The Origin of Severe Downslope Windstorm Pulsations

Abstract

Recently reported Doppler lidar observations of the downslope component of flow velocity made during the occurrence of a mountain windstorm at Boulder, Colorado, have established that such storms are characterized by an intense pulsation of windspeed with characteristic period(s) near 10 minutes. Scinocca and Peltier (1989) have independently shown such pulsations to be predicted on the basis of two-dimensional nonhydrostatic numerical simulations in which internal waves launched by stratified flow over smooth topography are forced to exceed critical steepness and, therefore, ?break.? In the present paper we analyze the physical mechanism that supports this pulsation. As we demonstrate, it is due to Kelvin-Helmholtz instability of the new (quasi-parallel) mean flow that is established in the lee of the obstacle by the wave, mean-flow interaction induced by wave breaking. As such the pulsation represents a secondary instability of the stratified flow in which the primary instability is that associated with the initial transition into the high drag, severe downslope windstorm state. This secondary instability also appears to play a role in determining the maximum intensity that the windstorm may achieve and, therefore, is a crucial ingredient in the wave-turbulence interplay that constitutes the mountain windstorm phenomenon.