我国西部高原大气边界层中的对流活动

利用 1 998年第 2次青藏高原野外试验中的多普勒声雷达探测、低空探测观测以及卫星观测资料对高原大气边界层内的对流现象进行分析研究。声雷达探测到了高原边界层内有强烈的对流活动。这种对流泡中心的垂直速度可超过 1m/s,并存在尺度为 1个多小时的周期性 ,表现为中小尺度的有组织的湍流活动。高原边界层强对流得以发展和维持的物理机制是 :强辐射加热、复杂的地形地貌形成的下垫面不均一性造成边界层斜压性、边界层内的平流活动等 ,这些现象都有利于对流的发展。在这些条件的作用下 ,边界层内可以产生一系列有组织的强湍流大涡旋活动 ,这些大涡旋形成的热泡在向上发展的过程中有的能够发生合并 ,变得更大也更为猛烈 ,达到凝结高度以上可形成对流云 ,并发生充分的对流混合。成云过程凝结潜热释放更有利于对流运动进一步发展 ,使对流云逐步发展成更大的对流云团 ,从而产生卫星云图中显示的云团发展过程。

Convective Activities in the Atmospheric Boundary Layer over the Plateau in Western China

The convective phenomena in the atmospheric boundary layer on the Tibetan plateau are analyzed based on data of Doppler sodars, the radiosonde system with high resolution, and satellite images during the second field experiment of atmospheric sciences on the Tibetan Plateau in 1998. Strong convective activities were detected by sodars. The vertical velocity in the thermal plume exceeds 1 m/s. The time scales of the convective plumes are about 1.4 hours and they appear as organized mesoscale and small scale turbulent motions. The physical mechanisms of the developing and maintaining of strong convection in the plateau boundary layer can be described as following. The strong convective activities are related with the intense total radiation in the plateau area, the strong baroclinity in the plateau boundary layer induced by the inhomogeneous heat status of the underlying surface caused by the complicated terrain, and the advection effects in the plateau boundary layer. With these effects, strong organized large eddies can occur in the boundary layer. The thermal plumes formed from the eddies develop upward and can be combined to form large thermal convective cells. Some convective cells can be combined with convective cloud clusters, in which the full convective mixture occurs when these eddies exceed the condensation level. The latent heat during the condensation process can help the cloud clusters to develop further, and the cloud clusters can become larger cloud groups, which can be seen in the satellite images.