触发四川盆地极端短时强降水的中尺度对流系统环境条件

Environmental Conditions of Mesoscale Convective Systems Triggering Extreme Flash-Rain in Sichuan Basin

为研究触发极端降水的中尺度对流系统(MCS)环境条件,利用常规探空和地面观测资料以及雷达组合反射率因子资料,分析2013-2019年5-9月41例触发四川盆地极端短时强降水的MCS的环流形势和中尺度物理量特征,并对MCS的成因进行初步探讨。结果表明:触发四川盆地极端短时强降水的MCS发生的天气形势可分为斜压锋生类和准正压类。斜压锋生类天气形势可分为低层切变型和西南低涡型,分别对应邻近层状云类和涡旋类MCS发生前的环流背景。准正压类天气形势可分为低槽东移型、两高切变型、东风扰动型,易发生平行层状云类、后向扩建类和尾随层状云类MCS。邻近层状云类和涡旋类MCS的抬升凝结高度和平衡高度高于平行层状云类、后向扩建类和尾随层状云类MCS。高温高湿环境是5类MCS发生发展的共同特征,涡旋类和邻近层状云类MCS环境的热力不稳定性相对较高,涡旋类和邻近层状云类、平行层状云类和后向扩建类MCS发生在低层水汽输送更为充沛的环境下。相较于其他3类MCS,强的垂直风切变更有利于后向扩建类和平行层状云类MCS的发展维持。在准正压类天气形势下,天气尺度强迫较弱,MCS形成和发展与低层流场、地面中尺度辐合线、地形强迫以及雷暴冷池有密切关系。

Due to discuss environmental conditions of mesoscale convective systems triggering extreme precipitation,using the conventional sounding and surface observation datasets,and composite radar reflectivity data,this study investigates the synoptic situation,mesoscale physical parameters and formation of 41 MCSs triggering extreme flash-rain between May and September during 2013-2019 in Sichuan basin.The results show that there are two synoptic situation configurations of MCSs:baroclinic frontogenesis and quasi-barotropic category.The baroclinic frontogenesis synoptic category can be classified into southwest vortex and low-level shear patterns,representing the circulation background before the occurrence of Training Line/Adjoining Stratiform(TL/AS)and Vorticity Stratiform(VS)MCSs,respectively.The quasi-barotropic synoptic category can be classified into low-trough eastward movement,shear between two high-pressure and easterly disturbance patterns.In these patterns,Parallel Stratiform(PS),Back-Building/Quasi-Stationary(BB/QS)and Trailing Stratiform(TS)MCSs are easy to occur.Comparing with PS,BB/QS and TS modes,TL/AS and VS modes have relatively high altitude of lifting condensation level and equilibrium level.The high temperature and high humidity environment is the common feature of producing and developing the five MCS modes.The higher thermal instability is benefit for VS and TL/AS modes,and the more vapor transport in low level can make for generating VS,TL/AS,PS and BB/QS modes.The strong vertical wind shear play a more important role in the development and maintenance of BB/QS and PS modes than that in the other 3 modes.In quasi-barotropic synoptic situation,the formation and development of MCSs have a close relationship with low-level stream field,surface mesoscale convergence line,topographic effect and thunderstorm cold pool under weak synoptic-scale forcing.