Orographic precipitation and kinematic flow structures of winter storms over the U.S. Pacific Northwest

Intensive analysis of observations and modeling studies from the Mesoscale Alpine Programme (MAP) and the Improvement of Microphysical Parameterization through Observational Verification Experiment II (IMPROVE II) field programs has lead to several refinements of conceptual models of orographic enhancement. In their review, Rotunno and Houze (2007) emphasize the importance of the first peak in terrain along the windward slope as a location of favored precipitation enhancement. The conceptual models derived from MAP and IMPROVE are based on a small set of cases under a limited range of environmental conditions. Operational data sets are less comprehensive than those obtained during major field programs but have the advantage of yielding a larger and more representative sample of storms and storm environments than multi-week field experiments.

We examine the exclusivity of favored orographic enhancement over the first peak in terrain using radar reflectivity and radial velocity data from the Portland, OR NWS WSR-88D radar for 117 winter season storms (1 November - 31 March) from 2003-2006. These data are used to determine the typical three-dimensional spatial patterns of precipitation and winds for this region and their relation to thermodynamic characteristics from the nearby NWS upper air sounding at Salem, OR. South and southwesterly storms represent 84% of storms in the Portland region. The distribution of storm volumes was strongly skewed toward smaller storms. These smaller storms had a wide range of stability, wind direction, and Froude number characteristics. Larger storms in the Portland region usually had winds from the southwest, were more stable, and had stronger cross-barrier wind speeds than the smaller volume storms. However, the occurrence of outliers to the above conditions, the small sample size of large storms, and the wide variation of the characteristics of smaller storms yields weak correlations between specific thermodynamic characteristics and storm volume.

In the Portland region, the location of favored precipitation enhancement along the windward slope of the Cascades occurs over different peaks depending on environmental conditions such as wind direction, stability, and cross-barrier wind speed. Additionally, the number of local maxima in precipitation frequency and their locations relative to the underlying ridges and valleys can differ based on the minimum dBZ threshold examined. For stable southwest storms which make up the majority of events near Portland, the frequency of 13 dBZ echo was enhanced midslope compared to over the first peak of terrain for Z ≥25 dBZ. For southwest storms with stronger wind and unstable conditions, the ≥25 dBZ precipitation frequency had local maxima over the first two peaks in terrain whereas for the same subset of storms the ≥13 dBZ precipitation frequency had its maximum at mid slope. While favored orographic enhancement over the first peak in terrain does occur, it does not do so exclusively. The complex, multi-ridge, 3D topography of the Cascades yields a superposition of localized enhancements that are similar to Colle's (2008) idealized 2D multi-ridge simulations.