WSR-88D radar rainfall estimation: capabilities, limitations and potential improvements

The WSR-88D Precipitation Processing Subsystem (PPS) represents the first attempt at network-wide operational radar-estimated precipitation in the United States. The system brings a dramatic advancement to operational flood forecasting when compared to earlier radars and rain gage networks. Neverthe-less, the system suffers from significant limitations inherent to the use of radar to estimate precipitation. These limitations have persisted despite nearly fifty years of research in the field. Much of the problem has been ascribed to the complex nonlinear relationship between radar reflectivity and rainfall rate at the surface. This relationship has been expressed empirically ( ' 'Z-R " power law) and many different ones have resulted from various experiments. Following suit, the WSR-88D software allows a change to the Z-R relationship for differing precipita-tion situations. A review of recent literature, however, reveals that the choice of an appropriate Z-R relationship is not the dominant issue in radar precipitation estimation, as was once widely believed. There are many precipitation estimation errors from radar, and they can often be larger than Z-R irrepresentativeness. These ~/Tors are briefly explored as they relate to the WSR-88D. It appears that overshooting of precipitation by the radar beam often produces the largest errors, usually causing precipitation underestimation. This is especially so at long ranges (> 60 nm), but can also occur at short ranges « 30 nm) because of the beam elevations used by the PPS. Overshoot is more pronounced in the cool season. Inadequate calibration ofWSR-88D network radars also appears to contribute large errors. Potential remedies for the major errors are reviewed and evaluatedfor their applicability to the WSR-88D. Recommenda-tions for implementation or further study are made based on this applicability. 1