0808号台风凤凰前部龙卷的环境场和雷达回波分析

Analysis on Environmental Conditions and Doppler Radar Data for Tornado Events in Front of Typhoon Fenghuang(0808)

用多普勒天气雷达、常规观测和地面加密观测资料对2008年7月29—30日发生在江苏的台风凤凰前部系列龙卷过程进行了详细分析。结果如下:该龙卷发生在台风前部风切变区里,低空较强的垂直风切变、极不稳定的大气层结和深厚的正涡度区,为龙卷的发生提供了热力和动力条件。龙卷回波的反射率因子特征表现为最大反射率因子梯度位于回波的后侧,一般都有弱回波区和后侧入流缺口相伴随。东台龙卷产生于飑线型弓形回波的顶点,具有相对较大的反气旋性切变。高邮龙卷中低层具有弱的气旋性涡旋,宝应龙卷回波的内部环流特征明显,有明显的中气旋结构。在业务工作中,强回波质心高度和垂直累积液态含水量的骤降、径向速度风场中气旋性涡旋的迅速发展是对龙卷的提前警戒很好的参考指标。

Tornadoes occurred in front of typhoon in Jiangsu Province on 29 to 30 July, 2008. A detailed analysis of the environmental conditions, structure and evolution of tornado is made based on the Doppler weather radar data, routine upper-air and surface observations, and intensive automatic weather station observations. The main results are as follows： The tornadoes occurred in the area with high value of low level vertical wind shear. The low lifting condensation level, unstable stratification and deeper positive vorticity all these factors supplied the thermodynamic and dynamical conditions for the occurrence of tornadoes. The tornado occurred at Dongtai is associated with a mesoscale squall-line, whereas the tornadoes occurred at Gaoyou and Baoying are associated with multi-cell convective storm. The rear part of the storm cell has a large reflectivity gradient, which is often accompanied by the weak echo structure and back-flank notch in the low level. Not all the tornadoes occurred in the storm-cell with meso-cyclone and tornado vortex signature. Some tornadoes happened in the storm-cell with weak cyclone vortex in the low level. When the altitude of strong echo center and the vertical cumulative liquid water falls suddenly and cyclone vortex develops rapidly, the tornado will occur very soon. In operational work, we shouldn’t alert the occurrence of tornadoes only depending on the meso-cyclone signature and tornado vortex signature. We should also combine the other products, for example, reflectivity, average radial velocity, storm structure, etc. It is possible to greatly enhance weather radar’s capability to detect the tornadoes.