山脉地形对热带风暴Fitow结构和运动影响的数值试验

热带风暴自东向西穿越琼州海峡时常常与海南西部的强天气相对应,尤其当风暴中心在海峡中部或海峡西端出口处有向西南方向的偏折时。Fitow(0114)是这类热带风暴的一个典型。通过对Fitow热带风暴的研究和分析,揭示了一个事实:Fitow在沿海峡西行过程中,其外围中尺度结构发生明显变化———风暴中心西南象限有一个中尺度对流(MCS)小涡生成和发展。受到这个诱生MCS小涡的“吸引”,Fitow在穿行海南岛北部和琼州海峡时,路径向西南方向偏折。数值模拟结果表明,海南岛中部的五指山地形对Fitow自东向西穿行海峡时的这种结构变化有显著影响:(1)当热带风暴Fitow(0114)自东向西穿过岛屿北部和琼州海峡时,其外围西北气流与山脉的辐合地带往往会诱生出中尺度强对流涡旋系统(MCS)。这种系统经过尺度分离和滤波处理后便会在山脉西北麓显现出来。(2)MCS小涡只生成在地形高度之下的大气层;地形高度之上并不显现这一小涡。用0高度作敏感试验的结果,在相同位置并不生成这种MCS小涡。(3)诱生小涡(MCS)的存在,对Fitow会产生“吸引”作用,使其向西南方向MCS所在处偏折。且MCS越深厚,维持时间越长,对Fitow中心的“吸引”程度越大,其中心向西南方向的偏折和移动越明显。0.0 km高度无MCS小涡时,Fitow中心并无这种偏折,而是向西北方向移动,在雷州半岛登陆。

THE NUMERICAL SIMULATION ON THE IMPACT OF TOPOGRAPHY ON THE STRUCTURE CHANGE AND MOTION OF TROPICAL STORM FITOW (0114)

The Hainan Island is one of the provinces where tropical cyclones made landfall frequently.There are 2.7 landfall tropical cyclones in average annually only less than Guandong province.A severe tropical strom,Fitow(0114) crossing Qiongzhou Strait would has a great structure change due to the topographic forcing.A mesoscale convective vortex system(MCS) could be appeared in the southwest periphery of a storm Fitow(0114) where is the convergence area between the mountain range and the NW flow of Fitow.This small vortex(MCS) would affect the structure change and motion of Fitow.Intensive conventional observations,satellite images and radar reflectivity data have identified the genesis and existence of the MCS in the northwest side of the mountain range.The numerical simulation was performed with PSU/NCAR MM5V3 model to study the genesis of the small vortex of MCS.The results of the simulation and filtering analysis show that a mesoscale convective vortex system(MCS) appeared clearly in a topographic convergent area caused by the peripheral flow field of Fitow and the mountain range which was favorable to form the MCS when Fitow was passing through the Qiongzhou Strait.The results also show that the MCS only occurred in the layer below the mountain altitude and disappeared in the level higher than the mountain.Higher altitude of the mountain is more favorable to the formation of MCS than the lower one.Many studies showed that tropical cyclone motion can be influenced by some mesoscale vorticies or topographic induced low in its periphery when it made landfall.One of the results from the simulation indicates that the MCS southwest to the Fitow's center caused the storm to turn direction towards southwest.It seems to be some attraction existed between the MCS and the Fitow.The another results showed that the Fitow would move towards northwest if without the MCS or the altitude of the mountain range in Hainan island is zero(no influence of the topography).