Climatological characteristics and inland penetration of atmospheric rivers over the western United States

Narrow corridors of intense water vapor transport known as atmospheric rivers (ARs) contribute to extreme precipitation and flooding along the Pacific coast, but knowledge of their influence over the mountainous interior of the western United States is limited. Here we use ERA-Interim analyses, Climate Prediction Center (CPC) precipitation analyses, and Snowpack Telemetry (SNOTEL) liquid precipitation equivalent (LPE) observations from mountainous regions to describe the characteristics of cool-season (November–April) ARs across the western United States, including their modification by major topographic barriers. 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. Farther 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.

We differentiate between inland-penetrating and non-penetrating ARs using low- (950-hPa) and mid-level (700-hPa) forward trajectories initiated within cool-season ARs as they approach the west coast of North America. These trajectories are then 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. At initiation, interior-penetrating AR trajectories are associated with a more amplified trough-ridge pattern over the northeastern Pacific and western U.S., more southwesterly (vs. westerly) flow, and larger water vapor transport (qu), particularly west of the Sierra Nevada. Such interior-penetrating AR trajectories most frequently originate along the Oregon coast, but the greatest fraction of trajectories that eventually penetrate into the interior is found along the coast of the Baja Peninsula. Due to large water-vapor depletion and other orographic effects, Inland- and interior-penetrating trajectories rarely pass over the "high" Sierra, helping to explain the reduced frequency and influence of ARs east of this range.

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 their inland penetration.