新疆克拉玛依强下坡风暴的机理研究

利用美国中尺度数值模式 WRF 对2013年3月7—8日克拉玛依强风进行了模拟，对下坡风发生、发展和结束3个阶段的三维结构特征进行了分析，并由此提出克拉玛依强下坡风的形成机制模型：上游地区出现中高层西南风、低层西北风并伴有强冷平流的配置，当风速不断增大时，气流能够翻越加依尔山在背风坡侧形成重力波，重力波相位向气流上游方向倾斜产生非线性效应，促进了波不稳定区域的形成并导致波破碎，形成湍流活跃层，不断把上层的能量向下传播；克拉玛依中低层形成三层夹心的大气层结稳定度分布，出现明显的过渡气流带从而导致强下坡风的形成；南北风分量在低层和中层符号相反，形成了临界层，不断吸收上层波能量并向地面传送，强下坡风暴不断维持发展。最后利用2006—2012年克拉玛依33个强下坡风过程中的探空观测资料对提出的形成机制进行了验证。

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 (V_max) 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.