An Intercomparison Study of NEXRAD Precipitation Estimates

Systematic biases in WSR-88D (Weather Surveillance Radar–1988 Doppler) hourly precipitation accumulation estimates are characterized from analyses of more than 1 year of WSR-88D data and rain gage data from the southern plains. Biases are examined in three contexts: (1) biases that arise from the range-dependent sampling of the WSR-88D, (2) systematic differences in radar rainfall estimates from two radars observing the same area, and (3) systematic differences between radar and rain gage estimates of rainfall. Range-dependent biases affect hourly rainfall accumulations products over much of the area covered by the WSR-88D. Significant underestimation of rainfall occurs within 40 km range of the radar due to bias in reflectivity observations at the higher elevation angles used for rainfall estimation close to the radar. Bright band and anomalous propagation (AP) lead to systematic overestimation of rainfall at intermediate range. Beyond 150 km in spring-summer and beyond 100 km in winter-fall, underestimation of precipitation is pronounced due to incomplete beam filling and overshooting of precipitation. Radar-radar intercomparison studies suggest that radar calibration is a significant problem at some sites. Anomalous propagation during clear-air conditions, a major problem with previous National Weather Service network radars, has been largely eliminated by the WSR-88D processing. AP remains a problem for cases in which AP returns are embedded in rain. Radar–rain gage intercomparison analyses indicate systematic underestimation by the WSR-88D relative to rain gages for paired gage-radar rainfall estimates. Analyses of spatial coverage of heavy rainfall, however, illustrate fundamental advantages of radar over rain gage networks for rainfall estimation.

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