Effects of Drop Size Distribution Variability on QPE/QPF in the San Francisco Bay Area

In this study, an improved Z-R relationship that best fits the rainfall behavior in the San Francisco Bay Area is being developed. It is well known that drop size distribution (DSD) variability can be a significant cause of error in quantitative precipitation estimation via radar and forecasting, especially in areas with complex topography and multiple climatic regimes. Using the NWS WSR-88D (KMUX), a newly installed X-band radar, a dense networking of tipping bucket rain gauges (i.e., 40 stations) and two Parsivel disdrometers in the Santa Clara Valley region, we intend to establish a raindrop size distribution database among different rainfall episodes in the Bay Area, as well as to evaluate cloud microphysics used in the Weather Research and Forecasting (WRF) model. This study will advance our understanding of DSD distributions over areas of complex topography. As a result of this research, the Santa Clara Valley Water District is provided with a robust areal rain estimate and forecast tuned specially for the southern San Francisco Bay Area.