“23.7”华北极端暴雨精细特征和天气学成因分析

Fine Characteristics of the July 2023 Extreme Rainfall in North China and Associated Synoptic Weather Patterns

中文关键词:  极端暴雨，华北  台风，地形  低空急流，台风，西太平副热带高压

英文关键词:extreme  rainfall, North  China, topography, low-level  jet, typhoon, western  Pacific subtropical  high

中文摘要:

      2023年7月29日至8月1日，华北京津冀地区遭受极端特大暴雨袭击，导致区域性洪涝灾害，造成重大经济损失和人员伤亡。[目的]为探明此次“23·7”华北极端降水过程的基本特征和形成原因，[资料和方法]基于地面自动站分钟级降水资料、风廓线雷达和雨滴谱仪观测及ERA5再分析数据，[结果]本文揭示了“23·7”华北极端暴雨的精细特征和天气学成因。[结论]结论表明：（1）此次降水过程持续时间长，累积量极大，特大暴雨落区、突破降水量历史极值的国家观测站站点密集分布在京津冀太行山近山地区，降水呈现显著极端性特征、区域性差异和阶段性变化。河北和北京太行山迎风坡地区出现持续性、强度相对稳定的强降水过程，雨滴谱分布偏向于高雨滴数密度、小雨滴直径的海洋型对流降水；京津冀平原地区总雨量明显弱于山区，但中尺度对流雨带活跃，降水阵性、对流性特征突出。（2）在“杜苏芮”台风残涡、西太平洋副热带高压、大陆高压等天气系统协同作用下，中层位势高度场出现罕见的“北高南低”加“西低东高”的稳定天气形势。北上“杜苏芮”残涡在高压坝以及太行山、燕山地形影响下移动缓慢，残涡中心北侧的地形障碍流与低空东南风之间形成稳定倒槽，京津冀地区出现持续性低层辐合，同时配合来自南海季风和东部洋面台风“卡努”的双路水汽输送，导致京津冀地区出现长历时、区域性极端暴雨过程。（3）在阶段性降水发展过程中，与台风残涡相关的倒槽、暖切变线、低空急流等精细结构调控了中尺度对流系统的组织和发展特征，其中31日早晨至上午京津冀地区出现超过22 m/s的超低空东南风急流活动，配合对流和地形抬升就等因素等影响，降水强度有明显增强，北京西部山前出现超100 mm/h的极端雨强。此次极端降水事件中的地形影响、中小尺度特征以及极端暴雨可预报性等问题尚待更深入研究。

英文摘要:

      From 29 July to 1 August 2023, the Beijing-Tianjin-Hebei region was struck by heavy to extremely heavy rain, resulting in severe flooding, causing huge economic losses and considerable casualties. Based on the minute-interval precipitation observation, wind profiler radar observation, raindrop spectrum observation, and ERA5 reanalysis, detailed characteristics and synoptic weather causes of the July 2023 extreme rainstorm in North China are analyzed and revealed in this paper. The results show that: (1) the precipitation process presents significant extremes and regional differences with varied moisture origins. The 4-day event accumulated tremendous rainfall with a wide coverage, and 86 national stations in the region registered record-breaking daily rainfall. In the western mountainous areas, precipitation was relatively stable and persistent, and the raindrop spectrum approximated the marine type, with high rain droplet number concentration and small droplet diameter. Over the eastern plain areas dominated by a lower troposphere southeasterly jet stream, however, active mesoscale convective rain bands appeared, and the raindrop spectrum was close to that of continental type with rainfall being short-lived and highly convective. (2) Under the rarely-seen stable weather pattern of “high in the north and low in the south” and “low in the west and high in the east” in the 500-hPa geopotential height field, a stable inverted trough was formed between the topographic barrier flow on the north side of the remnant vortex of Typhoon Doksuri and the low-level southeasterly jet stream, forming continuous convergence and updrafts. Water vapor transport from both the South China Sea and north of Typhoon Khanun over the East China Sea converged in the upper reaches of North China, sustaining the extreme rainstorm. (3) In the phased rainfall development process, the typhoon remnant vortex, inverted trough, low-level southeasterly jet, and warm shear lines modulated the organization and development of the mesoscale convective systems, and affected the rainfall location and intensity. On 31 July, a mesoscale easterly jet core exceeding 22 m s^(-1) occurred in the lower boundary layer, resulting in significantly enhanced convective precipitation reaching 100 mm h^(-1) in the west of Beijing. Nonetheless, more refined attributes of the various weather systems, the role of diurnal variation, as well as predictability of such an extreme rainstorm, need to be further studied.