The instability of Long's stationary solution and the evolution toward severe downslope windstorm flow. Part I: nested grid numerical simulations

Abstract

As a result of this increased resolution, three distinct stages of windstorm development are explicitly identified. In the first, convection acts to neutralize the region of overturned isentropes. During the next stage, a large-amplitude stationary disturbance develops above the lee slope of the topography. In time, small-scale secondary shear instability develops in local regions of enhanced shear associated with flow perturbations caused by the large-amplitude disturbance. In the final stage of development, these modes of shear instability evolve to larger spatial scale and come to dominate the flow in the mature windstorm state. These stages of development can be qualitatively and, to some extent, quantitatively reproduced in a parallel flow extracted from a cross section through Long's solution if a horizontally localized forcing, designed to enhance the vertical shear in the background wind field, is imposed. -from Authors