INVESTIGATION OF MULTIFREQUENCY MULTIPOLARIZATION RADAR SIGNATURES OFRAIN CELLS OVER THE OCEAN USING SIR-C/X-SAR DATA/

Radar signatures of rain cells are investigated using multifrequency/m ultipolarization synthetic aperture radar (SAR) im ages acquired from the space shuttle Endeavour during the spaceborne imaging radar-C/X-ba nd SAR (SIR-C/X-SAR) missions in April and October 1994. In SIR-C/X-SA R images, radar signatures of rain cells over the ocean usually consis t of irregularly shaped bright and dark patches that strongly depend o n radar frequency and polarization. The radar signatures of rain cells observed in SIR-C/X-SAR imagery of the ocean originate from (1) the s cattering and attenuation of the microwaves by raindrops and ice parti cles in the atmosphere and (2) the modification of the sea surface rou ghness 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 generat e turbulence in the upper water layer, which reduces the sea surface r oughness. Depending on the radar wavelength, ocean areas struck by rai n can have higher or lower normalized radar cross section (NRCS) than the surrounding rain-free area; in ocean areas where heavy rain is imp inging 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 rai n rate are obtained. The present analysis shows further that the prese ntly used wind speed retrieval algorithms for the scatterometers aboar d the ERS and ADEOS satellites may yield biased wind fields if several rain cells lie within a scatterometer resolution cell.