A modeling study on downslope windstorms

by Craig M. Smith

Institution: Oregon State University
Department: Atmospheric Sciences
Degree: PhD
Year: 2010
Keywords: downslope windstorms; Windstorms  – Mathematical models
Record ID: 1880345
Full text PDF: http://hdl.handle.net/1957/16235


The generation mechanism for downslope windstorms was shown to vary according to inversion height and strength using a series of numerical experiments. Strong low level inversions were dominated by interfacial waves on the inversion, while high level inversion and cases without an inversion were dominated by internal gravity wave (IGW) breaking. For cases with an inversion at intermediate heights the following mechanisms of downslope windstorm formation were explored and discarded: trapping of IGW energy in the lower layer by an inversion, nonlinear IGW breaking, and subcritical to supercritical transition of the interfacial mode on the inversion. The remaining mechanism of downslope windstorm formation for this case was shown to be a direct result of mountain wave induced instabilities on the inversion and associated coupling of the stagnation zone with the lee surface jet. The generation of the stagnation zone is due to imbalances in the momentum budget equation which lead to a pocket of neutrally stratified stagnant air which propagates back to the ridge and amplifies. Amplification of the perturbation and overturning of isotherms lead initially to buoyant production of turbulence and subsequently to shear production of TKE along the bottom of the stagnation zone and upper part of the surface jet. This shear production acts to grow the stagnation zone downstream, forcing the flow beneath the inversion into a lee jet beneath and leading to the creation of a downslope windstorm. The inversion instability mechanism suggests a scenario in which downslope windstorms may occur when neither a barotropic transition nor IGW breaking are predicted, and mesoscale models may not be able to adequately resolve relevant turbulent processes. Next, an analysis of observations is presented which explores the effect of surface heat fluxes on downslope windstorms. The observations of downslope windstorms on the Falkland Islands revealed a number of interesting characteristics. Most of the observed events are of limited downstream extent. Those that do extend far downstream tend be occur in conjunction with a strong low level inversion, while those of limited length more often occur in conjunction with weaker inversions. Finally, downslope windstorms often occur at night. Two specific case studies of events are presented which are indicative of these larger trends. A series of simplified numerical experiments are presented which explore the effect of surface heat fluxes on downslope windstorms using an eddy resolving model. Initially we focus on IGW breaking and lee wave rotors without an upstream inversion. In the basic case with no surface heating, a typical nonlinear wave response was produced, with a lee side surface jet and a large zone of stagnant air above the lee side of the mountain that eventually resulted in overturning of isotherms and generation of turbulence. Turbulence is generated initially by overturning isotherms in the nascent stagnation zone, and then by shear production along the edges of the stagnation zone,…