Helen台风(9505)异常路径的诊断分析与数值模拟

The Diagnoses and Numerical Simulation on the Unusual Track of Helen(9505)

南海台风西行北翘是小概率事件,预报具有一定的难度。作者应用中尺度模式MM5,对具有南海台风西行北翘路径特征的9505号强热带风暴(Helen)的异常路径进行了诊断分析和数值模拟。研究结果表明:副热带高压出现断裂,副高主体减弱东撤,导致Helen在断裂点处北翘,此异常路径是环境场与Helen本身非线性相互作用的结果。环境场的主要天气系统(如副热带高压、高空冷涡)与Helen之间的相互作用并未影响副热带高压薄弱地带的出现以及Helen在副热带高压断裂点处的北翘,但对Helen北翘后的路径有较为明显的影响。Helen的水平结构(包括风廓线的非对称结构、最大风速、最大风速半径等)变化对其异常路径影响有一定影响,其中风场的非对称结构变化对其异常路径的影响更为明显,当最大风速位于台风环流的东北象限,模拟得到的Helen路径无明显的北翘过程,为持续的西北行;当最大风速位于台风环流的西南象限,Helen路径为显著的西行北翘异常路径。

It is difficult to predict the track of typhoon moving westward and turning northward in the South China Sea. In the context of potential vorticity （PV）, the track of severe subtropical storm “Helen”（9505, briefly 9505TC） is investigated, which moved in the South China Sea with the track mentioned above. The diagnostic resuits of PV at 325 K isentropic surface reveal that the southerly in the east of 9505TC transports high PV into the subtropical high, leading to a rupture in the long, zonal subtropical high with the main of the subtropical high moving eastward, which results in the northward turning of 9505TC. After that, the trough with high PV over the Japan Sea at middle and high latitudes and the easterly wave system at low latitudes transfer positive PV advection into the subtropical high, on the one hand, to reduce the intensity of the subtropical high, on the other hand, to induce the southward moving of the subtropical high with strengthened southerly in the west. Finally there is a low meridional PV center in the subtropical high, and 9505TC finishes the northward turning steered by the southerly. Following the above, it can be considered that the rupture of the subtropical high and the breakpoint in the wind field are key factors to induce the abnormal track of 9505TC, and it is a result of nonlinear interaction between the environment filed and 9505TC. Using the nonhydrostatic version of meso-scale numerical model MMS, and introducing a bogus typhoon into the initial field, the numerical simulations are performed to investigate the abnormal track of 9505TC. In control experiments, the rupture in the subtropical high and the northward turning of 9505TC across the breakpoint are simulated successfully, and the landing point as well. Further sensitive experiments results display, though with a changed initial field by modifying the strength of the subtropical high and cold vortex, there still exists a rupture in the subtropical high during the integration, which induces the northward turning of 9505TC at the breakpoint. And the intensity change of the subtropical high and the deepening of cold vortex are not key factors responsible for the abnormal track of 9595TC, except for obvious effect on the track of 9505TC after its northward turning by virtue of interaction with 9505TC and other systems around it. In this paper, by modifying the maximum wind speed, maximum radius of wind speed, attenuation coefficient of the maximum wind speed and the asymmetry of wind field in 9505TC, the effect of the change of horizontal structure on the ambient field and the abnormal track of 9505TC is detected. It is found that the change of structure has some effect on the environmental flow and the northward turning of 9505TC cross the breakpoint, of which, the asymmetric structure in 9505TC has more remarkable effect on the abnormal track. When there exists an asymmetry of maximum wind speed in northeast quadrant, 9505TC will move northwestward without an obvious northward turning; conversely, 9505TC will move westward and turn northward in the South China Sea with an asymmetry of maximum wind speed in southwest quadrant.