Pulsating Downslope Windstorms

Abstract

The flow configurations that obtain in several severe downslope windstorm events generated over isolated topography are studied using a two-dimensional nonlinear anelastic model. A new high resolution simulation of the 11 January 1972 windstorm in Boulder, Colorado, constructed using a very large model domain, is shown to qualitatively reproduce the strong, quasi-periodic, 5?15 min transience in surface wind speed to the lee of the topography that was actually observed during this event. It is demonstrated that this transience is caused by the continuous generation of strong pulses of enhanced surface wind on the lee slope, which thereafter propagate downstream with individually constant speeds. An identical phenomenon is shown to be characteristic of the high drag regime in severe downslope windstorms simulated in flows characterized by upstream profiles having constant wind and stability. This newly discovered pulsation phenomenon is therefore a generic property of flows induced by the breaking of topographically forced internal waves.