A comparison of NEXRAD WSR-88D radar estimates of rain accumulation with gauge measurements for high- and low-reflectivity horizontal gradient precipitation events

Radar-estimated rainfall amounts from the NEXRAD Weather Surveillance Radar precipitation accumulation algorithm were compared with measurements from numerous rain gauges (1639 radar versus gauge comparisons). Storm total rai n accumulations from 43 rain events from 10 radar sites were analyzed. Thes e rain events were stratified into two precipitation types: 1) high-reflect ivity horizontal gradient storms and 2) low-reflectivity horizontal gradien t events. Overall, the radar slightly overestimated rainfall accumulations for high-reflectivity gradient cases and significantly underestimated accum ulations for low-reflectivity gradient cases. Varying degrees of range effe cts were observed for these two types of precipitation. For high-reflectivi ty gradient cases, the radar underestimated rainfall at the nearest ranges, overestimated at the middle ranges, and had fairly close agreements at the farthest ranges. A much stronger range bias was evident for low-reflectivi ty gradient cases. The radar underestimated rainfall by at least a factor o f 2 in the nearest and farthest ranges, and to a somewhat lesser extent at midranges.