季风涡旋对热带气旋生成影响的理想试验研究

An idealized simulation study of the impact of monsoon gyre on tropical cyclogenesis

利用新一代非静力平衡中尺度数值模式WRF_ARW(3.3.1版本)模拟季风涡旋中热带气旋生成的过程,从动力和热力作用两方面分析大尺度季风涡旋对热带气旋生成的影响。结果表明:从动力学角度来看,能提供较大环境场涡度的季风涡旋不利于扰动涡旋快速发展成热带气旋。初始阶段,由于季风涡旋尺度大,垂直涡度径向梯度弱。而垂直涡度径向梯度的强弱可以通过"涡度隔离"效应影响对流单体向涡旋中心的聚集合并过程。随着扰动的组织化,径向入流对涡度的平流作用越来越重要。对流单体相对最大风速半径的位置对热带气旋生成作用明显,当其集中在最大风速半径附近时涡旋容易快速发展。此外,环境场相对涡度与热带气旋的尺度存在显著正相关。初始尺度大的涡旋最终具有较大的外围尺度,其涡度的分布范围也更广。从热力学角度来说,较大的环境场相对湿度有利于热带气旋的生成。虽然较大的环境场湿度能够诱发较强的外围对流,但同时也会使最大风速半径以内存在丰富的对流,后者能够提供充分的内区非绝热加热,降低中心气压,促进涡旋发展。

The impact of monsoon gyre on tropical cyclogenesis is studied based on a series of idealized numerical experiments u- sing the non-hydrostatic Advanced Weather Research and Forecasting model （WRF version 3.3.1）. Numerical results show that the large environmental vorticity of monsoon gyre is not conducive to the rapid development of tropical cyclones. Although the monsoon gyre has large initial vorticity, the radial gradient of its vertical vorticity is relatively small due to its large initial size. The radial vorticity gradient can modulate the merging of convective cells through the ＂vorticity segregation＂ effect in the initial phase. As the disturbances organize, the inflow advection of vorticity becomes increasingly important. The position of convective cells relative to the radius of the maximum wind can influence the tropical cyclone genesis. Meanwhile, there is a significant positive correlation between the environmental vorticity and tropical cyclone size. The vortex, which has a large ini- tial size, can develop into a large tropical cyclone at the mature stage. From the thermo-dynamical point of view, a relatively large environmental relative humidity is favorable for tropical cyclogenesis since the large environmental relative humidity can induce sufficient convections inside and outside the radius of the maximum wind. The former can provide abundant diabatic heating, which further lowers pressure in the tropical cyclone center and thus is conducive to the rapid development of the tropi- cal cyclone.