Multi-frequency/multi-polarisation radar signatures of rain cells over the ocean measured by SIR-C/X-SAR

On synthetic aperture radar (SAR) images of the ocean, rain cells can often bedelineated as irregularly-shaped bright and dark patches. Such radar signatures have been observed, e.g., in SAR images from the American Seasat satellite and from the first and second European Remote Sensing satellites, ERS-1 and ERS-2. The present investigation uses data acquired by the multi-frequency / multi-polarisation SAR flown aboard the Space Shuttle "Endeavour" during the two Shuttle Imaging Radar-C / X band SAR (SIR-C/X-SAR) missions in April and October, 1994. The frequency and polarisation dependence of radar signatures of rain cells over the ocean is studied by analysing various SDR-C/X-SAR images. In particular, the normalised radar cross section (NRCS) at the different radar frequencies (L, C, and X band) and at different polarisations (HH, VV, and HV), as well as the phase difference between the HH and VV polarised backscattered radar signals at L and C band are analysed. The measured radar signatures of rain cells are compared with those calculated from theoretical models of the interaction of microwaves with water droplets, using data from laboratory experiments and from multi-frequency weather radar observations. The radar signatures of rain cells observed in SIR-C/X-SAR imagery of the ocean originate (I) from the scattering and attenuation of the microwaves by rain drops and ice particles in the atmosphere and (2) from the modification of the sea surface roughness induced by the impact of rain drops and by air motions (up-and downdrafts) associated with rain cells. Rain drops 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 normalised radar cross section than the surrounding (rain-free) area: In ocean areas where heavy rain is impinging on the water surface, the X and C band NRCS is usually enhanced, and the L band NRCS is reduced. From the phase difference between the HH and VV polarised signals, estimates of the rain rate can be obtained.