一次江淮地区MCV过程的数值模拟分析

Numerical simulation on a MCV case in Yangtze-Huaihe Basin

本文在前期统计工作的基础上,选取了一次典型的中尺度对流涡旋(MCV)个例,利用NCEP/NCAR再分析资料分析其背景场特征,并利用WRF数值模拟结果分析其成因及其触发"二次对流"的可能机制。结果表明:MCV发生前,江淮地区处于200 h Pa强辐散场中,高层抽吸作用明显,500 h Pa江淮西北部短波槽槽后不断有冷空气南下,加强该地区大气层结不稳定,850 h Pa湖北至安徽中部有切变线活动,这种高低层配置十分有利于MCV生成及对流发生;MCV生命史各阶段垂直输送项和涡管倾斜项呈反位相分布,而水平平流项和辐合辐散项的作用基本是相互抵消的,垂直输送项和辐合辐散项是MCV生成阶段中低层涡度的主要来源;MCV引发的"二次对流"出现在其生成阶段,且位于其南侧,MCV发展成熟后,对流迅速减弱;MCV的生成使南侧西南低空急流加强,伴随水平涡度的变化,"二次对流"的发生发展与水平涡度对应的垂直环流上升支有直接联系。

Based on the previous statistical analysis, a typical Mesoscale Convective Vortex （MCV） case was selected to investigate its background situation, possible causes and vertical structures by using NCEP/NCAR global objective analysis and WRF simulation data. Results indicate that before MCV forms, Yangtze-Huaihe Basin is in the position with strong divergence at 200 hPa, and dry cold air inva- sion increases potential instability of atmosphere structure. The shear line is active from Hubei to the center of Anhui Province, which is of great importance to the generation of MCV and new convection. The contributions from vertical advection and tilting counteract at different levels during each stage of MCV li- fetime while that from horizontal adveetion and convergence is opposite. The major vortieity in low layer during the development stage of MCV comes from vertical advection and convergence as a whole. The see- ondary convection is located on the southern part of MCV center and mainly occurs during the develop- ment stage of MCV. After the formation of MCV, the southwesterly airstream in its southern part enhances the jet in the lower level, which may contribute to the change of horizontal vorticity result from vertical wind shear and then lead to updrafts. The two updrafts caused by different low level jets together provide dynamic conditions for the secondary convection.