高云与高层垂直速度关系的个例研究II·动力学分析

Case Study on the Relationships between High Cloud and High-Level Vertical Velocity Part II: Dynamics Analysis

观测表明,高云的夏季块状分布和冬季带状分布,与低层赤道辐合带的夏季与冬季的形状十分相似;并且卷云和卷层云可以独立于深对流单独存在。作者对这两个观测分析结果进行动力学分析,结论如下:1)由于印度洋北面是青藏高原与亚洲大陆,夏季不能在北面副热带地区形成反气旋,从而印度洋赤道北面为西南气流,导致了赤道辐合带在该地区断裂并且相应的深对流在亚洲季风区的块状分布。2)利用斜压超长波理论,将Rodwell等的亚洲季风单向模型(即非绝热加热导致季风形成)作了修改,扩展为双向闭合模型。印度洋跨赤道偏南风产生大范围水汽辐合,其与地形的共同作用,产生了降水云系的高层加热,由于Sverdrup涡度平衡关系,导致了低层的偏南风而形成了一个相互作用的闭合过程,从而表明了亚洲夏季风是准定常的。3)通过详细分析涡度方程,证明除了恰好在赤道上之外,赤道辐合带上的水平辐合均会产生涡,并且这些涡由点涡(涡度的奇异部分)与各种尺度的涡(涡度的正则部分)组成。正涡度对应于云区,负涡度对应于晴空区,与赤道辐合带(ITCZ)的观测结果一致。4)由于辐合和切变产生涡,得到赤道辐合带和深对流的带状准定常维持的动力机制,即:由于赤道辐合带的辐合,其南北风辐合与东西风切变将产生涡,其与水汽的共同作用产生了深对流的上升降水云系,而降水云系的潜热诱导上升,进一步加强了水平辐合,从而表明了赤道辐合带的带状准定常维持的中介是不同尺度的涡。5)卷云和卷层云可以独立于深对流的原因是热带卷云和卷层云与流场是可以互相激发的,深对流不是其唯一的源。

According to the observations in Part Ⅰ, block pattern in summer and zonal belt pattern in winter concerning with tropical high clouds is like the corresponding patterns of Intertropical convergence zone （hereafter, ITCZ）, and Cirrostratus and Cirrus can form locally without deep convective cloud. In this paper, the above two resuits of observations are explained by means of dynamic analysis. The conclusions are as follows： 1） As the Tibet Plateau （Oinghai-Xizang Plateau） and Asian land are located in the north of the Indian Ocean, the subtropical anticyclone can not form, so southwest winds flow in the north of tropical Indian Ocean. Thus, the breakdown of ITCZ appears and the deep convective cloud region in the tropics resembles block pattern in the Asian summer monsoon region. 2） On the basis of ultra-long wave theory in the Baroclinic atmosphere, Rodwell et al.＇s one way model of the summer Asian monsoon, in which the formation of the Asian summer monsoon results from the diabatic heating, has been revised, and the two way close model is put forward, in which the combined effects of the southwest winds across the tropical Indian Ocean and the trapezoid orographic distribution result in the heating of precipitating cloud system （especially deep convective） in high level, which produces upward motion in high level. As a result of Sverdrup vorticity balance, the corresponding low level southwest wind forms. The dynamic and thermodynamic mechanism of quasi-permanent summer Asian monsoon is explained by this modified model. 3） By analyzing the baratropic vorticity equation, the production of vorticity results from the horizonal convergence except the equator exactly, and the vortex consists of vortex of various scale from the point vortex （the singular solution） to varioui scale vortex （regular solution）. The positive vorticity corresponds to the high cloud regions and the negative vorticity corre- sponds to clear regions respectively, which is almost same as the observation in ITCZ. 4） The mechanism of quasi-permanent zonal belt of ITCZ and deep convective in tropics is posed. As a result of the convergence of ITCZ, vortex results from the convergence of south wind and north wind and shear of zonal wind. Then, the joint effects of vortex and water vapor result in the vertical upward deep convective cloud systems, and its heating produces vertical motion which enhances the horizontal convergence as a close processes. Thus, the medium between ITCZ and deep convective in the tropics is the various scale vortex. 5） Cirrostratus and Cirrus can form locally without the appearance of deep convective cloud because high cloud and wind field can be excited mutually.