Orographic modification of convection and flow kinematics by the Oregon coastal range and Cascades during IMPROVE-2

This presentation describes the kinematic and precipitation evolution accompanying the passage of a cold baroclinic trough over the central Oregon Coastal Range and Cascades during 4-5 December 2001 of the second Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE-2) field project. In contrast to previously documented IMPROVE-2 cases, the 4-5 December event featured weaker cross-barrier winds (15-20 m s-1), weaker moist static stability (Nm < 0.006 s-1), and convective cells that preferentially intensified over Oregon's modest coastal mountain range. These cells propagated eastward and became embedded within the larger orographic precipitation shield over the windward slopes of the Cascades. The Weather Research and Forecasting (WRF) model (version 2.2) at 1.33-km grid spacing was able to accurately replicate the observed evolution of the precipitation across western Oregon.

As a result of the convective cell development, the precipitation enhancement over the coastal range (500-1000 m above mean sea level [MSL]) was nearly as large as that over the Cascades (1500-2000 m MSL). Simulations selectively eliminating the elevated coastal range and differential land-sea friction across the Pacific coastline illustrate that both effects were important in triggering convection and in producing the observed coastal precipitation enhancement. A sensitivity run employing a smoothed representation of the Cascades illustrates than narrow ridges located on that barrier's windward slope had a relatively small (< 5%) impact on embedded convection and overall precipitation amounts there. This is attributed to the relatively weak gravity wave motions and low freezing level, which limited precipitation growth by riming.

Hovmoller plots of reflectivity at Portland, OR (RTX WSR-88D radar) will highlight the importance of intermittent convective cells becoming enhanced over the coastal range during major storms over the Cascades.