广西两次系列龙卷过程多尺度对比分析

Multi-scale Contrasting Analysis of Two Tornadoes over Guangxi Province

利用常规观测资料、fnl资料和雷达等资料分析了2015年5月10日和2013年4月17日发生在桂林、柳州的龙卷天气形势及雷达产品特征,及其预警的可能性,表明:两次龙卷都发生于低压带中,0417过程的触发机制是边界层的小低涡,0510过程触发机制是低层切变。两次过程都有低层高温高湿的特征。由探空计算的CAPE值并不能体现几个小时后大气的对流不稳定,0510过程通过对CAPE值的订正可预见几小时后大气的对流不稳定性增强,0417过程通过判断龙卷发生区位于fnl分析场的CAPE值梯度大值区,也能判断几小时后对流不稳定可能加大。两次龙卷的雷达中气旋有共同特征就是入流与出流并不平衡,形成系列龙卷的中气旋比单个龙卷的中气旋发展更强,0417系列龙卷径向风切变最大达到36m/s(应该是对应龙卷的中气旋或微气旋的切变),不同于0510单个龙卷中气旋径向风切变最大只有15m/s。0417龙卷影响范围比0510过程大,持续时间更长。龙卷径向速度剖面显示,在低层3km以下有辐合,在6km有辐散,同时表现为倾斜。两次龙卷反射率剖面显示在龙卷的初生到最强阶段,反射率剖面具有悬垂特征,推测在龙卷发生的同时伴有冰雹。0417龙卷反射率剖面显示有强反射率核心迅速下降特征,有下击暴流相伴随。两过程都存在提前预警困难的问题,通过对温度对数压力图的订正和判断对流有效位能梯度大值区,将预测未来几小时对流有效位能的增幅,尽力提前预警龙卷,同时,通过对雷达产品的分析达到对龙卷临近预报的时间尽可能提前。

Based on conventional meteorological data, fnl data and Doppler weather radar data, the two torna- does occurred in Guilin and Liuzhou, Guangxi province on 17 April 2013 and on 10 May 2015 were analyzed. The results are summarized as follows. The two tornadoes both occurred in the low pressure zone. The triggering mecha- nism of the process on 17 April 2013 was the small vortex in boundary layer. The trigger mechanism of the process on 10 May 2015 was low-level shear. There were also high temperature and humidity characteristics at lower-level in these two processes. The CAPE value calculated by radio sounding data did not reflect convective instability in a few hours later. The convective instability can be predict to enhance in a few hours later through the correction of the CAPE value in the process on 10 May 2015. The tornado on 17 April 2013 was located in the CAPE gradient large value zone basing fnl data, so as to judge the convective instability may increase a few hours later. The inflow and outflow of the middle level cyclone of these two tornadoes were imbalanced. The middle level cyclone which led series tornados developed stronger. The maximum radial wind shear of the series tornados on 17 April 2013 was a- bout 36m/s, different that of the single tornado on 10 May 2015, only 15 m/s. The series tornados on 17 April 2013 influenced more large range than that of the single tornado on 10 May 2015, lasting longer duration. Radial velocity cross section shows convergence below 3 km and divergence at 6 km, with tilting characteristics. In the pri- mary stages to the strongest stage of tornadoes, the reflectivity cross section shows hook-shaped echo feature, ac- companying by the hail. The strong reflectivity core of the process on 17 April 2013 descended vertically quickly, accompanying by the downbursts. The two tornado processes were both difficult to bring forward the warning. Through revising the T-logp diagram and judging the CAPE gradient large value zone, the increasing of moist CAPE in the next few hours later can be predict, the tornado warning may be put forward early. Meanwhile, through the analysis of radar products the tornado forecastinK time may be as early as possible.