On radiative heating due to polar stratospheric clouds

Abstract

Recent satellite observations of polar stratospheric clouds (PSCs) and the theoretical explanation of their formation brought up questions on the radiative effects of those thin clouds. Optical parameters are calculated for selected stages of the cloud development and for wavelengths between I and 40 μn. Radiative transfer calculations yield absolute values of heating rates of about 0.03 K/day for the thickest polar stratospheric clouds observed by the SAM II satellite. The top portion of the cloud is characterized by cooling of the order of 0.01 K/day while thr lower and denser part of the cloud may experience either cooling or warming depending on the thermal structure and on the vertical distribution of clouds in the troposphere. Due to the correlation between cold stratosphere and warm troposphere, the radiative effect appears more sensitive to the distribution of cloud than to the strength of the surface temperature inversion. Unlike most tropospheric cases and under favorable conditions, the growth of cloud particles may reach equilibrium not only by depletion of ambient moisture but also by thermal equilibrium. The hypothetical fully developed cloud would lead to absolute values of the heating rates of the order of 1 K/day. The maximum warming is found in the wings of the I5 μm CO2 band and extends up to 24 μm, while some cooling occurs at the upper end of the thermal spectrum. A simple parameterization of optical properties of PSCs is presented for inclusion into broad band radiation models.