Climatological Characteristics of Atmospheric Rivers and Their Inland Penetration over the Western United States

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Thursday, 6 February 2014: 3:45 PM
Room C101 (The Georgia World Congress Center )
Jonathan J. Rutz, NOAA/NWS, Salt Lake City, Utah; and W. J. Steenburgh and F. M. Ralph

Narrow corridors of water vapor transport known as atmospheric rivers (ARs) contribute to extreme precipitation and flooding along the west coast of the U.S., but knowledge of their influence over the interior is limited. Here, we use ERA-Interim reanalysis data, CPC precipitation analyses, and SNOTEL observations to describe the characteristics of cool-season (November–April) ARs over the western U.S. We show that AR frequency and duration exhibit a maximum along the Oregon/Washington coast, a strong transition zone upwind (west) of and over the Cascade–Sierra Ranges, and a broad minimum that extends from the “high” Sierra south of Lake Tahoe eastward across the central Great Basin and into the deep interior. East of the Cascade–Sierra Ranges, AR frequency and duration are largest over the interior northwest, while AR duration is large compared to AR frequency over the interior southwest. The fractions of cool-season precipitation and top-decile 24-h precipitation events attributable to ARs are largest over and west of the Cascade–Sierra Ranges. Further east, these fractions are largest over the northwest and southwest interior, with distinctly different large-scale patterns and AR orientations enabling AR penetration into each of these regions. In contrast, AR-related precipitation over the Great Basin east of the high Sierra is rare. These results indicate that water vapor depletion over major topographic barriers is a key contributor to AR decay, with ARs playing a more prominent role in the inland precipitation climatology where lower or less continuous topography facilitates the inland penetration of ARs.