The Impact of Varying Environmental Conditions on the Spatial and Temporal Patterns of Orographic Precipitation over the Pacific Northwest near Portland, Oregon

Operational radar data from three winter seasons (2003-06) in Portland, Oregon, in the U. S. Pacific Northwest are used to describe how orographic precipitation varies with cross-barrier wind speed, 0 degrees C level height, and stability over the moderately wide (similar to 50-km half-width) Cascade Mountain Range. Orographic enhancement is specified in terms of location, frequency, and relative intensity of the reflectivity (precipitation field). The typical storm for the region, as defined by the 25th to 75th percentile characteristics, is compared to storms with <25th and >75th percentile characteristics for a given variable. About half of Portland-region storms have a low-level wind direction within a relatively narrow azimuth range. This subset of storms is used to examine the sensitivity of orographic enhancement relative to other environmental variables. Cross-barrier wind speed has a stronger role in determining the magnitude of precipitation frequency than either 0 degrees C level or stability. Cross-barrier wind speed and 0 degrees C level height have separate but comparable roles in determining the frequency of relatively heavier precipitation. The increase in precipitation frequency with stronger cross-barrier wind speed is partially attributed to the higher occurrence of intermittent convective cells intersecting the slope. The area where inferred riming growth occurs over local peaks on the windward slope broadens upslope as the 0 degrees C level height increases. In the Portland region, variations in the squared moist Brunt-Vaisala frequency yield smaller differences in the pattern and intensity of precipitation enhancement than either cross-barrier wind speed or 0 degrees C level height.