Analysis of Water Vapor Transport and Precipitation over the Russian River and Santa Ana River Watersheds in California

California water resources are heavily dependent on the precipitation associated with atmospheric rivers (ARs). Many studies use Integrated Water Vapor Transport (IVT) magnitude in forecasting impacts associated with ARs and precipitation. Few studies have focused on the vertical distribution of IVT and its direction relative to topography. This study is a 20-yr cool season (October–March) climatology that examines the vertical distribution of (IVT) in ARs over the Russian River (RRW) and Santa Ana River Watersheds (SARW). Cool-season precipitation data from PRISM is averaged over the RRW and SARW for the 20-year period 1999–2019. IVT magnitude and direction are calculated using water vapor flux and wind data from NASA MERRA in 50-hPa layers at 38°N 123.125°W for the RRW and 33°N 117.125°W for the SARW. AR events with IVT magnitudes >250 kg m-1s-1(IVT250 events) in the RRW generate up to ~70% of total annual precipitation and generate up to ~60% of total annual precipitation in the SARW. The vertical structure of IVT250 events are analyzed to investigate the distribution of water vapor flux in the lower troposphere (1000-700 hPa), the upper troposphere (700-300-hPa), the direction of IVT relative to terrain and precipitation totals. IVT concentrated in the 1000-700mb layer, and southwesterly ARs explains 73% of the variance of precipitation totals in the RRW and 48% of the variance in the SARW. Future work includes investigating the role of synoptic-scale forcing on seeding orographic precipitation to help explain the other 27% of variance.