The climatological characteristics and inland penetration of atmospheric rivers over the western United States.

摘要

This dissertation quantifies the climatological characteristics of cool-season (November-April) atmospheric rivers (ARs) over the western U.S., and identifies favorable conditions for the inland penetration of ARs approaching the U.S. west coast.;To quantify the climatological characteristics of cool-season ARs, they are objectively identified in the ERA-Interim reanalysis dataset on the basis of vertically integrated water vapor transport from November 1988 to April 2011. The climatological characteristics of ARs are generally largest along the coasts of Oregon and Washington, decreasing gradually southward along the coast, and rapidly eastward across the Sierra-Cascades Ranges. Over the interior, these characteristics are largest over the southwest and northwest, whereas they are smallest over the central and southern Great Basin, which lies downstream of the southern or "high" Sierra Nevada. The climatological characteristics of ARs over the western U.S. are strongly influenced by the climatology of landfalling ARs along the west coast and by water vapor depletion, particularly over high topographic barriers.;Data from 72-h forward trajectories initiated from 950 hPa within ARs near the North American west coast are used to identify conditions that are favorable for AR penetration into the interior. These trajectories are classified as coastal-decaying, inland-penetrating, or interior-penetrating based on whether they remain within an AR upon reaching selected transects over the western U.S. Conditions associated with interior-penetrating AR trajectories include a more amplified atmospheric pattern, stronger southwesterly flow, and larger amounts of water vapor transport. Interior-penetrating AR trajectories most frequently originate along the Oregon coast, but trajectories originating along the coast of the Baja Peninsula are most likely to eventually penetrate into the interior. Although coastal-decaying and interior-penetrating AR trajectories experience similar decreases in specific humidity, the latter experience offsetting increases in wind speed that allow them to maintain large water vapor fluxes. Interior-penetrating AR trajectories lose a smaller fraction of their initial water vapor because they have more to begin with. Hence, synoptic conditions favoring larger initial water vapor flux and allowing for increases in wind over the western U.S. appear to play an important role in the maintenance of ARs as they penetrate into this region.