广西沿海一次弱热带低压破纪录暴雨过程的诊断分析

Diagnosis and analysis of a record-breaking rainstorm process caused by weak tropical depression in Guangxi coastal

中文关键词:  破纪录暴雨，热带低压，边界层急流，动能，非地转风

英文关键词:Record-breaking rainstorm,Tropical depression,Boundary layer jet,Kinetic energy,Aageostrophic wind

中文摘要:

      [资料和方法]利用多源实况观测资料以及ERA5再分析资料，基于大气热、动力诊断方程，[目的]对2023年6月7－9日广西沿海破纪录热带低压暴雨过程的成因及低压维持的可能机制进行了分析，[结果与结论]结果表明：（1）在中高纬与低纬天气系统相互作用的背景下，热带低压在广西境内移动缓慢，盘旋少动，促成了此次强降水过程的发生。期间大气热力、水汽因子均伴有显著的异常性，整层水汽通量散度、大气可降水量以及K指数的标准化异常分别达到-5.5、3.2以及1.3。（2）降水最强时段集中于夜间至凌晨，准静止的中尺度对流云团以及对流系统“列车效应”使降水不断在局地累积，造成破纪录累积雨量。（3）7日（8日）夜间至8日（9日）凌晨，热带低压东南侧（东侧）暖式切变线（边界层急流）以及陆面摩擦为强降水的发生发展提供了动力条件。9日边界层急流辐合强于8日切变线辐合，锋生强迫更强。热带低压系统增强导致的气压梯度力增大以及位势高度经向平流对应的气压梯度力作功过程促进了局地动能增长，是边界层急流发展的原因。（4）热带低压环流不断将南海水汽卷入其内部，水汽辐合及垂直输送使得湿层不断增厚，有利于降水效率增大。持续的暖湿输送有利于不稳定层结维持，使大气低层对流不稳定结构贯穿整个降水过程。其中，垂直风切变和大气斜压性的有利配置使得9日层结不稳定特征较8日更强，与动力条件相结合促使9日出现更强降水。（5）热带低压系统的维持主要受水平风场的辐合效应支撑，地转风分量在整个过程中贡献相对较弱，非地转风分量的水平散度项则主导了局地涡度的变化，这与非地转风惯性旋转后向热带低压中心辐合有关。

英文摘要:

      Based on the atmospheric thermal and dynamic diagnostic equation, the cause of the record-breaking tropical depression rainstorm process in the coastal area of Guangxi and the possible mechanism of the maintenance of the depression in 7－9 June 2023 are analyzed by using multi-source observation data and ERA5 reanalysis data. The results showed that: (1) Under the background of the interaction between the mid-high latitude and low latitude weather systems, the tropical depression moves slowly and circles over Guangxi, which contributes to the occurrence of the heavy precipitation process. The atmospheric thermal and water vapor factors have significant anomalies during the process. The normalized anomalies of the vertical integral of vapor flux divergence, precipitable water and K index reach -5.5, 3.2 and 1.3, respectively. (2) The precipitation is strongest from night to early morning, and the quasi-stationary mesoscale cloud clusters and the convective system "echo training" caused continuous local accumulation of precipitation, resulting in record-breaking rainfall. (3) From the night of the 7th (8th) to the early morning of the 8th (9th), the warm shear line (boundary layer jet) on the southeast (eastern) side of the tropical depression and the land surface friction provided the dynamic conditions for the occurrence and development of heavy precipitation. The convergence of the boundary layer jet on the 9th was stronger than that of the shear line on the 8th, and the frontogenic forcing was stronger. The increase of pressure gradient force caused by the strengthening of tropical depression system and the meridional advection of potential height which corresponding the pressure gradient force doing work process promote the growth of local kinetic energy, which is the reason for the development of boundary layer jet. (4) The tropical depression circulation continuously sucked water vapor from the South China Sea, and the convergence and vertical transport of water vapor made the wet layer thicker, which is conducive to the increase of precipitation efficiency. Continuous warm and humid transport is conducive to the maintenance of unstable stratification, which makes the convective unstable structure at lower atmosphere run through the whole process. Among them, the favorable configuration of vertical wind shear and atmospheric baroclinicity make the stratification more instability on the 9th than that on the 8th, which combined with the dynamic conditions led to more intense precipitation on the 9th. (5) The maintenance of the tropical depression system is mainly supported by the convergence effect of the horizontal wind. The contribution of geostrophic wind component is relatively weak in the whole process, while the horizontal divergence term of the ageostrophic wind component leads the change of local vorticity, which is related to the convergence of the tropical depression center after the inertia rotation of the ageostrophic wind.