A Climatology of Atmospheric Rivers and Associated Precipitation for the Seven U.S. 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 consistently applied to global reanalysis from the Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2) to characterize ARs regionally across the continental United States (CONUS). AR detection is based on an objective identification algorithm integrating IVT magnitude thresholding, geometry, and directional requirements. Precipitation extremes are identified based on 3-day precipitation totals exceeding the 95^th percentile at each grid point. Regionally aggregated measures of AR characteristics, including length, direction, and magnitude, are computed across the CONUS at the grid point scale and summarized for each of the seven US National Climate Assessment (NCA) regions. Primary annual and seasonal AR moisture pathways are identified to characterize canonical AR characteristics within each NCA region. Basic geometry, location, and IVT characteristics of objectively detected ARs reveal regional and seasonal variability. 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. Linked AR precipitation results suggest that a substantial proportion of 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 in the East compared to the West where orographically enhanced wintertime precipitation extremes are dominant.