南海中层气旋的结构和演变特征分析以及数值实验

针对1980～1996年间发生在南海地区的两类中层气旋（MTC）典型个例分别进行合成分析，研究了它们的结构特点。研究发现，西风扰动类MTC在强度上要强于东风扰动类MTC；而东风扰动类在垂直三维结构上随高度向西南方向倾斜的程度远远大于西风扰动类，但它们都是一种冷心结构并且随高度向西南方向倾斜的系统。两类中层气旋的涡度、风速都在中层有最大值，对流层中层为辐合区，300hPa以上为补偿辐散区，垂直上升运动在400hPa达到最大。分析各自的演变过程，发现在MTC的发展过程中，西风扰动类MTC基本处于准静止状态略有西移，而东风扰动类MTC由于其发展过程的独特性，往往向西北方向移动并且移速较大。东风扰动类MTC所在的位置较西风扰动类MTC偏高，这是由于夏季南海地区处于西南季风控制下，东风气流所在位置偏高所致。我们还发现西风扰动类MTC西边的西风气流与MTC发展过程相对应有一个风速加大、维持和减弱的过程；东风扰动类MTC东面的东风气流也有类似过程。同时，与两类MTC相对应的代表副高强度的东风气流随MTC发展的演变表明，当有西风扰动类MTC时，副高有减弱的征兆，而有东风扰动类MTC时，副高并未减弱，反而东风有所加强。对1998年个例进行数值实验表明：MTC系统的对流加热在500hPa达到最大，且对流加热对MTC的发展具有很重要的作用，对流加热越弱，系统发展越弱。

On the Structure and Evolvement feature of Mid-Tropospheric Cyclones over South China Sea and Its Numerical Simulations

Through composite analyses of typical cases of MTC (Mid-Tropospheric Cyclones) occurred over South China Sea from 1980-1996, the structure features of MTC are studied. The results show that MTC of the westerly disturbance type are stronger than that of the easterly type in intensity. Both have the cold core and the maximum value in the mesosphere. But, the easterly type inclines much more south-westwards in vertical than that of the westerly one. Two types have the convergence in the mid-troposphere and the divergence above the 300 hPa pressure level. The up-rising velocity reaches maximum at 400 hPa. Analyzing their individual evolvement processes, the westerly type is mainly in the quasi-stationary situation and moves westwards slightly. But, the easterly type has its particular development process and often moves north-westwards quickly. The easterly type is in the higher location than that of the westerly type. This is because in summer South China Sea is controlled by south-westerly monsoon and the easterly flow is in the higher location. We also find both types have the increasing, maintaining, decreasing three sub-processes. Simultaneously, the evolvement process of the easterly flow which stands for the intensity of the subtropical high shows that, when MTC are in westerly type state, the subtropical high inclines to decrease, and when MTC is in the easterly type state, the subtropical high does not abate. According to the numerical experiment of the cases in 1998, it is shown that convection heating reaches the maximum value at 500 hPa and plays an important role in the developing process of MTC. The weaker convection heating, the weaker MTC system.