Challenges and Solutions for Integrating Multi-Frequency Radar Observations in Complex Terrain in the San Francisco Bay Area

Regions with complex topography, such as the San Francisco Bay Area, present a variety of challenges for obtaining accurate quantitative precipitation estimates (QPEs) with adequate spatiotemporal resolution. The Advanced Quantitative Precipitation Information (AQPI) project addresses these issues in the San Francisco Bay Area by integrating observations from multiple radar systems to fill in coverage gaps. Currently, three operational National Weather Service (NWS) radars, namely KBBX, KDAX, and KMUX, operating at S-band, provide coverage over this region. As part of AQPI, four scanning X-band radar systems (XSCW, XEBY, XSCV, and XSCR) have been strategically deployed to augment the NEXRAD observations. However, coverage gaps within the combined NEXRAD and X-band radar networks still exist to varying degrees across the entire AQPI domain. Individual radar sampling is affected by spatial distances between radar sites, along with timing differences between the individual radar scans, range degradation, and blockages in complex terrain. The latter issues especially result in under-sampling or even non-detection of precipitation, especially in shallow clouds dominated by warm rain processes. All these issues together pose a challenge in developing a unified QPE product that effectively utilizes complementary observations from disparate radar systems.

The current study aims at a synergistic approach to combine observations from all the NEXRADs and X-Band radars (7 in total) to create a near real-time composite QPE product. The methodology begins with the seamless blending of different radar observations to mitigate the challenges of beam blockages and beam overshooting. Differences in frequencies and spatial sampling between all the radars are addressed. The lack of synchronization among the NEXRAD scans further complicates the integration process and is addressed. A vertical profile of reflectivity correction scheme is implemented to mitigate bright-band contamination in regions of low environmental freezing levels. The final blended product removes sampling artifacts described and provides accurate QPE estimates with high spatiotemporal resolution, ultimately improving the spatial representation of precipitation over the entire domain. The QPE product is also evaluated on several precipitation events, and results are presented.