广东两次台风龙卷的环境背景和雷达回波对比

利用常规气象观测、广州多普勒天气雷达及NCEP/NCAR再分析等资料对比广东省佛山市2015年10月4日EF3级和2006年8月4日EF2级台风外围强龙卷过程。结果表明:两次强龙卷都发生在登陆台风的东北象限，低层辐合、高层辐散及中低空强劲东南急流在珠江三角洲叠加是其产生的相似环境背景。环境参数均表现为较小的对流有效位能、低的对流抑制与抬升凝结高度、强的垂直风切变和大的风暴相对螺旋度。两个龙卷母体均为微型超级单体，前者雷达回波强度更强，钩状回波特征更明显；都存在强中气旋和龙卷涡旋特征（TVS），中气旋都在中低层形成后，向更低层发展最终导致龙卷。TVS比龙卷触地提前1个体扫出现，或与龙卷触地同时发生，中气旋和TVS的底高和顶高均很低。但两次龙卷触地前后，前者中气旋和TVS的底高和顶高出现突降现象，而后者中气旋和TVS的底高和顶高一直维持较低高度。龙卷触地前后，两者风暴单体的最强切变均出现剧增现象，但前者TVS的最强切变更强，比后者大1倍以上。

Contrastive Analysis of Two Intense Typhoon-tornado Cases with Synoptic and Doppler Weather Radar Data in Guangdong

Conventional observations, Doppler weather radar and NCEP/NCAR reanalysis data are used to analyze two strong tornado events originated in the outside-region of typhoons on 4 October 2015(EF3) and 4 August 2006(EF2) contrastively. Results show that two strong typhoon-tornado events both occur in the northeast quadrant of landfalling typhoons, with some similar environmental conditions including low-level convergence, upper-level divergence and superimposition of strong southeast jet at mid and low level over the Pearl River Delta. The difference of circumstances is that two typhoons are in different weakening phase. The former typhoon has just landed and is more organized and stronger than the latter. Environmental parameters are shown to be relatively moderate convective available potential energy, low convection inhibition, low condensation uplift height, strong deep (0-6 km) and low level (0-1 km) vertical wind shear and large storm relative helicity (SRH). Storm relative helicity is a good indicator for the occurrence of supercell or mesocyclone. The larger the SRH is, the more likely a supercell or mesocyclone may form. Combining the northeastern quadrant of typhoon with the high SRH area, the area where typhoon tornadoes may occur could be determined to a certain extent. The two tornado storms are mini-supercells, and the radar base reflectivity factors of two tornadoes are similar to features of classical supercell such as the low-level warm-humid inflow gaps and hook echoes. The former (tornado parent storm on 4 October 2015) has stronger echo and more apparent hook echo features. Strong mesocyclones and tornado vortex signature (TVS) can be observed on radar speed chart in both tornado events, and mesocyclones form at mid-low level firstly, then develop to the lower level, resulting in tornadoes finally. TVS is observed either synchronously with the tornado touchdown or a volume scanning ahead. Vertical vorticity of TVS in the center of low-level mesocyclones is strong, and the bottom and top heights are very low in the mesocyclone and TVS. The difference of bottom/top height of the mesocyclone and TVS between two cases is that, the former presents an abruptly-drop phenomenon whereas the latter (tornado parent storm on 4 August 2006) maintains at low level before and after the tornado touchdown. Before and after tornadoes touchdown, the strongest wind shear of storms both increase sharply, but the wind shear in TVS is larger, which is about twice of the latter.