鞍型场对热带气旋移动路径影响的研究

鞍型场是一种特殊的流场,鞍点附近气压梯度很小,风场很弱,引导气流不明显。当热带气旋进入鞍型场时,会表现为停滞、打转、突然转向等异常路径。采用NCEP资料对2011年热带气旋"梅花"路径和大尺度形势场分析结果表明,在其整个生命史中出现三次移动路径转折,热带气旋进入鞍型场后,出现在鞍点附近停滞、移动缓慢、突然转向等异常路径。利用理想变形场和兰金涡旋相结合的鞍型场作为背景场,用梯度风平衡构造了理想热带气旋,分别考虑了科氏参数为常数和β平面近似两种情况,利用二维浅水模式对热带气旋处于鞍点和收缩轴附近不同位置的情况进行了数值对比试验。当科氏参数为常数时,处于鞍点位置的热带气旋在鞍点停滞;而当采用β平面近似时,处于鞍点位置的热带气旋向东北低压一侧移动,并和低压合并。当初始热带气旋处于鞍点南侧的收缩轴上以及偏东或偏西100 km时,会出现三种截然不同的移动路径。试验结果表明:鞍型场对热带气旋有"陷阱"作用。当热带气旋进入到鞍型场区并接近鞍点时,在β漂移和弱引导气流作用下,可能陷入鞍点附近区域,移动缓慢,出现异常路径,也可能沿着膨胀轴向偏东或偏西方向移动,使得热带气旋突然转向。热带气旋的移动对其和收缩轴的相对位置十分敏感,数十千米的偏差可能会导致截然相反的移动路径。

Study of Col Field Impact on the Moving Path of Tropical Cyclone

The col field is a special flow field characterized by weak gradient of pressure, winds and steering flow near the col point. When a tropical cyclone （TC） enters the col field, it may moves with abnormal tracks such as stagnation, swirling, abrupt turning, etc. By using NCEP reanalysis data, the track of 2011 TC ＂Muifa＂ and synoptic pattern are analyzed. It is shown that the track of ＂Muifa＂ shows three abnormal tracks during its life cycle. When the TC enters the col field, it stagnates, swirls and ab- ruptly turns near the col point. With the col background field combining the ideal deformation field and the Rankine vortices, considering the coriolis parameter is constant and β-plane approximation in both cases, an ideal TC with gradient wind balance constraint condition is introduced into a two-dimensional shallow water model. The TC tracks that the TC is located at the col point or the contraction axis of the col field are numerically simulated. When the coriolis parameter is constant, the TC which is located at the col point stagnates at the col point all the time, while it moves toward east by north and finally merges into the low when the β-plane approximation is used. The TCs which are located at the contraction axis, east and west to the contraction axis by 100 km show three different tracks. The simulation results show that the col field has ＂trap＂ effect on the TC. When the TC moves close to the col point, it may stagnate near the col point under the influences of β-drift and weak steering flow, or abruptly turn to east or west along the dilatation axis. The TC track is very sensitive to the distance between the contraction axis and the TC, dozens of kilometer deviation could lead to adverse track.