Remote Sensing of Ice Water Characteristics in Tropical Clouds Using Aircraft Microwave Measurements

Abstract

An ice water path retrieval algorithm, using airborne Millimeter-Wave Imaging Radiometer brightness temperatures at 89, 150, and 220 GHz, is developed for tropical clouds. This algorithm is based on the results of radiative transfer model simulations, using in situ ice particle properties measured from aircraft as model inputs. The scattering signatures at the 150- and 220-GHz channels are the primary inputs into the algorithm, while 89-GHz data are used for determining the nonice background radiation. The ice water path is first calculated from each of the 150- and 220-GHz scattering signatures, and then a combination of the two channels is used for the final retrieval, based on the consideration of the different channel sensitivities to the magnitude of the ice water path. The algorithm is evaluated by comparing the retrieved with in situ measured ice water paths for seven cases observed during the Tropical Oceans Global Atmosphere Coupled Ocean?Atmosphere Response Experiment (TOGA COARE). Theoretical analysis shows that the uncertainty due to particle size could be the largest error in the retrievals and this error could be as large as plus or minus 50%. As an application of this algorithm, the ice water path characteristics during TOGA COARE are studied, including assessment of the mean of ice water path, its frequency distribution, and its relationships with cloud-top temperature and liquid water amount. Although tropical clouds are the target of this study, this algorithm could be modified and extended to other climatological regions.