An Overview of a Heavy Precipitation Event over California in October 2021: Roles of an Extreme Atmospheric River and Explosive Extratropical Cyclone

An exceptionally strong landfalling Atmospheric River (AR) that reached AR-scale 5 (Ralph et al. 2019), along with a 950 hPa bomb cyclone over the ocean, impacted Northern California on October 24, 2021. The storm brought record-breaking extreme rainfall that helped end the wildfire season and mitigated a severe drought. The study provides a comprehensive multiscale analysis that documents the key factors that contributed to the extreme nature of this event. This includes an examination of cloud-precipitation characteristics using satellite data, the influence of an antecedent exceptionally intense AR on the explosive cyclogenesis offshore, and the interaction between the mesoscale dry intrusion and low-level enhanced moisture convergence in ARs. Comparison of model reanalysis data and satellite observations will also be shown to unveil the performance of a numerical model in representing the dynamics and physics of this event. The observational data utilized in the study included MODIS-Terra satellite cloud products, GOES-West satellite-derived wind, GPM precipitation, and precipitation data from California weather stations. Meanwhile, the model data used came from ERA-5 reanalysis and CFSv2 analysis. The study's findings indicate that the atmospheric river (AR) provided a significant influx of water vapor and latent heat that fueled rapid cyclogenesis, consistent with recent research demonstrating that the presence of an antecedent AR can accelerate cyclone deepening by 50% relative to cyclogenesis cases without one nearby (Zhang and Ralph, 2021). However, a comparison of model data and observational data revealed that the models underrepresented the total cloud cover, with ERA5 showing a low bias that was especially pronounced for low clouds behind a cold frontal boundary associated with dry intrusion. As a result, the reanalysis and satellite data exhibited a significant discrepancy in capturing the bomb cyclone's development.