An inversion based instability mechanism for downslope windstorms

A new mechanism of leading to the generation of downslope windstorms is proposed based on results from large eddy simulations (LES) of two dimensional ridge with upstream inversions of varying heights. This mechanism involves instabilities on the inversion which couple with shear production of turbulence resulting in a growing stagnation zone below which the flow is forced forming a lee surface jet. Momentum budget analysis show the creation of vertical profiles of velocity and buoyancy which are unstable with respect to perturbations by the combined action of pressure gradient forces and nonlinear advection associated with the flow response to the topographical perturbation. These forces act to split the inversion and slow the horizontal flow down, thus creating a pocket of neutrally stratified stagnant air. In this nascent stagnation zone instabilities spontaneously occur and grow, thus creating a large stagnation zone and associated lee surface jet. Previous suggested mechanisms of downslope windstorm formation of trapping of internal gravity (IGW) energy by the inversion, nonlinear hydrostatic IGW breaking and a subcritical to supercritical transition of the shallow water mode are explored with respect to the experiments presented. In cases with inversions present downslope windstorms may occur for small forcing, i.e. mountain height below which IGW breaking would not be expected to occur. Overall, the inversion instability mechanism, which many mesoscale models may not be able to adequately resolve, predicts the formation of downslope windstorms in cases where traditional theories of nonlinear IGW breaking and transition of the barotropic mode do not.