TESTS OF RAIN PROFILING ALGORITHMS FOR A SPACEBORNE RADAR USING RAINCELL MODELS AND REAL DATA PRECIPITATION FIELDS

This paper compares the performances of several algorithms for profili ng the rain rate from a spaceborne radar. Three techniques are conside red: single frequency (or SF) at 13.8, or 24 GHz; dual frequency (or D F) at 13.8 and 24 GHz; and dual beam (or DB) at 24 GHz. The beam footp rint resolution L is taken in the range 1.5-3 km. Other characteristic s of the radar are based on realistic values. In the evaluation of alg orithm performances, emphasis is put on the nonuniform beam-filling ef fect because of its direct impact on the required cross-range resoluti on. Sensitivity of the algorithms to the speckle noise, the radar cali bration error, and the variability in drop size distribution is also i nvestigated. The study is achieved by using a three-dimensional simula tion tool applied, first to model raincells, and then to precipitation field from real data. Raincells with exponential-or Gaussian-shapes, and various rain-late peaks and D/L ratios, where D is the raincell '' diameter,'' are explored. Real data consist of vertical cross sections within a tropical squall line, including deep convection and stratifo rm rain, observed in Florida by means of 3D scans performed by a groun d-based S-band radar. The results point out the merit-demerit balance of the various algorithms. Use of surface echo to constrain the total path attenuation is an efficient way to improve the SF algorithms. The DF algorithm has definite advantages, compared with the SF counterpar ts, to correct scaling errors. The DB algorithm has some specific adva ntages but requires higher measurement accuracy and cross-range resolu tion, and has poorer spatial resolution than those of other approaches . Possible impact of the results on the definition of future spaceborn e radars, and their data exploitation, is also discussed.