南亚急流波列在冬季华南准静止锋维持中的作用

Winter quasi-stationary front is a key weather system responsible for the freezing rain and snow storm disasters in South China. It results from the continuous conflict between the cold and dry air from mid-latitude inland area and the warm and wet air from low-latitude ocean. The strength of warm and wet southerly is closely related to the activity of South Asian jet wave trains. When the wave trains along the South Asian jet propagate downstream to East Asia, the India-Burma trough is deepened and the western North Pacific subtropical high (WPSH) extends northward. In between, abundant wet and warm air from the Bay of Bengal and the South China Sea is transported to South China by the southerly flow ahead of the trough and to the west of the WPSH. .Our previous study showed that the South Asian jet wave trains play a key role in the activity of the India-Burma trough and the winter precipitation over South China. They have distinct characteristics in multi-temporal and -spatial scales. The Rossby wave source, the excitation of disturbances, the propagation of disturbances along the South Asian jet, and their influences on the activities of troughs and ridges beneath the jet show great discrepancies in different scales..Though previous studies have highlighted the great importance of the South Asian jet wave trains to the maintenance of the winter quasi-stationary front in South China, the underlying mechanisms are not well understood. This study aims to investigate the impact of the South Asian jet wave trains on the position, strength, duration and structure of the winter quasi-stationary front, and to explore the roles of the wave trains and their multi-scale variations in maintaining the winter quasi-stationary front in South China and the underlying mechanisms. This study also aims to find out the precursor of the winter quasi-stationary front and to improve its prediction from the perspective of the South Asian jet wave trains.

冬季华南准静止锋是造成华南雨雪冰冻灾害的直接天气系统，它是北方干冷空气与南方暖湿空气持续交绥的结果，后者与南亚急流波列活动密切相关。当波列传播至东亚地区时，印缅槽加深东移，西太副高北扩，大量来自孟加拉湾-南海的暖湿气流往华南输送。.申请者前期研究发现该波列具有多尺度时空变化特征，在不同尺度上，扰动的信号源、激发机制、传播路径、及高、低层槽脊异常的配置存在明显差异，印缅槽的加深及槽前加强的水汽输送是多尺度相互作用的结果。本工作将以南亚急流波列作为研究主体，在分析波列对准静止锋的位置、强度、持续时间和结构的影响的基础上，揭示波列通过调控高、低层环流配置导致准静止锋维持的物理机制；探讨波列在准静止锋长时间维持中的多尺度相互作用，通过研究波列在各尺度上的信号源为冬季华南准静止锋活动寻找前兆信号并建立预报模型。

中国南方冬季强降水往往带来严重的社会影响，但其发生机制较少得到研究者的关注。来自大西洋的扰动可通过沿南亚急流波列传播至西北太平洋地区，它是大西洋扰动信号影响东亚冬季气候的重要桥梁。当波列传播至东亚地区时，印缅槽加深东移，为中国南方冬季降水输送大量水汽，对波列的触发过程、传播过程和气候影响的深入研究有助于提升东亚冬季降水的预报能力。本项目主要利用了日本气象厅提供的JRA-55逐日再分析资料，对南亚急流波列的活动特征、触发和传播机制以及对中国南方冬季降水的影响进行系统研究，得到的主要结论如下：.1..揭示了南亚急流波列的活动特征。南亚急流波列呈准相当正压结构，最强活动信号位于200 hPa，以10-30天的季节内活动周期为主。来自大西洋的扰动信号沿波列传播至西北太平洋约需8天，在波列东传的同时，各扰动中心加强东移，对传播路径上的天气/气候具有重要影响。.2..揭示了南亚急流波列的触发机制。NAO正、负位相都可以激发出相同位相的南亚急流波列，但NAO的空间结构存在差异。西北-东南走向的NAO负位相最容易激发出波列，NAO-位于高纬度正高度场中心的扰动信号上传，通过西北-东南向的垂直环流圈导致地中海西部高层辐散，激发出Rossby涡源/汇，扰动进入南亚急流并往下游传播。.3..揭示了南亚急流对东亚冬季强降水影响的物理过程。当扰动信号沿南亚急流波列东传至东亚地区时，既可以通过加深印缅槽，又可以通过激发西北太平洋反气旋，为南方地区输送大量水汽，加强华南准静止锋，导致冬季强降水的发生。但两者对应的波列纬向相互正交，相差1/4位相，从而导致的冬季强降水的落区分别位于华南南部和长江中下游。.以上研究成果对提供东亚地区冬季短期天气、气候预测具有重要意义，从南亚急流波列的角度揭示了低纬度冬季水汽输送系统变异的信号源以及物理过程，为冬季气候预测发掘新的预报信号。