Precipitation detection by the TOPEX/Poseidon dual frequency radar altimeter, TOPEX microwave radiometer, Special Sensor Microwave/Imager and climatological shipboard reports

The athors evaluate the ability of a dual-frequency radar (C and Ku band) altimeter to detect rain events. A TOPEX/Poseidon (T/P) altimeter rain flag for the year 1994 is compared to collocated rain rate (RR) from the Defense Meteorological Satellite Program´s Special Sensor Microwave Imager (DMSP SSM/I), as processed to the TOPEX/Poseidon passive radiometer´s (TMR) liquid-water content, and to a 34-year climatology of shipboard present-weather reports compiled by G. W. Petty (1995). The altimeter-SSM/I analysis is couched in terms of the tradeoff between the probability of a false positive and the probability of a failure to detect rain. The authors show that the ability of the SSM/I and TMR datasets to detect precipitation are closer to each other than to either the altimeter or the shipboard climatology, and this difference is accentuated at latitudes poleward of 45°. They argue that the different footprint sizes explain only part of this discrepancy. They propose that the difference at high latitudes is caused by the altimeter data´s sensitivity to snow. In order to detect precipitation (as opposed to detecting bad altimetric values or out-of-range altimetric corrections), a TMR-only flag with liquid-water content of 600 μm recovers too few rain events, 400 μm is close to climatological moderate-to heavy intensity rains, and 200 μm is close to rain of any intensity. For the same purpose, a combined altimeter and TMR flag, with a TMR threshold of 100 μm and with the Ku radar cross section 1.5 standard deviations below an average Ku/C curve, gives the best match for climatological precipitation of any intensity class