Multi-season observational study of precipitation structures along the Oregon Cascade windward slope

During the winter months, extra-tropical cyclones develop over the Pacific Ocean and move across the U.S. Pacific Northwest. The Coastal and Cascade mountain ranges modify and enhance the structure of the storms leading to enhanced precipitation. Portland, Oregon operational WSR-88D radar data is utilized to examine the spatial characteristics of precipitation structures over the windward slope of the Cascade Mountains. Data from operational soundings at Salem, Oregon and a vertically pointing MicroRainRadar in Portland are used to analyze the environmental characteristics upwind of the Cascades. Extreme storm events are associated with flooding and mudslides. All flooding events since 1998 had freezing level heights greater than the seasonal 50th percentile and 86% of flooding events had freezing levels that were higher than the 75th percentile. Vertically-integrated water vapor and rain layer depth are highly correlated (r=0.82). Seventy winter storms from 2005-2008 with low-level wind directions within 33° azimuth of south-southwest were selected for analysis. Examination of a narrow wind direction subset of storms reveals the sensitivity of precipitation to other environmental parameters.

Storm accumulated rainfall is the sum of each rain rate occurring during a storm times the rain rate's duration. The varying height of the bright band in the Portland region makes surface precipitation estimation difficult. We determine persistence of precipitation by calculating the exceedence frequency of reflectivity values > 13 dBZ and the relative intensity of precipitation using a ratio of exceedence frequencies for different thresholds. A statistical tool called a contoured frequency by distance diagram (CFDD, first described by B. Jewett, U. Illinois) is used to identify regions of orographic enhancement along the slope. CFDDs depict the distributions of persistence and intensity as a function of distance from the Cascade crest. Both persistence and intensity of precipitation have higher magnitudes for deep rain layers compared to shallow rain layers. Persistence generally increases moving eastward across the foothills. For shallow rain layer storms, persistence drops sharply over the upper portions of the windward slope (within 35 km of the crest). For deep rain layer storms, persistence is relatively constant across the upper windward slope until it starts to decrease 10 km from the crest. Both stronger persistence and intensity of precipitation contribute to the high rainfall accumulations of flooding storms compared to other deep rain layer storms.