北京地区夏季边界层急流的基本特征及形成机理研究

A Study of the Basic Features and Mechanism of Boundary Layer Jet in Beijing Area

首先指出了北京地区夏季边界层急流的基本特征,即北京地区边界层急流一般出现在白天高温背景下或发生局地暴雨的夜间,强度存在明显的日变化,垂直分布具有明显的“鼻状”结构特征,急流核高度一般为600～900m。从中尺度扰动方程出发,并通过天气过程演变实例,研究了地形热力作用、局地强降水在边界层急流形成过程中的作用,指出:(1)夏季高温背景下,平原与山区之间温度梯度方向、强度的变化,是造成低空风垂直切变加速或减速,即边界层急流形成或消失的直接原因,因此这种边界层急流的高度一般要低于山体的高度。(2)局地暴雨与边界层急流之间存在明显的正反馈现象:由于局地暴雨同时改变了对流层中层和近地面层气温的水平分布,这种热力强迫作用造成了边界层气流加速;反过来,边界层气流的加速又加强了急流前方的风速辐合——如果急流方向水平垂直于山坡,这种迎风坡上的辐合将更强,造成局地降水强度进一步增强。

The basic features of boundary layer jets ,which appear in Beijing area in some summer nights , have been investigated first. The investigation shows that the boundary layer jets often turn up during the clear nights after days with an extreme high temperature or local heavy rain, and there is obvious diurnal variation for their intensities. Their vertical wind structure is similar to nose, and heights of the strongest wind are about 600900 m. The thermodynamic effects of topography or local heavy rain have been studied by deducing meso\|scale equations and confirmed by weather cases. Some conclusions can be drawn: (1) The vertical wind shear is strengthened (the jet appears) or weakened (the jet disappears) with change of the direction and intensity of temperature gradient between mountain and plain. The reason why boundary jets come into being during the summer nights after days with an extreme high temperature is that the strongest temperature gradient pointing to plain area is easy to come into being. (2) There is obvious positive feedback between local heavy rain and appearance of boundary layer jet: condensation latent heat in the lower mid\|troposphere and rapidly decreasing air temperature in near\|surface with local heavy rain should bring forth obvious temperature gradients of opposite directions in different layers, and the opposite temperature gradients cause accelerated motion of air stream in the mid\|boundary layer together, so the boundary layer jets often reach the strongest after rain starts for 12 hours; however, the convergence in front of the jet will be strengthened with the accelerated stream and the rain intensity will be advanced.