[cam-users] longwave fluxes at top-of-model, FLUT versus FLNT

[Norm Wood]

I'm looking at results from a two-year run of CAM2.0 and comparing
FLUT (upwelling top-of-model longwave flux) with FLNT (net
top-of-model longwave flux). I expected them to be identical
(since I expected there to be no significant downwelling
longwave flux at the top of the model), but there's a slight
difference, with FLUT being larger than FLNT by about
1. W/m^2.

Looking through radclwmx(), it appears that the downwelling
longwave flux at the top-of-model is not zero (flnt and flut
are calculated in lines 875 - 883).  At first I thought
this might be an attempt to estimate the blackbody emission
downwelling from the sponge layer above the radiation model, but
after looking at other parts of the calculation in radtpl(),
I'm not sure that's the case. 

Thanks for any info.  The difference is probably not significant
in terms of total outgoing longwave, but it would be good to
have a clearer understanding of what the model is doing.

Norm

-- 
Norm Wood

     Department of Atmospheric Science
     Colorado State University
     norm@atmos.colostate.edu
     phone:  970-491-8587