Variability in Windward and Leeward Orographic Precipitation Ratios

Although the climatological ratio of lowland to mountain precipitation (hereafter the Orographic Precipitation Ratio, OPR) is relatively well known, its storm-to-storm variability is poorly documented and understood. As part of the Spring 2006 senior capstone project at the University of Utah, we examined storm-to-storm OPR variability across several mountain barriers of the western United States including the Washington Cascades, Oregon Cascades, Sierra Nevada of California, Wasatch Mountains of northern Utah, and Markagunt Plateau of southern Utah. Within the Washington Cascades, we also examined smaller scale variability across the Tatoosh Divide in Mt. Rainier National Park.

Daily windward and leeward OPRs were calculated for “wet days” using 24-h precipitation data from cooperative stations in the upstream lowlands, near the barrier crest, and in the downstream lowlands. To qualify as a wet day, both the low and high elevation sites needed to receive at least 2 mm of precipitation. Consistent with climatological precipitation gradients, the wet day mean and median leeward OPRs over the Cascades and Sierras were much larger than the windward OPRs. In contrast, the windward and leeward OPRs were similar across the narrower Wasatch Mountains, Markagunt Plateau, and Tatoosh Divide. The storm-to-storm variability in windward and leeward OPR was also larger for the wider barriers compared to the narrow barriers.

Large-scale analyses of individual and composite strong and weak OPR events suggest that strongly orographic storms in the Cascades and Sierra feature enhanced cross-barrier flow and moisture transport. In contrast, large-scale differences could not be discerned between strong and weak OPR events in the narrower Wasatch Mountains, Markagunt Plateau, and Tatoosh Divide. Additional work is needed to fully evaluate and understand storm-to-storm OPR variability over western U.S. mountain barriers, particularly those with narrow profiles.