Comparison of AQPI and NEXRAD Radar-Estimated Rain Rates during Two Extreme Atmospheric River Events over the Northern San Francisco Bay Area

The Advanced Quantitative Precipitation Information (AQPI) project has installed a network of strategically located X-band radars across the Greater San Francisco Bay Area. These radars complement the existing NEXRAD S-band network by filling horizontal and vertical gaps in coverage, and by operating at a very high spatial and temporal resolution, providing more detailed rainfall information across the region (Cifelli et al. 2024). This presentation will focus on AQPI performance in terms of X-band radar-estimated rain rates compared to those of the NEXRAD S-band network and local rain gauges during two cases of heavy precipitation. The first case, 24-25 Oct 2021, was driven by a historically strong early-season atmospheric river, which produced several periods of very high precipitation rates and storm-total precipitation records across the North Bay region. The second case, 21-24 Nov 2024, featured a long-duration atmospheric river event across the same area, which produced a 1000-year rain event in some isolated locales. In both cases, AQPI X-band rain estimates (both hourly rates and storm totals) matched rain gauge observations much more closely than those of the NEXRAD S-band network at most locations. This X-band advantage is greatest near the X-band location and decreases with distance from the radar, owing to radar beam attenuation. The X-band advantage is also greater during more intense rain rates. Hence, these additional radars greatly complement the existing network by providing higher-quality rain estimates in the densely-populated areas where they are located, with benefits towards any number of meteorological and hydrological applications. Future work includes a larger-scale statistical analysis of AQPI system performance across the Bay Area during subsequent winter seasons. More information is available at: https://cw3e.ucsd.edu/aqpi/.