强台风Chanchu（0601）的数值研究：转向前后内核结构和强度变化

A numerical study of Typhoon Chanchu (0601): The inner core structure evolution and intensity changes around its northward turn

中文关键词:  热带气旋， 内核结构， 强度变化， 涡旋， Rossby波

英文关键词:Tropical Cyclones, Inner core structure, Intensity changes, Vortex Rossby waves

中文摘要:

      使用FY卫星TBB资料和新一代非静力中尺度模式WRF分析南海强台风Chanchu（0601）“急翘”转向前后内核结构和强度变化过程。结果表明：转向后内核结构非对称特征明显。WRF数值模式较好地模拟出Chanchu强度和异常路径变化过程，再现了内核结构演变：转向前，垂直切变较弱，有利于快速加强，内核结构较为对称；转向后，垂直切变明显增大，强回波位于垂直切变下风方向的左侧，显示为内核非对称结构。使用傅立叶变换方法分解模拟结果中的雷达回波，发现眼壁和内螺旋雨带的2波非对称沿方位角移速与涡旋罗斯贝波（VRWs）的理论波速一致，Chanchu快速加强过程中断和强度维持的可能原因为：眼壁传播的VRWs受到外螺旋雨带的扰动以及涡旋倾斜加剧引起眼壁非对称性加强导致“急翘”时眼壁破裂，此后眼区和眼壁区水平混合过程加强，850 hPa眼区相当位温明显增加，抑制高层相对暖干空气和低层相对冷湿空气相互交换，使得随眼壁内侧下沉气流向下输送的暖干空气减少，低层增温作用减弱，快速加强过程中断；VRWs径向内传导致高值涡度由眼壁内侧向眼心传播，引起最大风速半径（RMW）内侧切向风速增大，RMW随时间向眼心延伸，眼壁进一步收缩，一定程度上抵消了垂直切变加大的负面影响，Chanchu维持强度。

英文摘要:

      In this study, the inner core evolution and intensity changes of the severe typhoon Chanchu (0601) was analysed using the FY satellite TBB images and the new generation mesoscale weather research and forecasting (WRF) model. This case experienced a sharp turn from a westward to a northward component motion with an end of its intensification. The TBB images showed that the inner core structures of Chanchu become asymmetric after the northward turn. The WRF model was used to simulate Chanchu at high resolution (3 km spacing) with 180 h integration performed. The intensity and track in simulation are close to the observed, and the model reproduces its inner core structure evolution well. The weak wind shear favored rapid intensification before its sharp northward turn. The wavenumber 1 asymmetry was dominant with a downshear left pattern of the inner core emerged as the wind shear was at its peak just after the northward turn. Its intensification then stopped under the negative impact of increasing shear. Fourier decomposition of the reflectivity from the model outputs revealed that the azimuthal propagation speeds of the wavenumber 2 asymmetry of the eyewall and inner spiral rainbands were consistent with those from the vortex Rossby waves (VRWs) theory. The VRWs features were used to explore the possible mechanism for the rapid intensification break and intensity maintenance. The analysis shows that the eyewall experienced a breakdown while the VRWs in the eyewall were perturbed by the outer spiral rainbands and vortex tilt occurred when northward turning. The equivalent potential temperature of the inversion level in the eye increased significantly at this time by growing horizontal mixing, which separated the warm, dry air above, from cooler, moist air below, leading to the reduction of the warm, dry air via transportation by the downdrafts along the inner edge of eyewall. As a result, the low level warming weakened and the rapid intensification stopped. And the inward transport of high vorticity, from the eyewall toward the eye owing to the VRWs radial inward propogation, caused an increase in the tangential winds within the radius of maximum wind (RMW), resulting in the RMW to extend inward and the contraction of eyewall. This feature compensated the negative impact of increased shear to some extent and remained the intensity.