SPACEBORNE RADAR SENSING OF PRECIPITATION ABOVE AN OCEAN SURFACE - POLARIZATION CONTRAST STUDY

This feasibility study explores the potential benefits of polarization adjustment for spaceborne radar sensing of precipitation. More specifically, the role of the wave polarization in separating or ''distinguishing'' ocean surface return from the hydrometeor echoes of a ''chirped'' signal is examined. To that end, experimental as well as computational data for the polarization scattering matrices of hydrometeors and ocean surfaces are obtained and used to calculate ocean and precipitation ''response'' to the transmitted pulse for various rain rates and incidence angles. The analysis is restricted to X and C bands, but simulations are performed for several signal-to-noise ratios, rain rates, and ocean surfaces. The problem is further restricted to the monostatic case (same polarizations for transmitter and receiver). Even when the ocean and hydrometeor echoes are mixed throughout the entire radar resolution volume, the results appear promising. It is found that polarization, which provides the best contrast between rain and ocean returns, varies from almost circular near nadir to elliptical at large off-nadir look angles of incidence (ellipticity of 23-degrees at a 40-degrees incidence angle). Calculations show an order of magnitude improvement in the ratio of the returns when compared with the traditional choice of HH (horizontal transmit and receive polarization). The improvement is largest for the range of angles between 15-degrees and 20-degrees but depends on the assumed rain rate and, in particular, on the ocean surface roughness. The general method described in this paper can be applied to many problems of radar and lidar meteorology, while the specific results reported here may have relevance for future precipitation measurement missions such as Tropical Rainfall Measuring Mission 2.