台风“山竹”登陆结构变化及造成广西强降水异常分布的成因分析

利用地面加密自动站观测资料、多普勒天气雷达及NCEP格点再分析资料，对2018年第22号台风“山竹”登陆前后环境场、动力热力场结构变化特征进行了分析，并初步探讨台风造成广西强降水分布异常的成因。(1) 200 hPa南亚高压稳定维持、西南季风与越赤道气流、西太平洋气流卷入提供水汽和能量是台风登陆后强度减弱缓慢的重要因素。(2)台风登陆移入广西过程中，其中高层θ_se漏斗状结构、正涡度柱状结构维持，东侧和北侧低层有深厚入流辐合上升，高层有明显出流。(3)水平风场的不对称分布导致台风东侧到北侧存在明显的风向切变和风速辐合，出现垂直螺旋度正值中心和强水汽辐合区，对应有螺旋对流回波带发展、维持，这可能是“山竹”强降水出现在台风偏北侧的重要原因。(4)云贵高原冷空气的侵入加剧了热力不稳定，加上地形抬升增幅作用，导致桂北和桂西持续强降水。(5) “山竹”降水分布与低层湿位涡负值区有较好对应，强降水主要出现在湿正压项(MPV1)负值中心附近和MPV1负值区与湿斜压项(MPV2)正值区相叠加的区域。 

CAUSE ANALYSIS ON THE STRUCTURAL CHANGE OF TYPHOON MANGKHUT DURING ITS LANDING AND THE ABNORMAL DISTRIBUTION OF HEAVY PRECIPITATION IN GUANGXI

Based on the intensified observation data from automatic weather stations, Doppler weather radars and the reanalysis data of NCEP grid point, the characteristics of structural changes of environmental field, dynamic and thermodynamic fields before and after the landfall of the 2018 typhoon Mangkhut (1822) were analyzed, and the causes of the abnormal distribution of heavy precipitation brought by typhoon were preliminarily discussed. The results are shown as follows. (1) A stable 200 hPa South Asia High, southwest monsoon, cross-equatorial airflow and the western Pacific airflow providing water vapor and energy were the important factors for the slowing and weakening of the typhoon after its landfall. (2) During the process of typhoon landing and moving into Guangxi, the funnel-shaped structure of high-level θ_se and positive vorticity column structure were maintained, while deep inflow convergence rose in the lower floors on the east and north sides, and there was obvious outflow in the upper floors. (3) The asymmetrical distribution of the horizontal wind field led to obvious wind direction shear and wind speed convergence from the east to the north side of the typhoon, and the occurrence of a positive vertical spirality center and a strong water-vapor convergence zone. Correspondingly, a spiral convection echo zone developed and maintained. All of these changes may be the main reasons for the occurrence of heavy precipitation north of the typhoon. (4) The invasion of cold air from Yunnan-Guizhou plateau aggravated the thermal instability, and coupled with the increase of topographic uplift, resulted in continuous heavy precipitation in northern and western Guangxi. (5) The precipitation distribution of Mangkhut corresponded well with the negative area of the low-layer moist potential vortex (MPV), and the heavy precipitation mainly occurred near the center of the negative area of the positive pressure term (MPV1) and the area where the negative area of the MPV1 superimposed with the positive area of the moist baroclinic pressure term (MPV2).