Characteristics of Precipitation Patterns in Moisture-Dominated Versus Wind-Dominated Atmospheric Rivers Over Western North America

We categorize atmospheric rivers (ARs) that make landfall between northern California and British Columbia during the period of 1980–2023 into two flavors: moisture-dominated (moist-ARs) and wind-dominated ARs (windy-ARs). Although moist-ARs and windy-ARs have similar duration and magnitude of integrated water vapor transport (IVT) at the AR core after they make landfall, their accompanying precipitation patterns are different. Over the duration of the AR, moist-ARs produce higher cumulative precipitation in inland and interior areas while windy-ARs produce higher cumulative precipitation in coastal areas. We explore the mechanisms leading to the precipitation pattern differences from both dynamical and thermodynamical perspectives. Moist-ARs are associated with a weaker but a more zonally extended anomalous trough at 500 hPa, which leads to stronger zonal wind and IVT that favor penetration of moisture and enhanced precipitation inland. Moist-ARs also have more moisture in the upper troposphere everywhere and stronger integrated water vapor and column relative humidity over the ocean and the interior. Windy-ARs are associated with an enhanced trough at 500 hPa and lower pressure at sea level, which promote a stronger low-level southerly wind component and colder temperatures. These together lead to stronger low-level IVT that are more perpendicular to the coastline and mountain ranges and stronger low-level relative humidity near the coastline, despite less total column moisture. Windy-ARs are also associated with stronger IVT convergence and stronger synoptic-scale ascent forcing near coastal areas than moist-ARs.