Modification of Precipitation in Mid-Latitude Cyclones Passing over a Coastal Mountain Range

Abstract

As mid-latitude cyclones pass over a coastal mountain range, the processes producing their clouds and precipitation are modified, leading to considerable spatial variability in precipitation amount and composition. Surface, radar, and aircraft observations from the 2015-16 Olympic Mountains Experiment (OLYMPEX) field campaign and realistic, high-resolution WRF model runs provided complimentary perspectives on the dynamic and microphysical processes associated with terrain-induced precipitation processes. To the first order, the location of the mid-latitude cyclone relative to the mountain barrier determines the likely modes of precipitation enhancement or lack thereof. The prefrontal, warm, and postfrontal sectors have distinct synoptic-scale environmental conditions (flow, stability, and temperature), which were shown to exert significant controls over the flow and microphysical response upon encountering complex terrain. Prefrontal sectors contained homogeneous stratiform precipitation with a slightly enhanced ice layer on the windward slopes and rapid diminishment to a lee rain shadow. A stably stratified coastal environment deflected the low-level flow in front of the barrier. Stationary mountain waves over the smaller-scale windward ridges had minimal impact on the overall precipitation pattern due to a lack of generation of cloud water or generation of small raindrops at low elevations. Warm sectors contained a broad spectrum of sizes and concentrations of raindrops where high precipitation rates were achieved from varying degrees of both liquid and ice precipitation-generating processes. Significant quantities of cloud water were produced over coastal foothills and lower-windward slopes, leading to a remarkable enhancement of precipitation on the lower-windward slopes. Enhancement in the ice layer occurred directly over the barrier where the ice particles were further advected downstream by cross-barrier winds and spilled over into the lee. As a whole, this dissertation demonstrates that precipitation enhancement over the Olympic Mountains depends on a complex mix of warm low-level rain processes and upper level ice processes. The relative importance of these processes depends on both the synoptic system and the position on the barrier. The greatest precipitation enhancement on the windward slopes occurred when both warm and ice processes were amplified by flow over the barrier and were acting together during the warm sectors of strongly-forced atmospheric river-type storms. The least precipitation enhancement occurred when the warm processes were absent. Lee side precipitation must overcome persistent descent on the downstream side of the higher windward ridges.