[cam-users] Single column radiation code

[Peter Langen]

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