Upstream Evolution of Landfalling Atmospheric Rivers and Their Impacts on Precipitation Distributions along the U.S. West Coast

Atmospheric Rivers (ARs) are responsible for a majority of global poleward moisture transport and can result in extreme precipitation events and flooding along the U.S. West Coast. ARs are long (>2000 km) and narrow (500–1000km) corridors of enhanced vertically integrated water vapor (IWV) and integrated water vapor transport (IVT) that may be found within a variety of synoptic scale flow patterns. Observational evidence suggests that ARs within different flow patterns may contain different water vapor source regions, different orientations, different IVT magnitudes, and may result in different precipitation distributions. This study uses a k-means clustering technique to objectively identify different flow patterns that contain landfalling ARs along the U.S. West Coast. For example, the clustering technique identified four different types of ARs that may make landfall along the north – central California coast: westerly, southerly, or southwesterly with moderate IVT values >200 kg/m/s or southwesterly with IVT values >400 kg/m/s. This project illustrates the upstream evolution of these different types of ARs, the evolution of water vapor within each type of AR, and the different precipitation distributions associated with each type of AR at landfall. The IVT magnitude and direction both have important implications on the resulting precipitation distribution. For example, the largest precipitation amounts are observed in locations where the IVT direction is parallel to upslope topography gradient, whereas the farthest inland extent of precipitation is observed in association with landfalling ARs that contain higher IVT magnitudes.