Comparison of two Jason-1 altimeter precipitation detection algorithms with rain estimates from the TRMM Microwave Imager

This paper evaluates and compares the ability of two different Jason-1 dual-frequency altimeter algorithms (referred as Tournadre's and Quartly's rain flags, respectively) to detect rain events in order to flag rain-contaminated altimeter range measurements. They are based on departures from a defined relationship between the Ku- and C-band radar cross sections observed in no-rain conditions. The algorithms' performances were assessed via collocations of these dual-frequency-based estimates with rain rates and a rain-no-rain flag from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI). The Jason-1-TMI analysis is built upon a yes-no discrimination, which is helpful in providing good insight into the altimeter rain detection flags' efficiency through estimations of the percentages of hits, misses, false alarms, and correct negatives when compared with TMI measurements. Tournadre's rain flag, based on a combination of altimeter and radiometer data, gives the best match with TMI estimates, compared to Quartly's, and also has a higher sensitivity to low-intensity rainfall.