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Cirrus Cloud Observations by the Millimeter-wave Imaging Radiometer During FIRE-ACE |
Laboratory for Hydrospheric Processes, NASA Goddard Space Flight Center,
Joint Center for Earth Systems Technology, University of Maryland
Data acquired by the Millimeter-wave Imaging Radiometer (MIR) on board the NASA ER-2 aircraft on May 26 and 27, 1998 are analysed for studying the effects of cirrus clouds. On board the same aircraft are Cloud Lidar System (CLS) and MODIS (MODerate-resolution Imaging Spectrometer) Airborne Simulator (MAS) that readily provide identification of cirrus clouds. The MIR has a new channel at 340 GHz for these measurements that gives a better sensitivity for cirrus cloud detection than previously available at frequencies at and below 220 GHz. Past results have shown that brightness depressions at and below the frequencies of 220 GHz occurred only for intense cases where CLS signals from the surface were totally attenuated. The new MIR observations in May 1998 show that moderate 340 GHz brightness depressions from the presence of cirrus clouds are clearly associated with CLS signal returns from the surface. A comparison of MAS and MIR images shows a close correlation between the 340 GHz brightness temperature and the MAS radiance at 1.88 and 10.4 micron. Furthermore, the MIR measurements from the 340 GHz and 183.3+/-7 GHz channel at 43 degree incidence angle are used to estimate the ice water path (IWP) and the median equivalent mass sphere diameter (Dme) of cirrus clouds. The estimated IWP and Dme values depend greatly on the assumption of particle size distribution.
