不同发展阶段对流云合并过程的数值模拟

使用MM5 (Mesoscale Model 5, 简称MM5) 中尺度模式和雷达组网产品,对2008年7月22日发生在安徽等地的一次强对流天气过程中对流云合并现象进行观测和数值模拟分析.观测结果表明,30 dBZ以上回波水平尺度约10 km,回波中心相距近20 km的小单体通过合并形成了水平尺度几十公里的大单体.首先是外围较弱的回波相连接,其次是中低层的强回波中心发生合并,合并后有雷达回波中的强回波面积增加等现象出现.对模拟结果和观测资料进行的对比验证的结果表明,模拟结果和实况特征基本一致.基于雷达观测结果和第三层细网格模拟结果,对两类不同发展阶段的对流单体之间的合并过程分析结果表明,当两个单体都处于相近的发展阶段,合并后单体发展增强；当一个单体强度大于另一个单体时,合并后一个单体得到增强,另一个单体减弱消亡.合并时,两云间下部的低压辐合区会有新的云水中心产生,前一类合并过程中,新产生的云水中心代替了原有的两个云水中心,而后者在合并时,新产生的云水和其中一个原有的云水发生了合并,而另一个云水中心减弱消散了.模拟分析结果还表明,对流云合并过程可引起回波增强、云顶抬高、云水、冰相物质含量增加、地面降水增加现象.

Numerical Simulations of Convective Cloud Merging Processes at Different Development Stages

Mergers of cells in a severe convective weather on 22 July 2008 are simulated and analyzed by Mesoscale Model 5(referred to as MM5)and radar network data.Observation results show that,the horizontal scale of the echo above 30 dBZ,which represent the small cells,is about 10 km,and the small cells that the echo centers are 20 km apart merge into a larger cell at dozens of km of horizontal scale.Mergers begin from the peripheral radar echo,and then strong central radar echo merges at the low level,at last,the acreage of strong radar echo increases after the merger.The contrast between the observations and the simulation results shows that they are consistent.Analysis on the simulation results oftwo kinds of cell mergers at different development stages based on the third network model output shows that,while the cell pairs are with almost the same intensity,cells would develop after merger;while one of the cell pairs is in stronger development however the other one weaker,the stronger cell would keep on development and the weaker cell would die out.During the merger,a new cloud water center appears in the low convergence region between the cell pairs,and would replace the two cloud water centers of the former cells,or the new cloud water center would merger with one of the old cloud water centers while the other old cloud water center disappears.The analysis of the simulation results also shows that,the cell merger would lead to the cloud top lifting and the increase in the radar echo,content of cloud water and ice,surface rainfall.