Improving Quantitative Precipitation Estimation in Complex Terrain over the San Francisco Bay Area Using Gap-Filling Radar Network

The operational S-band WSR-88D network forms the cornerstone of national severe weather warning and forecast systems. The San Francisco Bay Area is covered by two WSR-88D: KMUX and KDAX. However, these two radars are not sufficient to provide detailed precipitation information for quantitative hydrometeorological applications. In particular, the KDAX radar beams are partially blocked at low elevation angles due to the mountainous terrain, whereas the KMUX radar is deployed at an elevation of over 1000 m, which can easily overshoot precipitation during the winter storm season in Northern California.

As part of the Advanced Quantitative Precipitation Information (AQPI) project, high-frequency (i.e., C and X band) high-resolution gap-filling radars are being deployed over the Bay Area to improve precipitation observations and investigate the detailed precipitation microphysics over such complex terrain. To date, two X-band radars have already been deployed and collected a substantial set of precipitation measurements that contribute to the development of local radar rainfall algorithms. This paper presents the preliminary design of a real-time rainfall system for the gap-filling AQPI. In particular, the dual-polarization-based rainfall relations are developed. Case studies during the 2017-2018 winter storm season are presented. The high-resolution rainfall products are evaluated through cross-comparison with surface rain gauge observations. Results show that rainfall products generated by the AQPI radars have better performance compared to the operational products currently available in this particular domain.