基于SOM的长江流域持续性强降水过程典型环流的客观分型

Circulation Classification for Persistent Heavy Precipitation Processes in the Yangtze River Basin based on SOM

中文关键词:  长江流域持续性强降水，逐日环流型，异常环流的稳定性，自组织特征映射网络

英文关键词:Persistent heavy rainfall process, Daily circulation patterns, Stability of anomalous circulation, Self-Organizing Map

中文摘要:

      该研究利用1961-2021年逐日降水格点化观测资料和ERA5再分析资料，基于无监督深度学习的自组织特征映射神经网络（SOM）方法，将与我国长江流域夏季持续性强降水过程对应的大尺度环流客观划分成四种典型环流型（P1-P4）。各环流型呈现出的关键环流系统配置影响了异常雨带的形成及落区。P1和P3中高纬分别为典型的单阻型和双阻型环流形势，且西太平洋副高显著偏强并向西延伸。而P2和P4则表现出较为明显的低压异常，P2中的巴尔喀什湖到贝加尔湖以西为宽广低槽，贝加尔湖以东为脊区，形成入梅期间的稳定环流形势，长江流域受到低值系统的控制，副高位置靠南。P4在巴尔喀什湖以西和以东分别表现出位势高度异常偏低和偏高，长江流域表现为气旋性环流异常，同时副高位置靠北。P1、P2有来自高纬度冷空气的加持，而P3和P4的冷空气则较弱。以副高北跳为主要特征的东亚夏季风向北推进与四种环流型的出现以及相应的雨带位置有着紧密联系，P1、P2主要对应6月至7月上旬的持续性强降水，而P3、P4则主要对应的7月上旬至8月的持续性强降水，导致P1和P2的降水异常中心分别位于长江中下游的江南地区，P3和P4的降水异常中心则分别位于长江流域和长江以北地区。P1、P3的水汽输送相比另外两类明显加强，造成的降水强度也更强。此外，对典型环流型的稳定性分析表明，长江流域持续性强降水过程与上述四种典型环流型的稳定维持密不可分。持续性强降水过程中93.2%的环流表现出持续性的特征，P1和P2、P3和P4持续天数分别主要集中在5天、3天。从长期趋势来看，P1和P3环流型出现频次增多，而P2和P4则趋于减少。这意味着从有利环流的角度来说，持续性强降水倾向于在长江以南和长江中下游地区发生。综上，该研究为厘清持续性强降水对应典型环流的特征和变化趋势提供了科学依据。

英文摘要:

      Based on daily gridded in situ-observed precipitation data and ERA5 reanalysis data spanning from 1961 to 2021, this study employed an unsupervised deep learning method known as the Self-Organizing Map (SOM) to objectively classify the large-scale circulation patterns associated with summer persistent heavy rainfall processes in the Yangtze River Basin. Four typical circulation types (P1-P4) were identified, representing distinct configurations of key circulation systems that are closely linked to the formation and distribution of anomalous precipitation. P1 and P3 represent typical single and double blocking circulation patterns at the high latitudes, respectively. In these patterns, the Western Pacific subtropical high (WPSH) is significantly intensified and westward extended. In contrast, P2 and P4 showcase more prominent features of deepened low-pressure systems at the mid-to-high latitudes. In P2, a broad low trough extends from the Lake Balkhash to the west of the Lake Baikal, with a ridge to the east of the Lake Baikal, forming a typically stable circulation pattern of the Meiyu season. The Yangtze region is under the influence of the low-pressure system, while the WPSH is positioned to the south. P4 shows low and high geopotential height anomalies in the west and east of the Lake Balkhash, respectively, along with a cyclonic anomaly in the Yangtze region. The WPSH exhibits a noticeable northward shift in P4. P1 and P2 are supported by cold air from high latitudes, while P3 and P4 are weaker. Additionally, the northward advancement of the East Asian summer monsoon characterized by the northward jumps of WPSH is closely linked to the appearance of the four circulation types and the position of rain belts. P1 and P2 mainly correspond to persistent heavy precipitation from June to early July, while P3 and P4 correspond mainly to persistent heavy precipitation from early July to August. P1 and P2 result in heavier precipitation to the south of the middle and lower reaches of the Yangtze, whereas the heaviest precipitation center associated with P2 and P4 lie respectively in the Yangtze River and to the north of the Yangtze. The precipitation intensity of P1 and P3 is also strengthened as the water vapor transport is enhanced. The stability of these typical circulation patterns was further analyzed. During persistent heavy rainfall processes, persistence is observed on 93.2% of all the corresponding circulation patterns. P1 and P2 typically persist for around 5 days, while P3 and P4 tend to last approximately 3 days. Long-term trends indicate an increasing frequency of P1 and P3 occurrences, while P2 and P4 are gradually diminishing. This suggests an inclination for persistent heavy rainfall processes to occur in the southern and middle-lower Yangtze River regions from a favorable circulation perspective. In summary, this study reveals several meaningful results, which provide a scientific guidance for clarifying the characteristics and trends of the typical circulation patterns corresponding to persistent heavy rainfall.