Small-Scale Rain Nonuniformity and Its Effect on Evaluation of Nonuniform Beam-Filling Error for Spaceborne Radar Rain Measurement.

An observational method has been proposed to sample radar echo with high range resolutions using a ground-based meteorological radar. Utilizing this method, the rainfall echo data with a high range resolution of 125 m was obtained by using an X-band meteorological radar. The analysis of rain nonuniformity strength using these high-resolution radar data shows that the nonuniformity is significant and, even in an instantaneous field of view (IFOV) of 1 km, the reflectivity excursion above 10 dB is common. The simulation of the nonuniform beam filling (NUBF) error of the path-integrated attenuation (PIA) measured by the spaceborne radar has been also implemented using these data. The results show that the PIA encounters mainly underestimation and cannot be neglected; even in 0.5-km IFOV the underestimation can reach up to 50%. The correlation analyses show that the relative PIA error and the standard deviation of rain rate have a power-law relationship with quite good correlation, which might be used to partially correct this error. The simulation also shows that it is very important to use the high-resolution data in studying the NUBF error of the next-generation spaceborne radar with a higher across-beam resolution (e.g., below 3 km).