长江中下游地区梅雨期线状中尺度对流系统分析Ⅱ:环境特征

Analysis of the linear mesoscale convective systems during the meiyu period in the middle and lower reaches of the Yangtze River. Part Ⅱ: Environmental characteristics

利用2010年6—7月长江流域雷达拼图、各种观测资料和NCEP逐日再分析资料,合成分析了长江中下游地区梅雨期线状中尺度对流系统的环境特征及典型个例。结果表明,长江中下游地区梅雨期线状中尺度对流系统环境风相对对流线分量的垂直分布是决定线状中尺度对流系统组织模型的重要因子。尾随层状降水中尺度对流系统和邻接层状单向发展中尺度对流系统垂直对流线的风分量在对流层是从前向后,而前导层状降水中尺度对流系统垂直对流线的风分量在对流层中低层为从前向后,至中高层转为从后向前,这3类中尺度对流系统平行对流线的风分量都随高度明显增加;尾随层状降水中尺度对流系统和前导层状降水中尺度对流系统对流层风垂直切变主要是垂直于对流线,邻接层状单向发展中尺度对流系统则主要是平行于对流线方向。平行层状降水中尺度对流系统和准静止后向建立中尺度对流系统对流层平行于对流线的风分量占绝对优势,且随高度增大,前者增大得更显著,垂直于对流线的风分量较小。7月8日邻接层状单向发展中尺度对流系统形成于地面冷锋北侧的高湿、不稳定的环境中;6月7日准静止后向建立中尺度对流系统形成于一个变化的高温高湿的地面环境场中。高温高湿环境是各类线状中尺度对流系统发展环境的共同特征。

Using the composite radar reflectivity data, the operational observation data and the NCEP daily reanalysis data, the envi- ronmental characteristic of the line MCSs that occurred in the middle and lower reaches of the Yangtze River in June and July 2010 is compositely analysed. The results show that the middle- and upper-tropospheric storm-relative flow field is of primary importance in de- termining the organizational mode of the linear mesoscale convection. The trailing stratiform MCS and the training line adjoining the stratiform MCS show front-to-rear storm-relative winds in the troposphere, while the leading stratiform MCS shows lower-tropospheric front-to-rear storm-relative winds and upper-tropospheric rear-to-front storm-relative winds. The low-and mid-level wind shears are largely parallel to convection line for the training line adjoining the stratiform MCS but perpendicular to convection line for the trailing stratiform MCS as well as the leading stratiform MCS. The parallel stratiform MCS and the quasi-stationary back-built MCS experienced significant middle- and upper-tropospheric line-parallel storm-relative winds with the increasing magnitude with the increasing height. The training line adjoining the stratiform MCS on 8 July formed on the cool/north side of a cold front and in unstable air conditions. The quasi-stationary back-built MCS on 7 June formed in a changing high moisture and high θe surface environment. The most favorite environment for producing and developing the linear MCSs is that of high temperature and high humidity.