一次华北强对流风暴的中尺度特征分析

Analysis of the Mesoscale Characteristics About a Severe Thunderstorm in North China

利用非静力平衡模式(MM5v3.5)对2001年8月23日影响华北地区的一次强对流风暴成功模拟的基础上,对其中的中尺度重力波系(MGWs)特征进行分析,并与美国中东部地区MGWs进行比较。结果表明:这次MGWs波长约为100～120km,相速约为21～28m·s-1,伴随对流风暴的发展维持时间超过6h。太行山地形强迫和对流是产生MGWs的重要机制,虽然大气层结并不经常具有波导机制,但在地形强迫、对流和切变气流提供波动能量情况下,低层MGWs仍可维持较长时间。太行山触发的华北MGWs与美国西部高原引发的中东部地区MGWs具有相似的地形作用特征。在对流初始发生和对流单体合并阶段,对流和MGWs构成一个类似正反馈机制的耦合系统,waveCISK机制可能发挥着重要作用;成熟阶段的对流风暴,由于中高层潜热释放和湿下沉气流加强,波动传播加快,引起MGWs和对流风暴分离并减弱。MGWs在触发新雷暴,并将其组织成带状对流系统过程中可发挥一定作用。上述现象较好地验证了MGWs的概念模型以及对流与重力波相互作用的理论。

A severe thunderstorm that occurred on 23 August 2001 in North China is simulated with a mesoscale model. The downslope wind from the Taihang Mountains is the key mechanism to initiate the convection, on the other hand, the terrain can induce unbalanced low\|level flow, which plays an importance role in generating the mesoscale gravity waves (MGWs) by convergence and divergence in the lower troposphere. The MGWs wavelength is 100120 km, and the propagation speed is 2128 m·s -1 . The explosive convection can initiate MGWs in the higher troposphere and enhance the low\|level MGWs. Convective and shear instability can provide the wave energy. During the initial and merger stages, the convection and MGWs probably interact with each other constructively. Particularly, the line of deep convection seems to provide the waves energy through enhancing mass convergence/divergence, the waves tend to organize convective elements into a line structure, the wave\|CISK is a noticeable mechanism, and the wave/convection interaction verifies certain theories of gravity wave propagation and the vertical structure conceptual model. After the storm moves into the Shandong area and reaches a maturation period, the moist downdraft appears, the MGWs disunite from the thunderstorm and move faster, and the MGWs weaken gradually because of absence of wave duct and energy supply.