Rain measurement with SIR-C/X-SAR

Rain measurements by ground-based radar are unavailable in many parts of the world, especially the oceans. Because many SARs will be flying in space, the authors (a group headed by Dr. F. Li of JPL) performed an experiment using the SIR-C/X-SAR radars to test the ability of spaceborne SARs to measure rain rates. SAR resolution in rain is degraded by Doppler shifts due to turbulent motion of the rain (Atlas and Moore, 1987). The wide vertical beam required to achieve SAR ground coverage means that one must accept a path-integrated estimate of the rain rate. The authors used two approaches to rain measurement using the SIR-C/X-SAR radars pointed well off vertical. Over the western Pacific Ocean, they used the strength of the rain echo to estimate rain rate using an appropriate Z-R relation, as do ground-based radars. To separate rain echo from surface echo, one must estimate the surface contribution to the signal. Over the Amazon rain forest where the shadows are obvious they used the reduction of signal in the shadow due to attenuation through a rain storm