Ambient Factors Controlling the Wintertime Precipitation Distribution Across Mountain Ranges in the Interior Western United States. Part I: Insights from Regional Climate Simulations

摘要

This study analyzes the control of upstream conditions on the distribution of wintertime precipitation across mountain ranges in the interior western United States using 10 winters of high-resolution regional climate model data. Three mountain ranges, the Wind River Range, the Park Range, and the Teton Range, are selected to explore the statistical relations between the precipitation distribution and upstream wind, stability, and cloud conditions. A 4-km-resolution simulation is used for the former two ranges, and a 1.33-km-resolution simulation driven by the 4-km-resolution simulation is used for the Teton Range, which is smaller and steeper. Across all three mountain ranges, the dominant factor controlling precipitation is the mountain-normal low-level wind speed. Statistically, stronger wind results in heavier precipitation and a lower upwind precipitation fraction. The low-level wind generally veers with height during precipitation events, but the amount of veering does not unambiguously affect the precipitation distribution or intensity. The more the terrain blocks the upstream flow, the more the precipitation shifts toward the upstream side of the mountain and the weaker the overall precipitation rate is. A higher cloud-base temperature and a lower cloud-base height typically are associated with heavier precipitation. Deeper clouds tend to produce heavier precipitation and a slightly lower windward/leeward contrast. Convective precipitation proportionally falls more on the lee slopes than stratiform precipitation. The upstream and macroscale cloud conditions identified herein predict both the mean precipitation rate and the upwind precipitation fraction very well for the three ranges studied here.