Radiative Heating Rates for Saharan Dust

Abstract

A combined longwave and shortwave radiative transfer model was used to determine effects of Saharan dust on the radiative fluxes and heating/cooling rates in the atmosphere. Cases are treated for cloud-free and overcast conditions over the ocean and for cloud-free sky over desert. A benchmark comparison, made for the cloud-free ocean case between our calculations and those from Wiscombe’s detailed model, yielded results which were in close agreement. For moderately heavy dust amounts commonly measured over the Sahara and the eastern tropical Atlantic Ocean, typical calculated aerosol heating rates for the combined longwave and shortwave spectrum were in excess of 1 K day⁻¹ for all three cases for most of the atmosphere beneath the top of the dust layer (500 mb). For the ocean case, maximum heating rates are found near the level of maximum concentration (700 mb), and also near the surface beneath the Saharan air layer (below 900 mb). Net fluxes determined at the top of the atmosphere for the ocean cloud-free case were very insensitive to changes in dust optical depth. For the cloudy oceanic and desert cases, the reflectivity of the earth-atmosphere system diminished with increasing dust optical depth and approached that for the ocean case at large optical depth. In all three cases, the dust reduced the downward radiative flux into the ocean or desert while at the same time it increased the heating in the atmosphere, thus indicating a stabilizing effect by dust on the temperature lapse. However, further speculation concerning climatological significance of these results must be tempered by a need for further study of interactions between aerosol heating and atmospheric circulations, and between aerosols themselves and cloud microphysical processes.