Numerical study of severe downslope winds at Kelamayi, XinJiang

Severe downslope winds are observed at Kelamayi, XinJiang with serious damages caused, but the mechanism for its occurrence is not clear. Recently, a strong downslope wind occurred at Kelamayi on 7-8 Mar 2013, with a maximum wind speed (Vmax) exceeding 25.8 m/s at the Kelamayi district and exceeding 35.2 m/s at the Baijiantan district, severe wind persisted for more than ten hours. We use a mesoscale numerical model (WRF) to simulate the windstorm, the model successfully reproduces the major features of the observed downslope winds. Based on the simulative results, we propose a mechanism for Kelamayi downslope windstorms: the environmental conditions of upstream is characterized by the lower level northwestly and the middle-higher level southwestly, and, when wind speeding up, the flow with enough kinetic energy is able to overcome the static stability up and over the mountain with the result that a distinct gravitational wave dominates in the lee of mountain. The phase of gravity wave tilt to the upstream area, then wave breaks and a well-mixed stagnant layer above the crest level appears. Transitional flow is clear between the turbulent flow and lower well-defined stable layer, which causes strong downslope winds developed along the lee slope of the mountain range. A critical layer is present in the mountain lee characterized by low level northwestly and upper level southwestly, and, below the mean-state critical layer, the isentropes began to descend over the crest of mountain and continued descending until the flow reached the base, and thus the energy from the upper level could transfer to the surface. At the end, we use the observed sounding data from the 33 severe downslope wind cases between 2006 and 2012 to verify the proposed mechanism for the Kelamayi downslope windstorms.