Wave Ducting in a Stratified Shear Flow over a Two-Dimensional Mountain. Part II: Implications for the Development of High-Drag States for Severe Downslope Windstorms

Abstract

Abstract In this study, it is found that the discrepancies among earlier studies of severe downslope windstorms are caused by the use of the critical level height (zc), instead of the low-level uniform flow–layer depth (z1), as an indicator to determine the optimal conditions for the occurrence of high-drag states. It is determined that once the wave breaking occurs, it induces a critical level and establishes a flow configuration favorable for wave ducting in the lower uniform wind layer, which determines the phase of reflected waves. Flow regimes of high- and low-drag states for a two-dimensional, nonrotating flow with uniform static stability and a basic-state critical level over a mountain were also determined as functions of nondimensional mountain height (h), Richardson number (Ri), and nondimensional z1 in the terrain-following coordinates (σ1). The authors found that 1) the critical h for high-drag state increases as Ri increases when σ1 is fixed, 2) the critical h for high-drag state increas...