A modeling study on downslope windstorms

Graduation date: 2010The 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\ud 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.\ud 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.\ud A series of simplified numerical experiments are presented which explore the effect of surface heat fluxes on downslope windstorms using an eddy resolving\ud 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, especially at the interface with the surface jet on the lee of the slope. Over time, a series of trapped waves or rotors formed in the lee of the ridge, with the first rotor representing the separation of the lee side jet from the mountain slope.\ud Weak surface heat flux was found to reduce rotor strength and increase rotor heights. Strong surface heating prevented the formation of rotors entirely and produced a much weaker downslope wind event. Experiments with heating confined to either the upstream or downstream side of the mountain suggest that locally generated turbulence on the downstream side of the mountain is more important in controlling the rotor behavior than turbulence advected over the mountain from the upstream boundary layer. The application of moderate surface cooling led to a train of lee side waves capped by a jet of increased streamwise winds. Increased stratification from surface cooling was found to be important in the formation of a stably stratified undulating jet, which capped the rotors. Shear between this jet and the rotors generated increased turbulence in the rotors\ud themselves, while buoyant destruction of TKE in the rotor downdrafts acted to maintain the rotor circulations for a longer distance downstream from the ridge.\ud Next a series of experiments were presented which explored the effect of surface heat fluxes which occur in conjunction with a strong ridgetop level inversion. Surface heating was shown to reduce lee jet length due to transport of TKE over the ridge and buoyant production of TKE in the lee of the ridge. Surface cooling applied to a low inversion case with a fully turbulent neutrally stratified boundary layer resulted in an increase in jet length. The katabatic contribution to this jet length was fairly significant far away from the slope, which seems to be in agreement with observations from the Falkland Islands.\ud The series of cases presented with a medium height level inversion presented an even more extreme example of surface heating leading to a complete absence of downslope windstorms. It was suggested that this is due to the lack of mountain wave induced perturbation on the inversion. When surface cooling was applied to a medium inversion case with a fully turbulent neutrally stratified boundary layer, the lee flow transitioned to downslope windstorm. The transition which occurred resulted in a jet which, besides additional near surface stratification, was not much stronger than the no heating case and katabatic contributions to the cooling case were minimal.\ud A number of forecasting considerations are implied by this research: downslope windstorms over a low ridge may preferentially occur at night, daytime events may be of limited downstream extent, and downslope windstorms which\ud occur in conjunction with an inversion may not be adequately predicted by IGW breaking or interfacial wave considerations, or non-eddy resolving mesoscale models