The Limitations of the WSR-88D Radar Network for Quantitative Precipitation Measurement over the Coastal Western United States

In 1996, the last of the 136 operational Weather Surveillance Radar-1988 Doppler (WSR-88D) Next Generation Radar radar sites in the contiguous United States was put into operation, completing a major component of the National Weather Service (NWS) modernization program. This radar network, along with improvements in profilers and weather satellites, represents a continuing shift from in situ to remotely sensed measurements. The benefits of the WSR-88D in the western United States have been substantial, leading to improvements in short-range forecasts and contributing to a greater understanding of many regional weather phenomena (e.g., Colle and Mass 1998; Doyle 1997). Most recent radar application research has evaluated the capabilities of the NWS network east of the Rocky Mountains, with a majority of these studies focusing on severe, convective-type events. As shown below, the often-encouraging results from these studies are not representative of radar capability over mountainous regions, especially for estimating quantitative precipitation. The figure provided in Klazura and Imy (1993), a portion of which is shown in Fig. 1, is probably the most commonly cited estimate of radar coverage over the conterminous United States. In establishing the boundaries of radar coverage, Klazura and Imy (1993) determined the unblocked areas at 10 000 ft (3.05 km) above the radar sites; based on this criterion, there is radar coverage over a vast majority of the western United States. However, this approach produces an unrealistically optimistic estimate of useful radar coverage for quantitative precipitation measurement over the mountainous West. For example, previous studies of orographic precipitation (Houze et al. 1981; Marwitz 1983; Rauber 1992; Bruintjes et al. 1994) show that much of the orographic precipitation enhancement occurs within a 1–2-km layer above terrain. Precipitation along the West Coast tends to be shallow and stratiform, and many western radar sites are The Limitations of the WSR-88D Radar Network for Quantitative Precipitation Measurement over the Coastal Western United States