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

[Vishal Mehta]

Hi all,

the idea of converting elevation to degrees did'nt work out.

i managed to project my latlong dem (30min, approx 960m resolution)  to
lambert conformal conic, so elevations, and coordinates are both in meters.

then i ran r.sun with a fixed albedo of 0.23

r.sun elevin=gtopo30 aspin=gtopoasp slopein=gtoposlope
alb=0.23beam_rad=beam1 diff_rad=diffuse1 refl_rad=reflect1 day=15

i got some output that looks right for beam and diffuse radiation: however,
the reflected radiation map is almost all zero. there are some small
positive values near mountainous regions (~50 Whr/(m2-day)).

since the albedo was 0.23, and the average global radiation i get is about
5000 Whr/(m2-day), i cant understand why the reflected radiation (Ri)is all
zero. The manual says something about a factor related to 'fraction of
ground viewed by an inclined surface' rg(gN).

i.e. Ri = albedo* global radiation * rg(gN)

so I assume then that this rg(gN) factor  must be causing the problem..

does anyone have any idea why this may be happening and what to do about it?

also, i wanted to report that what i'm getting for beam, diffuse and
reflected output now with lambert projections and the original 30min (960m)
resolution float dem,
is almost the SAME as in the beginning of this saga, when i was using
latlong at coarse 0.5 degree resolution and integer dem! The difference may
only be in the output when using the -s flag for accounting for shading.
However, the -s flag option took some half hour to run at the 960m
resolution..

There are many other things that happened in between that were funky, but i
guess i should save you all from that stuff:)

cheers,
vishal



On 8/28/06, Glynn Clements <glynn at gclements.plus.com> wrote:
>
>
> Vishal Mehta wrote:
>
> > so should i change the Proj_units file to:
> >
> > unit: degree
> > units: degrees
> > meters:  111300 * cos(y())
>
> It won't matter in this case; both r.slope.aspect and r.sun handle
> lat/lon locations separately.
>
> --
> Glynn Clements <glynn at gclements.plus.com>
>
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