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

[William Collins]

Norm:

The downward emission equals the emission from an isothermal 
layer above the top interface in the model.  The temperature of that
isothermal layer is the same as the temperature of the top interface.
For the purposes of computing its emissivity, the water vapor mixing
ratio is held constant from the top interface to the top of the atmosphere.
Analogous simple assumptions are applied to CO2.

Bill

On Tuesday 22 October 2002 12:55 pm, Norm Wood wrote:
> 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

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Bill Collins             | wcollins@ucar.edu
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