冬季北京强降雪过程水汽在空中的表现

Analysis of vapour conditions for 5 heavy snowfall processes in Beijing in 2009

文中应用探空资料和常规气象数据,分析了2009年冬季北京连续发生的几次强降雪天气过程,对各次降雪天气过程的水汽在空中的分布、近地面层平流输送、中高空平流输送及水汽累积过程进行了研究。结果表明:北京冬季较强降雪前的水汽在空中有两种表现:一是水汽在高空先达到饱和,二是近地面层的水汽先达到饱和。我们试称之为低源水汽和高源水汽。湿层的上下变化主要取决于垂直运动发生的高低,而垂直运动发生的高低由天气系统的位置决定。高层有水汽辐合时,水汽由上而下发展,当低层有水汽辐合时,水汽由上而下发展。水汽的变化反映出上升运动情况。两种水汽表现所对应的上升运动中心高度的位置有明显的差异,高源水汽上升速度中心位置较高,而低源水汽的上升速度中心高度位置较低。在这两种水汽输送过程中,上升运动的表现和水汽积累并不完全相同,但无论哪种输送,降雪时,湿层都表现为上下贯通,而且湿层基本上要接地。

There were five heavy snowfalls in Beijing in winter of 2009. To study vapour source of the snow in Beijing process ,we analyzed the vapour source, distribution of vapour in the air, transportation and accumulation process of vapours in the processes of the 5 heavy snowfalls in Beijing in 2009. The study discovered two types of vapour source for heavy snowfalls in the winter in Beijing： one was the advection transportation of vapour in the air, the other was the advection transportation of vapour in ground layer. The change of humidity layer was de- cided by vertical motion. The height of the vertical motion was decided by the location of the weather system. When the water vapor converged in the high, the water vapor grew from upper to low. When the water vapor con- verged in the low, the water vapor grew to the opposite. The change of the water vapor reflected the vertical mo- tion. The processes of the two - type vapour transportation, the performance of ascending motion and accumula- tion were not exactly the same. In addition, the eastern air under the special terrain of Beijing could both trans- port and lift the vapour, and it was also an important factor affecting the vapour condition of snowfall in Beijing. The wetting layer must be vertically connected, and was normally grounded.