Longwave radiative cooling rates in axially symmetric clouds

The effects of cloud sides and of cloud top perturbations on radiative cooling rates are examined using an approximate model of longwave radiative transfer that ignores the effect of scattering on net flux divergence. Tuned monochromatic calculations are used for isothermal clouds, and spectral integration from 8 to 13.4 μm for nonisothermal clouds. Water vapor and cloud droplet effects within the cloud are included. Atmospheric conditions outside the cloud are fixed appropriate to a midlatitude summer atmosphere. The distribution of cooling rates in isolated cylindrical clouds shows local maxima at the cloud top and sides as well as modest warming of the base. These rates also depend on cloud and surface temperature. For a 286 K cloud above a 302 K ground, the local cooling rates were typically −34 K h−1 at the cloud top and −14 K h−1 at the cloud sides, with a typical local heating rate of 8 K h−1 at the cloud base. The gradient in cooling rate near the cloud boundaries is very sharp and high rates occur mainly within ≈20 m of the boundary. The possibility of sustained cloud side cooling leading to low‐level convergence and enhanced cloud development is noted. Changes in cloud top cooling rates due to positive (lifting) or negative (sinking), axially symmetric perturbations to the top of horizontally extensive stratiform clouds were also considered. Significant effects were found that do not cancel when averaged over both positive and negative perturbations. A lifting of the cloud top does not increase the cooling rate, whereas the peak cooling rate decreases rapidly in depressions. For small perturbations, ≈10 m, this may tend to inhibit the growth of negative perturbations. For larger perturbations, ≈100 m, changes to the radiative cooling rates within the positive perturbations and next to the negative perturbations may act to sustain the perturbation and promote its growth.