[Wrf-users] SWNDOWN & CLDFRA

Mikhail Titov TitovM at ap.aurecongroup.com
Wed Oct 13 15:52:50 MDT 2010

Dear WRF help and WRF users,

I tried to calculate direct solar irradiance, which usually is measured by 
a pyrheliometer (or calculated
from satellite data) and corrected Registry.EM (and recompiled WRF) to 
dump in output SWDOWNC (short-wave down radiation flux
for clear sky) and CLDFRA (fraction of clouds in each cell for every 
level). CLDFRA can be only 0 (no clouds) or
1 (clouds in spatial grid cell) and I'm not sure how to calculate sensible 
cloud fraction in every spatial grid cell
as the simple counting of 0 and 1 for certain number of levels (for 
instance 15) and division of number of 1 on total number
of levels (CLDFRA = SUM(1)/15) doesn't have any physical seance to me.

In any way using cu_physics = 1 for grids 1 & 2 (27 and 9 km) running 
3-grid WRF I have all zeros for SWDOWNC (but normal time trends
for SWDOWN) and 0/1 for CLDFRA. I removed cumulus (cu_physics = 0 for 
grids 1 & 2) but it didn't change anything and
I still have no values for SWDOWNC (./read_wrf_nc -m wrfout... gives for 
SWDOWNC minimum = maximum = 0.0) and
still have 0/1 values for CLDFRA and non-zero RAINNC. 
So this trick doesn't help and I'm not sure that I can switch off anything 
else in physical parameterisation without termination of WRF run.

If to guess that in every spatial cell of WRF grid for every time step 
there is always as minimum one CLDFRA = 1
it is logical to suggest that SWDOWNC = 0.0 all the time for all surface 
cells (as direct solar irradiance is equal to
zero in pyrheliometer measurements if cloud index = 1). Or maybe SWDOWNC 
short-wave radiation down flux is
not calculated in WRFv3.0.1 (just defined)? I didn't dig deep inside 
Fortran code.

Maybe anyone has already had these problems with SWDOWNC & CLDFRA in WRF? 
Maybe it is easier to write a little program to calculate short-wave down 
radiation flux (for clear sky) and cloud fraction without WRF?

Any help is highly appreciable.

Thanks a lot,
Dr. Mikhail Titov | Energy  | Aurecon 
Ph: +64 3 366 0821 ext.9231 DDI +64 367 32 31 | Fax: +64 3 379 6955 | Mob: 
+64 21 106 5563 
Email: TitovM at ap.aurecongroup.com 
PO Box 1061, 122 Gloucester Street | Christchurch 8140 | New Zealand 
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