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Physical Retrievals of Over-Ocean Rain Rate from Multichannel Microwave ImageryMicrowave rain rate retrieval algorithms have most often been formulated in terms of the raw brightness temperatures observed by one or more channels of a satellite radiometer. Taken individually, single-channel brightness temperatures generally represent a near-arbitrary combination of positive contributions due to liquid water emission and negative contributions due to scattering by ice and/or visibility of the radiometrically cold ocean surface. Unfortunately, for a given rain rate, emission by liquid water below the freezing level and scattering by ice particles above the freezing level are rather loosely coupled in both a physical and statistical sense. Furthermore, microwave brightness temperatures may vary significantly (approx. 30-70 K) in response to geophysical parameters other than liquid water and precipitation. Because of these complications, physical algorithms which attempt to directly invert observed brightness temperatures have typically relied on the iterative adjustment of detailed micro-physical profiles or cloud models, guided by explicit forward microwave radiative transfer calculations. In support of an effort to develop a significantly simpler and more efficient inversion-type rain rate algorithm, the physical information content of two linear transformations of single-frequency, dual-polarization brightness temperatures is studied: the normalized polarization difference P of Petty and Katsaros (1990, 1992), which is intended as a measure of footprint-averaged rain cloud transmittance for a given frequency; and a scattering index S (similar to the polarization corrected temperature of Spencer et al.,1989) which is sensitive almost exclusively to ice. A reverse Monte Carlo radiative transfer model is used to elucidate the qualitative response of these physically distinct single-frequency indices to idealized 3-dimensional rain clouds and to demonstrate their advantages over raw brightness temperatures both as stand-alone indices of precipitation activity and as primary variables in physical, multichannel rain rate retrieval schemes. As a byproduct of the present analysis, it is shown that conventional plane-parallel analyses of the well-known foot-print-filling problem for emission-based algorithms may in some cases give seriously misleading results.
Document ID
19970027529
Acquisition Source
Headquarters
Document Type
Reprint (Version printed in journal)
Authors
Petty, G. W.
(Purdue Univ. West Lafayette, IN United States)
Date Acquired
August 17, 2013
Publication Date
January 1, 1994
Publication Information
Publication: Meteorology and Atmospheric Physics
Publisher: Springer-Verlag G.m.b.H. and Co. K.G.
Volume: 54
Subject Category
Meteorology And Climatology
Report/Patent Number
NASA-CR-203998
NAS 1.26:203998
Accession Number
97N72292
Funding Number(s)
CONTRACT_GRANT: NAGw-2984
Distribution Limits
Public
Copyright
Public Use Permitted.
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