浙江省梅汛期两次暴雨过程热力动力机制诊断

Diagnosing Thermal and Dynamic Mechanisms of Two Meiyu Rainstorm Processes in Zhejiang Province

利用常规气象观测资料、衢州S波段双偏振雷达产品资料、中国自动站与CMORPH降水产品融合的逐时降水量资料、NCEP GDAS/FNL 0.25°×0.25°再分析资料,对浙江省2020年梅汛期两次暴雨(6月30日和7月7日)进行热力动力机制诊断,结果表明:(1)“0630”暴雨由东北冷涡后部冷空气与西南暖湿气流交汇形成的东北—西南向冷切缓慢南压配合低空急流东移引发,“0707”暴雨由东亚大槽后部弱冷空气与副高北侧暖湿气流交汇形成的稳定的东西向切变配合低空急流东移而造成。(2)锋生函数诊断分析表明“0630”过程锋生作用更强,冷暖气流交汇更显著,锋区斜压性更强;水汽通量辐合区和大气可降水量大值区偏北是“0707”暴雨落区偏北的重要原因。(3)“0630”过程对流层低层存在条件对称不稳定,“0707”过程为对流不稳定层结。(4)两次过程的集合动力因子信号与降水强度相关性具有以下特征:共性为湿热力平流参数和广义Q矢量散度与降水相关性最高,其大值区与未来6 h强降水区对应较好;特性为广义对流涡度矢量垂直分量在“0630”过程中与降水相关性较好,说明对流系统更为深厚,中尺度系统活动更为活跃;水汽散度垂直通量在“0707”过程中与降水相关性更佳,是由于该过程的降水主要和天气尺度系统作用下引起的强辐合上升运动相关。

Using the conventional meteorological observations,Quzhou S-band dual-polarization radar products,hourly precipitation data from China automatic stations merged with CMORPH precipitation products and NCEP GDAS/FNL 0.25°×0.25°reanalysis data as well as synoptic method and dynamic diagnostic method,this paper diagnoses two Meiyu rainstorm processes that occurred on 30 June 2020 and 7 July 2020 in Zhejiang Province,respectively.The results show that:(1)The interaction between the cold air behind northeast cold vortex and the southwest warm moist airflow generates a cold shear stretching from northeast to southwest.The cold shear gradually moves southward together with the low-level jet moving eastward and finally incurs the 30 June rainstorm.By contrast,the interaction between the weak cold air behind East Asia deep trough and the warm moist airflow from the north side of the subtropical high generates a stable transmeridional cold shear,which moves eastward accompanied by the low-level jet and eventually causes the 7 July rainstorm.(2)Diagnostic analysis of frontogenesis function shows that the frontogenesis is much stronger in the 30 June process and that the interaction between cold and warm airflows as well as the baroclinicity of frontal zone is much stronger.However,more northward location of the rain belt in the 7 July process attributes to the more northward locations of both the moisture flux convergence zone and the large-value zone of precipitable water vapor.(3)In the 30 June process,there exists a conditional symmetric unstable stratification in lower troposphere,while convective instability stratification exists in lower troposphere in the 7 July process.(4)Correlations between ensemble dynamic parameters and precipitation intensity are as follows:The common point is that both moist thermodynamic advection parameter and generalized Q vector divergence show the highest relevance with precipitation,which means that the large-value zones of the above two parameters have good correspondences with the heavy precipitation regions in the next 6 hours.Their differences lie in the fact that the high correlation between vertical component of generalized convective vorticity vector and the 30 June rainstorm indicates deeper convective systems and more active meso-scale systems,while the good relationship between vertical flux of moisture divergence and the 7 July rainstorm can be attributed to strong converged ascending motion caused by synoptic scale systems.