Lower stratospheric radiative heating rates and sensitivities calculated from Antarctic balloon observations

Abstract

Temperature, water vapor, and ozone profiles from balloon soundings at McMurdo Station, Antarctica, during winter 1994 were used to calculate the diabatic heating rate in the lower stratosphere. These three variables represent the necessary inputs to a clear‐sky radiative heating rate calculation. Accurate profiles of these variables without recourse to ancillary climatological databases provide a means of testing the importance of realistic water vapor and ozone distributions to the lower stratospheric heating rate. Water vapor at wavelengths greater than 17 µm contributes as much cooling as CO2 in the lower stratosphere during winter. Dehydration begins in midwinter and causes a decrease in the cooling rate (increase in the heating rate) by 30%. Ozone depletion affects both longwave and shortwave heating rates in the altitude region where the ozone loss occurs. The longwave heating decreases by 0.3 K d−1 (potential temperature) and is comparable in magnitude to that of the decrease in shortwave heating for a balloon sounding in early October. Significant longwave heating increases of 0.6 K d−1 also occur above the region of ozone depletion as a result of higher penetration of upwelling radiation emitted by the surface. Sensitivity studies of the impact of tropospheric clouds at McMurdo show that the lower stratospheric heating rates can decrease by 0.08 to 0.26 K d−1 (vertically averaged) (18 to 60%), depending on the cloud parameterization used. Time series of the calculated heating rate at McMurdo show the effects of dehydration, ozone loss, and the increase of the heating rate (decrease of the cooling rate) as the temperatures decrease and approach radiative equilibrium through the winter.