LONGWAVE RADIATIVE COOLING RATES IN AXIALLY-SYMMETRICAL CLOUDS

The effects of cloud sides and of cloud top perturbations on radiative cooling rates are examined using an approximate model of longwave rad iative transfer that ignores the effect of scattering on net flux dive rgence. Tuned monochromatic calculations are used for isothermal cloud s, and spectral integration from 8 to 13.4 mu m for nonisothermal clou ds. Water vapor and cloud droplet effects within the cloud are include d. Atmospheric conditions outside the cloud are fixed appropriate to a midlatitude summer atmosphere. The distribution of cooling rates in i solated cylindrical clouds shows local maxima at the cloud top and sid es as well as modest warming of the base. These rates also depend on c loud 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 an d -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 cl oud boundaries is very sharp and high rates occur mainly within approx imate to 20 m of the boundary. The possibility of sustained cloud side cooling leading to low-level convergence and enhanced cloud developme nt is noted. Changes in cloud top cooling rates due to positive (lifti ng) or negative (sinking), axially symmetric perturbations to the top of horizontally extensive stratiform clouds were also considered. Sign ificant effects were found that do not cancel when averaged over both positive and negative perturbations. A lifting of the cloud top does n ot increase the cooling rate, whereas the peak cooling rate decreases rapidly in depressions. For small perturbations, approximate to 10 m, this may tend to inhibit the growth of negative perturbations. For lar ger perturbations, approximate to 100 m, changes to the radiative cool ing rates within the positive perturbations and next to the negative p erturbations may act to sustain the perturbation and promote its growt h.