Analysis for the rapid intensification of typhoon Mekkhala in southern Taiwan Strait
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摘要: 利用欧洲中期天气预报中心ERA-interim再分析资料、中国气象局台风最佳路径资料、NOAA逐日最优海表温度（OISST）、Himawari-8卫星观测资料以及中国地面自动气象站观测资料等，分析了台风“米克拉”近海强度预报的难点，并研究了导致“米克拉”在台湾海峡南部快速增强的环境因子，探讨了“米克拉”在较强环境风垂直切变下快速增强过程中对流非对称性分布特征。结果表明：（1）台风“米克拉”在较强200—850 hPa环境风垂直切变下在台湾海峡南部海域发生了快速增强，并以峰值强度在福建登陆，非常少见，造成预警时间短，强度预报难度大；（2）有利的海洋热状况和大气环流环境条件，如中国南海北部海温异常偏高，南亚高压南侧东风急流与“米克拉”相互作用引起的强烈高层出流以及强劲稳定西南季风气流带来的充沛水汽输送，均对台风“米克拉”在台湾海峡南部海域快速增强起重要作用；（3）台风“米克拉”快速增强过程中，传统业务主要关注的200和850 hPa之间的环境风垂直切变较强，但从环境风的垂直结构分析发现切变主要集中在对流层中、高层，而中、低层切变较小，且中、高层环境风垂直切变对台风增强的抑制作用相对中、低层切变要弱；（4）深对流在台风“米克拉”快速增强过程中表现出明显的非对称分布特征，主要集中在环境风垂直切变的顺切变一侧以及切变的左侧，并且伴随深对流由顺切变一侧向逆切变一侧气旋式爆发传播，期间台风“米克拉”高、低层涡旋中心的垂直倾斜度明显降低，有利于“米克拉”快速增强。关于不同层次环境风垂直切变对台风强度变化的影响机制以及较强环境风垂直切变下台风增强过程中对流非对称的爆发特征，今后将结合更多个例进行深入研究。Abstract: Using the interim reanalysis data of European Centre for Medium-Range Weather Forecasts (ERA-Interim), the tropical cyclone best track data from Shanghai Typhoon Institute of China Meteorological Administration, the NOAA daily optimum sea surface temperature (OISST), Himawari-8 satellite data, and observations collected at automatic weather stations (AWS) in China, the difficulties of the intensity forecasting of typhoon " Mekkhala " in the southern Taiwan Strait are analyzed. The environmental factors and the asymmetric distribution of the convective burst during the rapid intensification of " Mekkhala " in the southern Taiwan Strait are further studied. Results are as follows: (1) The rapid intensification of typhoon " Mekkhala " in the southern Taiwan Strait under the strong 200—850 hPa vertical wind shear and its landing in Fujian province at peak intensity are very rare, which resulted in a short warning time and great difficulties in intensity prediction. (2) The favorable ocean heat condition and large-scale environmental conditions such as the abnormal warm sea surface temperature in the northern South China Sea, the strong upper-level outflow caused by the easterly jet to the south of the South Asian High, and the abundant and stable southwesterly monsoon water vapor transport all play an important role in the rapid intensification of typhoon Mekkhala in the southern Taiwan Strait. (3) The traditional environmental vertical wind shear (200—850 hPa) is very strong, yet the vertical distribution of the environmental wind shows that the environmental vertical wind shear is mainly concentrated in the middle and upper levels, while the shear in the middle and lower levels is relatively weak. However, the vertical wind shear in upper and middle levels has relatively little inhibition on the typhoon intensification. (4) During the rapid intensification of typhoon Mekkhala, the distribution of deep convection presented obvious asymmetric distribution characteristics. The convection was mainly concentrated in the down shear side and the left side of the environmental vertical wind shear (200—850 hPa), accompanied by cyclonic propagation of convective burst from the down shear side to the up shear side, and the tilt of typhoon significantly reduced. Further studies on the impact mechanism of environmental vertical wind shear at different levels on typhoon intensity change and characteristics of asymmetric convective bust during typhoon intensification under strong environmental vertical wind shear will be conducted in the future based on more cases.
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图 1 台风“米克拉”路径 （a） 和强度变化 （b，中心附近最大风速：黑色曲线，单位：m/s；中心最低气压：红色曲线，单位：hPa） （图中时间为世界时 （月日时））
Figure 1. The best track （a） and intensity change （b） of typhoon Mekkhala （the maximum wind speed near the typhoon center：back line，unit：m/s；the minimum central pressure：red line，unit：hPa） （the time in the figures is Coordinated Universal Time （UTC））
图 2 2020年8月11日08时 （北京时） 自动气象站观测的平均风 （a，单位：m/s ，6级及其以上） 和1 h极大风 （b，单位：m/s ，6级及其以上） 的分布，（c） 登陆点附近站点59330 （位置为图2a十字） 观测1 h阵风 （蓝色曲线） 和气压 （红色曲线） 的变化，（d） 台风“米克拉”过程降水量分布 （单位：mm）
Figure 2. Distribution of average wind （a，unit：m/s ，above 6 grade） and 1-h gust wind （b，unit：m/s，above 6 grade） at 08 （BT） 11 August 2020；（c） changes in the 1 h gust wind （blue curve） and pressure （red curve） of the Station 59330，which is near the landing point （location is denoted by cross in Figure 2a）；（d） distribution of precipitation during the impact process of typhoon "Mekkhala " （unit：mm）
图 3 1949—2021年由中国南海北上进入台湾海峡的台风路径 （4个增强台风路径 （彩色） 和其他36个台风路径 （灰色））
Figure 3. Tracks of typhoons that moved northward from the South China Sea into Taiwan Strait during 1949—2021，including tracks （colored） of four intensified typhoon and the other 36 tracks （gray）
图 4 台风“米克拉”强度变化过程中环境风垂直切变 （a，200—850 hPa：黑色；200—500 hPa：绿色； 500—850 hPa：红色） 和环境风的垂直分布 （b，风速：色阶） 的分布
Figure 4. Changes in the environmental vertical wind shear （a，unit：m/s，back：200—850 hPa；green：200—500 hPa；500—850 hPa） and environmental wind at vertical levels （b，shaded） during the intensity change period of typhoon Mekkhala
图 5 2020年8月9日海表温度 （a） 和海表温度距平 （b） 的分布（单位：℃）
Figure 5. Distribution of sea surface temperature （a） and sea surface temperature anomalies （b） at 9 August 2020（unit：℃）
图 6 200 hPa流场和等风速线 （ a、b，≥12 m/s，色阶） 和 850 hPa水汽通量 （c 、d，单位：g·hPa /（m2·s）） （a、c. 2020年8月10日00:00 UTC），b、 d. 2020年8月10日21:00 UTC））
Figure 6. 200 hPa streamline and wind speed （a，b. shaded areas are for wind speed ≥12 m/s） and 850 hPa moisture flux （ c，d. unit：g·hPa /（m2·s）） （a，c. 00:00 UTC 10 August 2020；b， d .21:00 UTC 10 August 2020）
图 7 台风中心附近 （ ＜400 km） 平均散度 （a，单位：10−5 s−1） 和水汽通量散度 （b，单位：10−5 g·hPa /（m2·s）） 垂直分布
Figure 7. Vertical distribution of average divergence （a，unit：10−5s−1） and water vapor flux divergence （b，unit：10−5 g·hPa /（m2·s） ） near the typhoon center （＜400 km）
图 8 Himawari-8 可见光通道反照率 （a、b、 c） 和红外通道亮温 （d、 e、 f） 的分布 （a. 2020年8月10日03:00 UTC，b. 2020年8月10日06:00 UTC， c. 2020年8月10日08:00 UTC，d. 2020年8月10日12:00 UTC，e. 2020年8月10日18:00 UTC；f. 2020年8月10日21:00 UTC；黑色箭头线表示200—850 hPa环境风垂直切变的方向，不带箭头线与切变线垂直，将台风中心附近区域分为4个象限 （顺切变右侧 （DR）、顺切变左侧 （DL）、逆切变右侧 （UR）、逆切变左侧 （UL））
Figure 8. Distributions of albedo （a，b，c） of visible channel and brightness temperature （d，e，f） of infrared channel of Himawari-8 （a. 03:00 UTC 10 August 2020，b. 06:00 UTC 10 August 2020，c. 08:00 UTC 10 August 2020，d. 12:00 UTC 10 August 2020，e. 18:00 UTC 10 August 2020，f. 21:00 UTC 10 August 2020；the black arrow line represents the direction of vertical wind shear between 200 hPa and 850 hPa，and the line without arrow is perpendicular to the shear line；the region near typhoon center is divided into four quadrants （down shear right （DR），down shear left （DL），up shear right （UR），up shear left （UL））
图 9 975 hPa风场 （矢量） 和400 hPa位势高度 （等值线） 的分布 （红色和黑色圆点分别为400 hPa和975 hPa涡旋中心，黑色线为高低层涡旋中心的连线；a. 2020年8月10日06:00 UTC，b. 2020年8月10日12:00 UTC，c. 2020年8月10日18:00 UTC）
Figure 9. Distributions of 975 hPa wind field （vector） and 400 hPa geopotential height （contour） （the red and black dots denote the vortex centers at 400 hPa and 975 hPa，respectively； and the black line is the connecting line between the vortex centers at the upper and lower levels； a. 06:00 UTC 10 August 2020，b. 12:00 UTC 10 August 2020，c. 18:00 UTC 10 August 2020）
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