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Formation mechanisms of persistent extreme precipitation events over the eastern periphery of the Tibetan Plateau: Synoptic conditions, moisture transport and the effect of steep terrain
青藏高原东部边缘持续极端降水事件的形成机制、天气条件、水分输送和地形的影响
The eastern periphery of the Tibetan Plateau (EPTP) is prone to frequent and severe Persistent Extreme precipitation (PEP) events in summer. Given the complexity of weather systems and the intricate nature of terrain over this region, the generation and development mechanisms of the PEP in the EPTP remain to be determined. In this study, the formation and persistence mechanisms are further explored from the perspective of the general features of large-scale circulations, moisture source diagnosis and the influence of steep topography by using a thermal-dynamical diagnosis method, a mesoscale numerical simulation and a Lagrangian identification of the main moisture sources. The results show that during PEP events, the corresponding atmospheric circulation systems are characterized by an anomalous Rossby wave train at middle levels, an intensified westerly jet, an eastward extension of the South Asian high and a westward extension of the western Pacific subtropical high, all of which are possible to facilitate the development of potential instability and anticyclonic convergence of water vapor transport. The evaporative moisture sources from the Southeastern Asia (SEA), Bay of Bengal (BOB), and Southern China Sea (SCS) are remarkably enhanced during PEP events, with a contribution of 56.44%, which is 28.5% higher than the climatic mean. The further sensitive experiment indicates that the steep terrain could enhance the continuous transport of positive potential vorticity and the downward propagation of upper-level isentropic potential vorticity disturbances, thereby playing an essential role in the triggering and development of PEP events.
近日，《大气研究》（ Atmospheric Research）期刊刊载题为“青藏高原东部边缘持续极端降水事件的形成机制、天气条件、水分输送和地形的影响”（Formation mechanisms of persistent extreme precipitation events over the eastern periphery of the Tibetan Plateau: Synoptic conditions, moisture transport and the effect of steep terrain）的文章，研究人员利用热力诊断方法、中尺度数值模拟和主要湿源的拉格朗日识别，从大尺度环流的一般特征、湿源诊断和陡峭地形的影响等角度，探讨了青藏高原东部边缘持续极端降水事件形成和持续机制。
Fig. 1. (a) The geographical location (blue column) and topography (color shaded areas, m) of the eastern periphery of the Tibetan Plateau (EPTP), (b) the dis- tributions of the composite average rainfall amounts (mm d-1) during the summer (June, July and August) of 1980–2021 (contours) and 60 persistent Extreme precipitation (PEP) events (shaded areas) over the EPTP, (c) the distribution of the accumulated frequency of persistent Extreme precipitation at various stations during the summer (June, July and August) of 1980–2021, and (d) interannual variations of persistent Extreme precipitation frequency (bars) and regional mean precipitation (dash lines) over the EPTP.
研究指出，在青藏高原东部边缘的持续极端降水事件中，大气环流系统的结构具有显著的异常特征。具体来说，500hPa位势高度表现出类似于Rossby波列的异常结构，西风急流增强，同时伴有南亚高压（SAH）的东延和西太平洋副热带高压（WPSH）的西延。在这些异常大气环流的影响下，青藏高原东部边缘在对流层表现出明显的潜在不稳定性。收敛的反气旋水汽输送，加上增强的西南水汽，为持续极端降水提供了有利的水汽条件。就持续时间较长的事件而言，SAH和WPSH之间的经向距离较近，同时伴有较强的冷暖气流辐合和较长的低涡和水汽辐合持续时间。研究还指出，青藏高原东部边缘的持续极端降水事件同时受到东亚季风和南亚季风的影响。具体而言，东亚夏季风发挥了主导作用，通过东亚和南海路径将水汽输送到青藏高原东部边缘。此外，印度（南亚）夏季风输送的水汽在较短的时间尺度（4-6天）上也起到了支撑作用（16.38%）。总的来说，在持续极端降水事件期间，来自东南亚、孟加拉湾和中国南海的蒸发产生的水汽源显著增强，总贡献率为56.44%，比气候平均值高28.5%。
此外，地形敏感性测试表明，陡峭的地形在持续极端降水事件的触发、增强和维护中起着重要作用。一方面，它在青藏高原东部边缘形成了能量高度不稳定的温暖潮湿环境，南北气流的充分辐合和低层水汽的气旋辐合更有利于极端降水的触发。另一方面，陡峭的地形导致正位涡的形成、正涡的持续输送和高层等熵位涡扰动的向下传播，进一步促进了降水的增强和维持。
延伸阅读：https://www.sciencedirect.com/science/article/pii/S0169809524001236#f0025（田静编译）
