Mesoscale Processes in Atmospheric Rivers Impacting Southern California

Atmospheric rivers (AR) are the dominant driver of precipitation in Southern California, yet their regional impact has received less research attention than in Northern California. This is partially due to the fact that Northern California AR events are more frequent than their Southern California counterparts, and are depended upon by the entire state for water resources. Furthermore, the mesoscale processes that contribute to weather-related hazards during heavy precipitation periods in Southern California ARs have not featured prominently in research publications, despite their importance for regional forecasters. This research utilizes data from multiple NEXRAD radar sites, vertically profiling S-band and FMCW radars, and the dual frequency radar onboard the GPM satellite to observe mesoscale processes in Southern California AR events during the 2016/2017 winter season. The prevailing synoptic conditions during several case studies are additionally investigated relative to 95^th percentile precipitation events from 1981-2017. There appear to be several “flavors” of synoptic AR conditions that are historically important for Southern California, and differences between the Jan. 23 and Feb. 17, 2017 case studies highlight the importance of both large-scale and mesoscale influences on the region’s extreme precipitation events. For example, the Feb. 17, 2017 AR event was enhanced by a mesoscale frontal wave, featured substantial embedded convection, and relied relatively little on strong bulk upslope flux of AR moisture to produce extreme precipitation. Case study events were also simulated using a configuration of the Weather Research and Forecasting model that has been optimized for AR simulation over California (WestWRF), but has not been specifically investigated for Southern California ARs. WestWRF simulation of mesoscale features in these case studies is validated using the aforementioned radar data, with the objective of determining event hindcast skill. Sensitivity tests for microphysics and boundary layer parameterizations, as well as sea surface state, were additionally performed with the goal of identifying feasible changes to the WestWRF near real-time configuration that may improve Southern California AR forecasts. Mesoscale processes within Southern California ARs deserve increased attention as they are an important influence on regional impacts and substantially increase forecast challenges.