A Study of Cloud–Generated Radiative Heating and Its Generation of Available Potential Energy. Part I: Theoretical Background

Abstract

The theory is presented of the effect of radiative heating and cooling by clouds on the available potential energy (APE). This provides a measure of the influence of clouds on the general circulation. Absorption and scattering of solar radiation and absorption and emission of longwave radiation are considered. It is shown that the cloud radiative contribution to the generation of APE is determined by the net cloud radiative heating and the efficiency factor, which is a function of the temperature distribution of the atmosphere. Cloud classes are defined in terms of cloud top heights and optical thickness. Within each class, the microphysical and macrophysical properties are used in a two-stream radiation computation with 37 spectral intervals in the shortwave and 50 in the longwave ranges. The probability of occurrence of each cloud class is used in the computations to account for nonlinearities between cloud parameters and the radiation field. Results are presented for low and middle clouds effect on each of three atmospheric layers: 1000?500 mb, 500?100 mb, and 100?1 mb. The cloud radiative heating is found to be a single function of cloud optical thickness for all classes. It is shown that low clouds cool the lower layer and, to a smaller degree, the middle layer. Midclouds coal the middle layer more strongly and heat the low layer. Thus, low clouds at low latitudes destroy APE and midclouds generate APE. The clear sky state and surface properties are found to have only secondary influence on the results. A concept is developed to relate the cloud radiative heating to cloud heights and optical depths; this can be estimated from satellite measurements, such as those which will be produced by the International Satellite Cloud Climatology Project. Thus, given such measurements, the impact of cloud radiative heating on the general circulation can be inferred.