[cam-users] Single column radiation code

[Peter Langen]

Dear CAM-users,

We figured out the answer to the question I posted last week: The 
variables rel, rei and fice, which influence the clouds' optical 
characteristics, are temperature dependent. When we fix these variables 
the expected result (of no change in surface SW when only T is varied) 
was found.

The influence of varying relative humidity on aerosol properties was 
also suggested as a reason for our result. However, in the CRM model, 
this effect is not present since in the aerosol calculations RH is 
specified to 0.8.

Thanks for all your help,
Peter Langen


Peter Langen wrote:

> Dear CCM and CAM users,
>
>    We are doing experiments with the 1D NCAR single column
> radiation code (CRM). We used vertical reference profiles of
> temperature, humidity, clouds and liquid water paths, obtained from a CCM
> run, averaged over a certain period. Temperature and
> humidity were then perturbed using profiles obtained from another
> run. The model was set up to run at 80N and the profiles used
> (temperature, moisture etc) were typical for these latitudes.
> Our typical temperature perturbation was equal to zero at the
> ground, positive in the mid-troposhpere, with a maximum of ca.
> 1-1.5K at around 500 mb and negative in the stratosphere (with
> a minimum of -1K). In our first experiment we perturbed the
> temperature profile only, keeping moisture, clouds and LWP fixed
> at their references. We got a net radiative heating tendency at
> the surface, with lower net LW (directed upwards) and, surprisingly to 
> us,
> a slightly increased SW at the surface (directed downwards). The total
> tendency (+0.8 W/m2) thus consists of +0.54 (LW) and
> +0.26 (SW). The response in the LW was expected, but we were
> surprised by the change in the shortwave, given the fact that
> moisture, clouds and LWP were fixed. The surface pressure was
> kept constant as well, so that it is not changes in the mass of the
> atmosphere giving the result.
>
> Can anybody come up with an explanation for the change in the SW?
>
> Our idea was that the absorption may depend on the temperature. Is this
> true? Is this a real physical phenomenon or a model artefact?
>
> Thanks,
> Peter Langen and Vladimir Alexeev
>
> -- 
> Peter Langen
> Department of Geophysics, NBIfAFG
> University of Copenhagen
> ph: +45 3532 0568
>
>
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-- 
--
Peter Langen
Department of Geophysics, NBIfAFG
University of Copenhagen
ph: +45 3532 0552