一次罕见飑前强降雹超级单体风暴特征分析

Case analysis of a large hail-producing severe supercell ahead of a squall line

2009年6月5日,受一个飑线前超级单体雷暴的影响,上海部分地区出现了直径25—30mm的冰雹,随后,飑线尾随该超级单体扫过上海,造成大风、雷电和强降水天气。基于常规天气观测、多普勒天气雷达、风廓线仪和自动气象站等资料分析发现,该超级单体发生在东北冷涡西南侧的高空强冷平流与低空暖平流形成的强不稳定层结背景下,超级单体风暴发生、发展在飑线前的暖区中,经过由"热岛效应"和海陆风锋共同形成的低空辐合线时明显加强发展;该风暴呈现出"指"状、"楔"状、弱回波区(WER)等超级单体雷达反射率特征,"指"状回波处出现了中气旋的径向速度特征,具有标志大冰雹的三体散射长钉(TBSS)特征回波,通过三体散射长钉多普勒速度发现了大冰雹的下降区和增长区。分析还表明:东北冷涡西南侧横槽南摆导致中空降温,0℃层和-20℃层高度明显下降,为冰雹的空中增长提供较好的温度环境条件,较低的0℃层也保证冰雹在空中下落中融化较少。双风廓线仪对比观测表明,超级单体发展的低空风场环境中具有较大的垂直风切变和风暴相对螺旋度,中尺度对流系统与环境场的相互作用形成了有利于风暴发展和维持的正反馈机制。飑线前超级单体雷暴与飑线主体关系密切,起到类似"箭"与"弓"的引导作用,飑线主体的一部分进入超级单体所遗留下的"冷"区后明显减弱,东侧入海后也逐渐减弱,其余部分仍在发展加强;最终,强风暴逐渐减弱,超级单体特征也开始消失,飑线与之合并形成新的"人"字型中尺度对流系统,新的"弓"形回波带与原回波带相比移动方向发生右偏,因此,飑前超级单体在飑线主体移动和演变的临近预报中有重要指示意义。

A large hail-producing supercell developed ahead of a severe squall line around Shanghai, China on 5 June 2009. The supercell and its interaction and relationships with the squall line are analyzed using the conventional weather observations, Doppler radar data, wind profiler data, and automatic weather station data. The results show that the supercell initiated and developed in the warm sector ahead of the squall line, in a strong unstable environment which was caused by strong cold advec- tion at high levels over warm advection at low levels. The storm intensified while passing through a surface convergence zone induced jointly by an urban heat-island effect and a sea-breeze front. Some characteristics of a classic supercell were found, such as low level hooks and pendant echoes, the weak echo region, and a meso-scale cyclone, etc. A Three Body Scatter Spike （TB- SS） representing large hail in the sky was detected extending downrange behind a reflectivity （above 60 dBz） core. Descending and ascending large hail areas are detected based on the velocity of TBSS. An upper trough of a cold vortex in northeastern Chi- na brought strong cold advection and pressed the freezing level to lower. This provided a suitable ambient temperature structure for the hail embryo enhancement. Dual-wind profiler observation comparisons show ~hat the ambient wind field of the supercell could produce more low level vertical wind shear and storm relative helicity. On the other hand, the mesoscale convective sys- tem （MCS） might change the ambient wind structure. As a result of the interaction of the MCS and the environment, a positive feedback effect made the MCS into a long-lived system. The supercell＇ s motion implied the motion of the squall line with a leadership mechanism of an ＂arrow＂ relative to a ＂bow＂ suggested. After arriving at the supercell＇s cold air pool, some parts of the squall line decayed gradually. Finally, the supercell weakened and its supercell characteristics disappeared as it moved over the East China Sea. The squall line combined eventually with the remains of the superceII storm to become a line echo wave pattern （LEWP）, and the bow echo of the LEWP changed its motion direction from the northwesterly to the northerly. A super-cell ahead of a squall line might provide some useful operational nowcasting guidance for squall line motion and evolution.