Improving Operational Radar Rainfall Estimates Using Remote Sensing Observations over Complex Terrain in Northern California

The San Francisco Bay Area in Northern California is covered by two standard S-band NEXRAD radars: KMUX and KDAX. However, the KDAX radar is located over 80 km from the closest portions of the Bay Area and the KDAX radar beam is partially blocked at low elevation angles. The KMUX radar is closer but is deployed at an elevation of over 1000 m compared with the highly populated urban regions which are near sea level. As a result, it is challenging to use these radars to observe low-level atmospheric conditions and provide forecasters with detailed information for flood mitigation, reservoir operations, and emergency response. The operational rainfall methodology applied for these radars is unable to capture the complex rainfall microphysics induced by the orographic enhancement due to the surrounding mountainous terrain.

This study aims to improve operational radar rainfall estimates using auxiliary remote sensing observations. In particular, a number of S-band profilers have been deployed to investigate the vertical structure of precipitation at various locations in this complex terrain region. The representative vertical profiles of reflectivity measured by the profilers, which can better characterize the low-level rainfall microphysical structure, are incorporated in NEXRAD radar data processing and the derivation of improved rainfall products. In addition, high-frequency (i.e., X-band) high-resolution gap-filling radars are being deployed to improve tracking of incoming storms and provide high-resolution coverage over populated and flood-prone urban areas throughout the Bay region. To date, two X-band radars have been deployed and collected a substantial set of precipitation measurements that contribute to the development of local radar rainfall algorithms. This paper details the applications of vertical pointing profilers and X-band scanning radars in enhancing monitoring and quantitative estimation of precipitation over the Bay Area. The product performance is demonstrated through cross-comparison with rain gauge observations. Results show that rainfall products derived with the aid of additional remote sensors have better performance compared to the operational radar products currently available in this particular domain.