Reflection of Hydrostatic Gravity Waves in a Stratified Shear Flow. Part II: Application to Downslope Surface Windstorms

Abstract

A model of continuous partial reflection of hydrostatic gravity waves, developed in Part I, is applied to the Klemp and Lilly model of downslope surface windstorms. It is shown how the magnitude and the location of the downslope winds both depend on details in the vertical structure of the background basic flow and static stability profiles. Moreover, maximum speeds tend to be located between the half-width and the peak of a bell-shaped obstacle, and decay rapidly downslope to relatively small magnitudes at the base of the orography. Vertical profiles of the reflection coefficient are determined, using data from rawinsonde soundings for three different cases of moderate to strong windstorms observed at the Front Range of the Colorado Rockies and in the lee of the Pyrenees. The present results tend to support the results obtained by Klemp and Lilly: 1) the atmospheric structure conducive to wind enhancement is associated with a relatively low static stability in the middle and upper troposphere with higher static stabilities aloft and in a relatively shallow ground-based layer; 2) the magnitudes of the reflection coefficients associated with this characteristic atmospheric structure are sufficient to produce relatively high wind speeds. A principal weakness in the model is the absence of a mechanism that can account for the observed wind speeds at the base of the orography. Practical difficulties in model evaluation are associated with: 1) the extreme sensitivity of the model predictions to atmospheric structure, particularly low wind speeds; 2) the inclusion of upstream blocking effects; and 3) the inclusion of conditionally unstable layers in the determination of the reflection coefficient. However, the present study bridges the gap between layered and continuous models of wave reflection and provides a firmer foundation upon which the partial reflection mechanism can be evaluated against other quite different models of surface wind enhancement.