台风“艾云尼”（2018）外围两次近距离龙卷的环境条件和雷达特征

THE ENVIRONMENTAL CONDITIONS AND RADAR CHARACTERSTICS OF TWO TORNADOES IN THE PERIPHERY OF TYPHOON EWINIAR IN 2018

关键词:  天气学  台风龙卷  X波段双偏振雷达  雷达特征  钩状回波  中气旋

Key words:  synoptic analysis  typhoon tornado  X-band dual-polarization radar  radar characteristics  hook echo  mesocyclone

摘要:

2018年6月8日在距台风“艾云尼”中心80 km、160 km的广州市南沙区横沥镇、佛山市南海区大沥镇两地罕见地先后出现了龙卷天气。利用X波段双偏振雷达组网、广州S波段双偏振雷达、风廓线雷达和区域加密自动站等观测资料对两次近距离台风龙卷过程的环境条件和雷达特征进行了分析。环境条件分析表明，两次龙卷发生地位于低层西南急流和东南急流辐合区，所处环境为弱的对流有效位能（CAPE）、低的抬升凝结高度和强的低层垂直风切变环境中，0～1 km垂直风切变值超过15×10-3 s-1。中小尺度雷达特征分析表明：（1） 两地龙卷由台风外围微型超级单体引起，超级单体在发展强盛阶段有钩状回波、入流缺口、中层回波悬垂等典型特征，最强反射率因子55～60 dBz，强度≥50 dBz强回波发展高度在4 km以下，微型超级单体有水平尺度2～3 km的中气旋，由于速度模糊影响，仅在南海龙卷发生前9 min广州S波段雷达能自动识别中气旋。（2） 与南沙龙卷相联系的中气旋核心高度低，强度进一步加强紧缩导致龙卷发生；而与南海龙卷相联系的中气旋从中层发展，中气旋加强紧缩下降到更低导致龙卷发生。（3）两地弱龙卷发生时广州和南海双偏振雷达没能捕捉到龙卷碎片（TDS）特征，南海X波段雷达能提前30 min监测到入流急流，提前27 min探测出钩状回波等特征，并通过分析ZDR弧和KDP弧可判断低层强盛的上升气流和强的垂直风切变利于风暴的发展。（4） 佛山四部X波段组网雷达反演的1 km水平风场可分析出小尺度涡旋结构，对应钩状回波尾端有强的风向切变，这对龙卷发生地点的判断和风暴的流场结构有较好指示意义。

Abstract:

On June 8, 2018, tornados occurred in Nansha (Guangzhou) and Nanhai (Foshan), only 80km and 160km away from the center of typhoon “Ewiniar”, respectively. The environmental conditions and radar characteristics of these close-range typhoon tornadoes are analyzed by using the observation data of an X-band dual-polarization radar network, a Guangzhou S-band double polarization radar, a wind profile radar and regional intensive automatic stations. Our analysis shows that the two tornadoes occurred in the convergence area of a low-level southwest jet and a southeast jet, and the environment was a weak convective effective potential (CAPE), a low uplift condensation height and a strong low-level vertical wind shear. The vertical wind shear value of 0 ~ 1 km exceeded 15×10-3 s-1. The analysis of radar characteristics at medium and small scales shows that: (1) The two tornadoes were caused by a supercell in the periphery of the typhoon, and the supercell had some typical characteristics such as hook echo and mid-level echo in the strong development stage. The strongest reflectivity factor was between 55~60 dBz, ≥50 dBz strong echo development height was below 4 km, and micro supercell had horizontal scale 2~3 km mesocyclone. Due to the speed blur effect, the Guangzhou radar automatically identified the mesocyclone only 9 minutes prior to the occurrence of the Nanhai tornado. (2) The core height of the mesocyclone associated with the Nansha tornado was low, so the strength of further tightening led to the development of the tornado, and the mesocyclone associated with the Nanhai tornado developed from the middle of the cyclone and then intensified contraction and gradually reduced to a lower level, leading to the occurrence of the tornado. (3) The two polarization radars in Guangzhou and Nanhai failed to capture the characteristics of the TDS, but the radar in Nanhai could detect the hook echo, inflow stream and inflow notch characteristics of tornado storm in advance, and through analyzing the ZDR arc and the KDP arc, it can be concluded that the low-level prevailing ascending air and strong vertical wind shear are favorable conditions for the further development of the storm. (4) The 1 km horizontal wind field retrieved by the four X-band network radars in Foshan captured the small scale vortex structure, and there was a strong wind direction shear corresponding to the end of hook echo, which are good references for judging the location of tornado and the structure of storm flow field.