An observational and numerical study of windstorms along the western side of the Washington Cascade Mountains

Abstract

This dissertation investigates western Washington Cascade windstorms by utilizing various data sources such as conventional synoptic observations, profilers on both sides of the Cascade Mountains, a WSR-88D Doppler radar located in northern Puget Sound, and non-hydrostatic model simulations down to 1 km resolution. First, the lee side wind event of 12 February 1995 is described, since it represents an excellent case of a strong event, with gusts of 35-40 m s$\sp{-1}$ along the foothills downwind of Stampede Gap, the major mesoscale passage way through the in the Cascades.A major objective of this study is to determine the role of the three-dimensional topography of the Cascades on these lee side windstorms and the relative importance of gap flow and mountain wave accelerations. Low-level model trajectories for the 12 February 1995 event show a tendency for the air to pass westward through the major gaps in the Cascades. However, model simulations in which Stampede Gap is filled show that removing the lower passes of the gap ($$13 m s$\sp{-1}$) when an environmental critical level exists below 400 mb. Even without a critical level, reverse shear or a sharp reduction in stability above crest-level can lead to enhanced mountain wave amplification and strong lee side winds for strong cross-barrier flow events. An environmental critical level is less important for windstorm development for cases with weaker cross barrier flow ($<$10 m s$\sp{-1}$), which accounts for the majority of the observed windstorm events. Idealized simulations also show that significant lee side winds (20-30 m s$\sp{-1}$) do not develop for cases without initial cross-barrier flow until the pressure difference across the Cascades approaches 15 mb and the depth of the cold air exceeds the terrain height of Stampede Cap ($\sim$1500 m).