副热带高压对登陆台风等熵面位涡演变影响的数值模拟研究

Numerical Simulation of the Effect of Subtropical Anticyclone on the Evolvement of Potential Vorticity of Landfall Typhoon on Isentropic Surface

选取1997年第11号台风"温妮"为研究个例,通过中尺度模式MM5模拟再现了该台风登陆后经历初期减弱、变性及变性后再次发展的演变过程。采用Davisetal.(1996)提出的片段位涡反演方法,提取具有副热带高压物理意义的位涡扰动,采用片段位涡反演的方法,改变模式积分初始时刻台风东部副热带高压强度,并引入Ertel等熵面位涡收支方程,深入分析不同强度的副热带高压环流系统在登陆台风结构演变的过程中等熵面位涡的守恒性,以及守恒性与非守恒性相对作用的大小。研究表明:台风北上深入内陆的过程中,高空槽大值位涡源源不断的输送使得对流层低层西北侧位涡增长,台风中心上空的辐散形势有利于台风强度的再次增强。由于摩擦和非绝热加热的存在,对流层位涡局地变化主要决定于位涡的水平平流(守恒项)、位涡的垂直平流、加热的垂直微分(非守恒项)的分布。台风经历变性及再增强的过程中,其影响范围内位涡守恒性经历了先减弱后增强的过程,非守恒项中位涡的垂直平流能较好地描述对流层中层位涡局地变化趋势,而加热的垂直微分则在对流层低层和高层表现良好。副高强度的加强使台风加速北上,加快了台风变性速度,高层位涡的向下输送明显提前且强度增强,位涡守恒性的破坏、重建也相应提前,位涡垂直平流的整层负值减小,加热垂直微分对对流层低层位涡增长的正贡献加强,且持续时间更长。

Typhoon Winnie （1997） is taken as an example, and the entire process including initial weakening, extratropical transition and redeveloping after landfall is reappeared using the MM5 model. Using the piecewise potential vorticity （PV） inversion advanced by Davis et al.（1996）, the authors pick out the PV break with the physical meaning of subtropical anticyclone and change its intensity to get different initial fields. The conservational character and the adiabatic and diabatic effect with different intensity of eastern subtropical anticyclone during the entire process are explored by using the Ertel potential vorticity budget equation on isentropic surface The results show that transportation of high PV from the upper to lower troposphere makes local PV increase in the lower troposphere, the divergence is favorable to redeveloping of typhoon. Because of the existence of friction and diabatic heating, the local change of PV is mainly decided by the horizontal advection of PV, vertical advection of PV, vertical differential of heating. The conservational character undergoes the weakening and redeveloping in the area of typhoon during the extratropical transition. The vertical advection of PV describes the local change of PV in the middle troposphere better and the vertical differential of heating is better for that in the upper and lower troposphere. The stronger subtropical anticyclone may quicken the northward speed and extratropical transition of typhoon, and the transportation of upper PV is earlier and the intensity is stronger. The destroying and rebuilding of PV conservation is also earlier. The whole-level negative vertical advection of PV is also weaker and the positive effect of vertical differential of heating on local increasing of PV in the lower troposphere is larger and last longer.