Retrieval of Ice Cloud Parameters Using the Advanced Microwave Sounding Unit

Abstract

An algorithm is developed to derive cloud ice water path (IWP) and ice particle effective diameters De from the advanced microwave sounding unit (AMSU) measurements. In the algorithm, both IWP and De are related to the ice particle scattering parameters, which are determined from the AMSU 89- and 150-GHz measurements. The ratio of the scattering parameters measured at two frequencies provides a direct estimate of De. IWP is then derived from the scattering parameter at 150 GHz with the derived De and the constant bulk volume density. A screening procedure is developed to discriminate the scattering signatures between atmospheric clouds and surface materials. The major error sources affecting the retrievals are identified. The errors of retrieved effective diameter are primarily controlled by the errors in estimating cloud-base brightness temperatures at 89 and 150 GHz and the errors of the bulk volume density. It is shown that De possibly contains an error of 5%?20%. For the retrieval of cloud ice water path, the errors are influenced by the uncertainties in estimated cloud-base brightness temperature, retrieved particle effective diameter, and particle volume density. A 30% error in bulk volume would alone result in a 25% error in retrieved IWP. The algorithm is applied for various weather events and can primarily detect the precipitating ice clouds as well as thick nonprecipitating clouds because of an increasing sensitivity of AMSU measurements at 150 GHz to smaller particle sizes. These results demonstrate the use of 89- and 150-GHz channels for studying the ice cloud properties and their spatial variability under various atmospheric environments.