2018年5月江苏极端降水事件发生前副热带高压异常及原因分析

Analysis of anomalies and causes of subtropical high before the extreme precipitation events in Jiangsu in May 2018

本文利用NCEP/NCAR提供的2.5°× 2.5°全球再分析数据,以2018年5月江苏两次极端降水事件发生前副高异常变化为研究对象,根据全型涡度方程定量计算了凝结潜热分布不均引起的涡源对副高迅速演变的诱发作用。研究发现,120°E处500 hPa 5月第1候副高脊线多年平均位置位于16°N附近,而2018年同期120°E的脊线则位于19°N附近,呈明显偏北的状态。2018年5月第1候东亚500 hPa位势高度距平场表现出南高北低的形态,有利于我国华东地区成为暖湿空气和干冷空气的交汇区,构成了江苏5月两次极端降水过程的有利环流背景。与对流层中层环流异常对应的是,同期115°~125°E之间850 hPa上8 g·kg ^-1 等比湿线位于28°N附近,较多年气候态偏北15°,强降水区内同期850 hPa比湿较往年偏多2~4 g·kg ^-1 ,相应距平百分率可达50%~75%。且110°~120°E之间θ se 的340 K等值线5月第1候多年气候态位于13°N以南,但2018年同期却偏北至25°N附近,暖湿气团北进有利于强降水的发生。副高西伸北抬前,副高主体西侧和北侧均有凝结潜热加热区存在,说明潜热加热与副高演变关系密切。垂直剖面表明600 hPa为凝结潜热加热中心,向上加热率随高度减小,因此500 hPa处潜热加热率垂直梯度为负,使得500 hPa成为负涡源所在。因凝结潜热分布不均产生的负涡源,1~2 d便可形成与副高自身十分接近的负涡度值,足以诱发副高突变,该时间尺度与副高真实演变时间相符。负涡源中与凝结潜热垂直分布不均相关的部分起主要作用,而与凝结潜热水平分布不均相关的部分同时期产生的负涡度最多仅为前者的1/3左右,对副高突然西伸的作用较小。与凝结潜热相关的负涡源作为引发西太平洋副高异变的可能原因,其与副高的关系仍需进一步研究。

The 2.5°×2.5° global reanalysis data provided by NCEP/NCAR were used in this paper. The anomalies of subtropical high before the two extreme precipitation events in Jiangsu in May 2018 were studied, and the inducing effect of vorticity source caused by the inhomogeneous distribution of condensation latent heat on the fast evolution of subtropical high was calculated quantitatively by the complete-form vorticity equation. The results showed that the average position of the subtropical high ridge on 500 hPa at 120°E at the first pentad in May was near 16°N, while the ridge at 120°E in the same period of 2018 was near 19°N, evidently moving towards the north. The 500 hPa geopotential height anomaly field in East Asia at the first pentad in May 2018 was high in the south and low in the north. This was conducive to forming an intersection of the warm and wet air and the dry and cold air in East China, thus providing a favorable circulation background for the two extreme precipitation events in Jiangsu in May. Relative to the middle-level circulation anomaly of the troposphere, the 8 g ·kg ^-1 equal specific humidity line at 850 hPa between 115°E and 125°E was near 28°N, moving 15° towards the north compared with the perennial climatic state. The specific humidity at 850 hPa in the area with heavy precipitation is 2-4 g ·kg^-1 more than that in the same term in previous years, and the corresponding anomaly percentage is up to 50%-75%. The 340 K isoline of θ se between 110°E and 120°E at the first pentad in May was located on the south of 13°N by referencing to the climate states of many years, but it was near the 25°N in the same period of 2018. The warm and wet air mass moving northward is conducive to generating the heavy precipitation. Before the subtropical high stretches westward and rises on the north, there is a condensation latent heating zone on the west and the north of the main body of subtropical high, indicating a close relation between the latent heating and the evolution of subtropical high. The vertical section shows that 600 hPa is the heating center of condensation latent, and the heating rate decreases as the height increases. Therefore, the vertical gradient of the latent heating rate at 500 hPa is negative, making 500 hPa the origin of the negative vortex source. The negative vortex source generated by the inhomogeneous distribution of condensation latent heat can form a negative vorticity value that approaches to the subtropical high itself within 1-2 days, so it is enough to induce the abrupt change of subtropical high. This time scale is consistent with the real evolution time of the subtropical high. The part that is related to the vertical inhomogeneous distribution of condensation latent heat in the negative vorticity source plays a prominent role, while the part that is related to the horizontal inhomogeneous distribution of condensation latent heat only generates about one third of negative vorticity source of the former at most, which has a minor effect on the abrupt western stretching of subtropical high. The relationship between the negative vortex source associated with the condensation latent heat, as the abrupt change of subtropical high in western Pacific, with the subtropical high still needs further study.