A Climatology of Atmospheric Rivers and Associated Precipitation for the Seven US National Climate Assessment Regions

Atmospheric Rivers (ARs), long and narrow filamentary regions of enhanced vertically integrated water vapor transport (IVT), play an important role in regional hydrometeorological extremes and the global water cycle. Here an AR detection algorithm is applied to global reanalysis from Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2) to characterize ARs regionally across the continental United States (CONUS). Regionally aggregated measures of AR characteristics, including area, direction, magnitude, and associated precipitation quantities, are computed across the CONUS at the grid point scale and summarized for each of the seven US National Climate Assessment (NCA) regions. Climatologically, AR frequency maxima tend to occur in the fall and winter in the West, spring in the Great Plains, and fall in the Midwest and Northeast. Basic geometry and IVT characteristics of objectively detected ARs have regional and seasonal variability. Seasonal AR IVT magnitude, direction composites reveal northeastward directed moisture fueled by the tropical Pacific in the Northwest and Southwest with multiple seasonally-varying moisture sources, from the Pacific and Atlantic, observable in the Southern Great Plains and Midwest. Linked AR precipitation characeristics suggest that a substantial proportion of 95th percentile extreme precipitation days are associated with ARs over many parts of CONUS, including the eastern US. Regional patterns of AR-driven precipitation suggest that complex dynamic, seasonally-varying, synoptically-driven transport mechanisms are important across the East compared to the West where orographically enhanced wintertime precipitation extremes are dominant.