A Novel Ice-Cloud Retrieval Algorithm Based on the Millimeter-Wave Imaging Radiometer (MIR) 150- and 220-GHz Channels

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Abstract
A novel microwave technique for simultaneously retrieving cirrus ice water path (IWP) and characteristic ice particle size is described. The retrieval algorithm exploits radiance measurements made at 150 and 220 GHz by the airborne Millimeter-Wave Imaging Radiometer (MIR). Other MIR channels additionally are used to test for the presence of liquid clouds and precipitation, which otherwise would have a contaminating effect on the retrievals. Forward radiative transfer modeling was used to generate a two-dimensional retrieval table in which brightness-temperature depressions (relative to clear-sky values) for both microwave channels were recorded as functions of IWP and characteristic particle size for gamma distributions of ice particles. Retrieval errors due to particle shape, size distribution, clear-sky water vapor variability, cirrus-cloud altitude variability, and instrument noise were estimated using Monte Carlo analysis. Particle shape uncertainty is believed to be the dominant source of retrieval error. The technique is demonstrated using MIR data recorded on the National Aeronautics and Space Administration ER-2 aircraft during the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment experiment in the tropical western Pacific Ocean in 1993. The retrieval technique with MIR data is suited only to high-IWP clouds with large ice particles, such as thick frontal cirrus and convective anvils. The general methodology, however, is applicable to higher frequencies that have greatly increased sensitivity to thinner cirrus.

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