2023年6月一次北部湾低压极端暴雨过程天气气候特征

Weather and climate characteristics of an extreme rainstorm process caused by a tropical depression in the Beibu Gulf in June,2023

利用逐日再分析资料,对2023年6月7-10日北部湾低压持续性极端暴雨过程的天气气候特征进行研究。结果表明,(1)持续性极端暴雨前期和期间,副热带高压异常偏北偏西,位置稳定。孟加拉湾-南海-菲律宾一带热带辐合带活跃,南海西部有低压系统生成,并移入北部湾。(2)季节内振荡(MJO)处在第2、3位相,受其东传影响,中高纬度环流形成的槽脊位置有利冷空气南下影响广西。中层500 hPa我国东北地区附近有东北低涡维持,低涡后部强盛偏北风引导冷空气南下,受偏北偏西的副高阻挡,有小股冷空气渗透南下影响广西。(3)低层850 hPa印度洋越赤道气流和西南季风强盛但活动偏南,印度洋-中南半岛-南海的西南风急流受台风“古超”阻挡转向成南风,使气流在北部湾形成气旋性弯曲。广西西北部有弱东北风,增强北部湾周边气流的气旋性辐合。(4)广西南部和北部湾一带位于200 hPa东亚副热带高空急流右侧与850 hPa西南风低空急流左侧,高空辐散和低层辐合,1000~300 hPa水汽通量辐合区水汽条件充沛。中、低纬度天气系统的协同作用,为这次持续性极端暴雨提供有利的稳定大尺度环流背景,以及充分的热力、动力条件。

The weather and climate characteristics of the persistent extreme rainstorm process of a depression in the Beibu Gulf from 7 to 10 June 2023 were studied using daily reanalysis data.The results show that:(1)the subtropical high was abnormally north-westerly and in a stable position during the pre-and period of the persistent extreme rainstorm.The tropical convergence zone along the Bay of Bengal-South China Sea-Philippines was active,and a low-pressure system was generated in the western part of the South China Sea and moved into the Beibu Gulf.(2)The intra-seasonal oscillation(MJO)was in the 2nd and 3rd phases,influenced by its eastward transmission,and the position of the trough-ridge formed by the mid-and high-latitude circulation was favourable for cold air to move southward to affect Guangxi.There was a northeast low-vortex maintaining near the northeastern China at 500 hPa,and the strong northerly wind behind this vortex guided the cold air to the south,which was blocked by the north-westerly subtropical high,causing a small cold-air infiltration to affect Guangxi to the south.(3)The trans-equatorial airflow in the Indian Ocean and the southwesterly monsoon were strong at 850 hPa in the lower layers,but their activities were skewed to the south,and the southwesterly jets of the Indian Ocean-Central South Peninsula-South China Sea was blocked by Typhoon Guchol and shifted into southerly winds,which led to the cyclonic convergence of the airflow around the Beibu Gulf.There is a weak northeasterly wind in northwestern Guangxi,which enhanced the cyclonic convergence of the airflow around Beibu Gulf.(4)The southern part of Guangxi and the Beibu Gulf area were located to the right of the 200 hPa East Asian subtropical high-level jet stream and to the left of the 850 hPa southwesterly jet streams,with high-level divergence and low-level convergence,and abundant water vapour conditions in the 1000~300 hPa water vapour flux convergence zone.Overall,the above collaborative effect of the mid-and low-latitude weather systems provided a favourable and stable large-scale circulation background,as well as adequate thermal and dynamical conditions for this persistent extreme rainstorm.