Impact of Assimilating X-band Weather Radar Network Observations on Quantitative Precipitation Forecasts over the AQPI Domain

The Advanced Quantitative Precipitation Information (AQPI) project aims to improve precipitation monitoring and forecasting in the San Francisco Bay Area by combining data from various radar systems, including S-band and X-band radars. This project is particularly important as the AQPI domain covers most of the region's flood-prone watersheds. The area is monitored by three NEXRAD S-band weather radars, with four additional X-band radars deployed and operational as part of the AQPI initiative.

This research focuses on assimilating both S-band (NEXRAD) and X-band weather radar reflectivity and radial velocity measurements into the Weather Research and Forecasting (WRF) model to evaluate the impact of X-band data on improving Quantitative Precipitation Forecast (QPF) accuracy over the San Francisco Bay Area. Two different experiments are conducted: the first will assimilate only NEXRAD measurements, and the second will assimilate both NEXRAD and X-band radar measurements simultaneously. The 3DVAR technique is employed for radar data assimilation, with the North American Mesoscale Forecast System (NAM) providing initial and boundary conditions for the WRF model configured over the AQPI domain.

The study compares QPF from the AQPI-WRF model, which incorporates both S-band and X-band measurements, with QPF from the High-Resolution Rapid Refresh (HRRR) model, where only NEXRAD data is assimilated. The X-band radar network covers the lower altitudes of the AQPI domain, offering precise hydrometeorological data over the study domain. This additional low-altitude information from X-band radars is expected to enhance the initial conditions and forecasting accuracy of the AQPI-WRF model. Available rain gauge data across the AQPI domain will be used for validation. An overview of the assimilation process and detailed QPF evaluation results are presented.