Investigation of multifrequency/multipolarization radar signatures of rain cells over the ocean using SIR‐C/X‐SAR data

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Abstract
Radar signatures of rain cells are investigated using multifrequency/multipolarization synthetic aperture radar (SAR) images acquired from the space shuttleEndeavourduring the spaceborne imaging radar‐C/X‐band SAR (SIR‐C/X‐SAR) missions in April and October 1994. In SIR‐C/X‐SAR images, radar signatures of rain cells over the ocean usually consist of irregularly shaped bright and dark patches that strongly depend on radar frequency and polarization. The radar signatures of rain cells observed in SIR‐C/X‐SAR imagery of the ocean originate from (1) the scattering and attenuation of the microwaves by raindrops and ice particles in the atmosphere and (2) the modification of the sea surface roughness induced by the impact of raindrops and by wind gusts associated with rain cells. Raindrops impinging on the sea surface generate ring waves, which enhance the sea surface roughness, but they also generate turbulence in the upper water layer, which reduces the sea surface roughness. Depending on the radar wavelength, ocean areas struck by rain can have higher or lower normalized radar cross section (NRCS) than the surrounding rain‐free area; in ocean areas where heavy rain is impinging on the sea surface, the X‐ and C‐band NRCS is usually enhanced, and the L‐band NRCS is reduced. From the phase difference between the horizontally and vertically copolarized signals, estimates of the rain rate are obtained. The present analysis shows further that the presently used wind speed retrieval algorithms for the scatterometers aboard the ERS and ADEOS satellites may yield biased wind fields if several rain cells lie within a scatterometer resolution cell.

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