一个长生命期中尺度对流系统维持机制的研究

A study on maintain mechanism of a long life-cycle mesoscale convective systems

利用FY-2C多通道卫星云图、雷达资料和自动气象站记录等非常规观测资料,研究2008年4月12—13日形成于广西中部、移过广西东南部和广东西部到达南海的一个单独且生命期持续长达25 h的中尺度对流系统(MCS)的维持机制。水汽图(WV)分析显示4月12日14时前,对流层中上层存在一个明显的由湿变干过程,与此同时从红外(IR1)、中红外(IR4)和可见光(VIS)云图的合成分析中检测到低空有一条从北部湾延伸到广西东南部持续稳定的中尺度水汽输送带,在高空干区与低空湿区重合的广西东南部上空形成了强位势不稳定区,为MCS的形成和维持提供了有利环境条件。根据自动气象站记录分析结果,准静止锋在缓慢南移过程中连续抬升触发深对流而形成MCS,是一个典型的锋面抬升触发过程。分析表明,MCS能维持长生命期主要有利因素是:(1)在高空气流由湿变干的条件下,来自北部湾海面持续的低空中尺度水汽输送带给广西东南部提供充足的水汽、热量并形成大气位势不稳定层结,较长时间保持的位势不稳定层结为MCS的维持提供了有利环境条件;(2)稳定而持续缓慢南移的准静止锋在MCS西南边不断触发新的对流单体,这种后向传播方式触发的新对流单体并入MCS后,补充已减弱东移的旧单体,使MCS具有持续生命力。最后,给出了MCS的概念模型。

This paper describes the analysis of a MCS long life cycle in the south of China during April 2008. Using FY-2C geostationary satellite multi-channel cloud images, radar observations, records obtained from automatic weather stations andothers irregular observations, a long life cycle mesoscale convective systems（MCS） was studied to analyze the maintainable mechanism of MCS, whichoccurred in the middle part of Guangxi on 12 April 2008, and then moved to the southeast of Guangxi,the western Guangdong and The South China Sea. The MCS lasted 25 hours, and hence may be classified as a long life cycle. Section lines of TBB were given in the hourly cloud images of water vapor （WV） on April 12, from 05：00 to 20：00 BST, to analyze the TBB character change over time. Images show that there is an apparent course from wet to dry state in upper troposphere before 14：00. Simultaneously in the lower troposphere, a mesoscale water conveyor belt which was sustained and stable and extended from Beibu Gulf to southeast of Guangxi could be detected from the composite cloud images of infrared（IR1）and near infrared （IR4） and visible light （VIS）. In the dry area of upper air and wet area of low altitude, an instability area was formed in the southeast of Guangxi, which favored the formation and maintenance of MCS. It could be concluded from records of automatic weather stations that while the quasistationary front moving slowly from the north to the south, continuously lifting the wet warm air and triggering the deep convection, a MCS was formed. Characterized by the relationship between MCS forming course and the changes of atmospheric pressure, air temperature, wind velocity, precipitation, it could be regarded as a typical lifting and trigger course of the front. Results show that the main favorable factors maintaining a MCS long life cycle can be concluded in 2 points as below. Firstly, while the upper air changed from wet to dry, the water vapor and heat were carried from Beibu Gulf by a continuing mesoscale vapor conveyor belt in the lower troposphere to form the unstable atmospheric structure, providing an advantageous condition for MCS maintenance. Secondly, old convection cells of MCS gradually weakened in the way to the east,and were constantly replaced by new ones occurring in thesouthwest of the quasi stationary front moving slowly southwards provided MCS with energy to survive. A concept model of MCS is presented in the essay.