数值剖析0411号热带气旋位置不连续变化和强度突变

NUMERICAL DIAGNOSIS OF THE ABRUPT CHANGE OF TROPICAL CYCLONE 0411

为探究竟0411号热带气旋(TC0411)出现数小时内向北"走"了300多公里且急剧加强的异常现象,根据TC0411经向环流特征(辐合抬升ω<0的水汽多来自偏南风v>0)及其与第2类条件不稳定(CISK)机制的密切关系,运用反映南海经向环流的线性诊断模式(包含边界效应、各稳定度及17个内力因子),对TC0411进行定量诊断。结果表明:南海东北部深厚强盛的偏南风急流及其造成的弱惯性稳定度为TC0411发生发展(向北不连续位移)提供了有利水汽输送和抬升(驱动)条件。其中,对偏南风有突出正贡献的边界效应与5°N西南夏季风和西太平洋东南风的辐合有关;驱动地转南风的气压梯度力与中南半岛季风槽、西太平洋高压和高层东风扰动有关。驱动非地转南风的内力因子主要是与斜压不稳定有关的平均经向温度平流、与西南风急流切变场有关的涡动西风动量经向输送、与夏季风环流和高层东风扰动相互作用有关的平均西风动量纬向输送及与非对称动力热力场有关的涡动温度平流。TC0411向北不连续位移实质为22.7°N附近生成新热带低压(TD)和19°N附近TD消亡的结果。2004年7月26日20时—27日02时激发22.7°N附近上升运动(19°N附近下沉运动)的主要因子是平均经向温度平流和涡动西风动量水平输送(平均西风动量纬向输送和涡动温度平流)。27日02—14时使新TD加强为热带风暴(TS)并维持的主要因子是潜热加热和平均经向温度平流。根据以上分析,建议业务预报应着眼于深厚强盛西南风急流和温度平流的共同作用。

The killer tropical cyclone 0411 struck the eastern coast of Guangdong abruptly from 00：00 UTC to 06：00 UTC 27 July 2004 due to its significant intensification and sudden ‘northward jump＇ from around 19°N to 22.7°N. Characterized by the ascending of moist southerly （i. e. the ingredients of CISK mechanisms）, a linearized model of 106° - 126°E averaged meridional circulation is applied to a quantitative diagnosis for revealing the abrupt change. The linearized model includes boundary effect, effects of hydrodynamic stabilities and 17 internal dynamic and thermodynamic processes. This study reveals that a strong southerly jet with weak zonallyaveraged inertial stability in the Northeastern region of the South China Sea provided abundant moisture and lifting （driving） conditions favorable for the geneses and development （northward jump） of TC. The geostrophic component of the southerly was driven by the pressure gradient force associated with the monsoon trough, the western Pacific subtropical high, and the upper level easterly disturbance. Across the southern boundary （5°N） of the study area, the convergence of the southwesterly from the Southern Hemisphere and the southeasterly from the Western Pacific equatorial region also made partly contribution to the moist air lifting around 22.7°N. The ageostrophic component of the southerly was driven by the zonally-averaged meridional temperature advection due to the baroclinic instability, the meridional transport of eddy zonal momentum resulted from the wind shear of the southwesterly jet, the zonally-averaged zonal transport of zonal momentum associated with the interaction between the summer monsoon circulation and the upper level easterly disturbance, and the eddy modes of horizontal temperature advections related with the inhomogeneous wind and temperature distributions, and moreover, the first （last） two factors also initiated （suppressed） the new TD growth around 22.7°N （ the old TD around 19°N） through driving the rising motion （sinking motion） there. The intensification of new tropical cyclone around 22.7°N was tightly associated with the latent heating and zonally-averaged meridional temperature advection. Based on the above diagnostic results, this study suggests that forecasters should pay more attentions to the interaction between the deep/strong southwesterly jet and temperature advections in order to predict the similar cases of TC sudden change effectively in the future.