Rainfall over oceans: Remote sensing from satellite microwave radiometers |
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| Authors: | C Prabhakara G Dalu R Suhasini J J Nucciarone G L Liberti |
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| Institution: | (1) Present address: NASA/Goddard Space Flight Center, Code 913, 20771 Greenbelt, MD, USA;(2) Present address: Istituto di Fisica dell'Atmosfera, Rome, Italy;(3) Present address: USRA, NASA/Goddard Space Flight Center, Greenbelt, USA;(4) Present address: Hughes-STX Corporation Lanham, USA |
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| Abstract: | Summary Microwave radiometer brightness temperature (T
b) measurements obtained from satellites over the oceans in dual polarization, at frequencies ranging from 6.6 to 85 GHz, reveal information about the rain and precipitation sized ice. These multifrequency measurements are composited from observations made by the Scanning Multichannel Microwave Radiometer (SMMR) and the Special Sensor Microwave/Imager (SSM/I). TheT
b measurements at 37 GHz, having a field of view (fov) of about 30 km, show relatively strong emissions due to rain, reaching values as large as 260 K over the tropical and mid-latitude rainbelts. Only marginal effects due to scattering by ice above the rain clouds are revealed. At frequencies below 37 GHz, where the fov is much larger than 30 km and the extinction is weaker,T
b is significantly smaller than 260 K. Additional information content about rain, at these low frequencies, is not appreciable. On the other hand, at 85 GHz (fov  15 km), where the extinction is very strong, the sea surface below the clouds is often masked and scattering due to ice above the rain clouds is vividly noticed. However, these high frequency measurements do not yield direct information about rain below the clouds.Recognizing the above merits inherent in the 37 GHz observations the SMMR and SSM/I data at this frequency are utilized to develop and empirical method to retrieve rain rate over oceans. In this method it is assumed that over an oceanic area, the statistics of the observedT
b must be derivable from the statistics of the corresponding rain rates. Furthermore, the underestimation of rain rate, arising from the inability of the radiometer to respond sensitively to rain above a given threshold is empirically rectified with the help of two parameters that depend on the total water vapor content in the atmosphere. Rain rates deduced over the oceans around Japan using the SSM/I data, when compared with those measured by radars that are calibrated against rain gauges, show a good correlation; there is, however, a systematic overestimation. Seasonal mean maps of the rainfall over the global oceans based on SMMR data compare favorably with climatological rain maps over the Atlantic and Pacific Oceans developed by Dorman and Bourke (1979, 1981).With 16 Figures |
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