment:8
 "r.sun2 commissioning trials"

just use the -s shading flag and the rest will
happen automatically.


> I was wondering whether you could
> help me to
> solve two problems: i) I have not quite yet understood what
> I should
> do to specify a raster containing data on cloudiness when
> using the module
> eg. r.sun elevin=altmap aspin=aspmap slopein=slopemap +
> cloudmap??

from the help page:
>  Besides clear-sky radiations, the 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 pro-
>  vided as raster maps with spatial distribution of these coef-
>  ficients  separately for beam and diffuse radiation (see Suri
>  and Hofierka, 2004, section 3.2).


I've now updated the module in trunk and devbr6 to
(a little) better explain that.


> ii)
>  I am interested in a montly average of daily sum of solar
> radiation,
> and I only have montly cloud cover data.  I suspect
> that this is not
> ideal when calculations by r.sun are done on daily basis.
> Do you confirm? Is there a way around this?

If it's all you have to work with, it's all you have
to work with & you'll have to pretend like every
day is like the climactic average.

some LiCOR light data or even battery voltage from an installed
solar panel at a weather station can give you an
idea about which days are cloudy, and by how much.


Hamish