A Study of Cloud-Generated Radiative Heating and Its Generation of Available Potential Energy. Part ii: Results for a Climatological Zonal Mean January

Abstract

Cloud-generated radiative heating is computed for January zonal mean conditions for low and midclouds. For both cases, the strongest influence was found to be in the low troposphere, with marked differences in signs and magnitudes. Optically thin low clouds generate net radiation cooling in the atmosphere at all latitudes. As cloud optical thickness increases, the shortwave absorption becomes more important. Midlevel clouds generate a net radiative heating for the low troposphere at latitudes 40°S to 40°N. Poleward of 40°, midlevel clouds generate a net cooling for the lower troposphere. At extratropical latitudes, both cloud classes generate net radiative cooling. In the tropics, the effect of low clouds changes from net cooling to net heating as the optical thickness increases, and midclouds cause net heating. A mechanism is described whereby this dependence produces a strong positive feedback effect on the development of sea surface temperature anomalies in the tropical oceans, as during an El Niño event. The cloud pattern changes over the East Pacific Ocean during the onset of an El Niño result in a strong increase in radiative heating which is at least as large as the latent heat release. The generation of zonal available potential energy (ZAPE) by net radiative heating due to clouds is computed for January mean conditions. In terms of the effects of clouds on the general circulation, the globe can be divided into two regimes. The first regime consists of the portions of the Earth south of 40°S and north of 30°N, i.e., the extratropics. In this regime, both low and midclouds generate ZAPE. Their effects increase with latitude, independent of optical thickness. The other regime is the belt between 40°S and 30°N, i.e., the tropics and subtropics. Here, the effect of clouds on ZAPE is less than half that in the extratropics. However, a change in optical thickness and cloud class will significantly contribute to ZAPE. Any increase in ZAPE will intensify the Hadley circulation, linking tropical sea surface temperature changes to the extratropical circulation.