广西贵港地区极端高温日的时间变化特征及其环流背景

极端高温事件是我国南方夏季频发的天气灾害，区域性明显，持续性高温事件的环流背景及其影响机理值得深入研究。基于台站观测资料和欧洲中期天气预报中心的再分析资料，对1961—2010年广西东南部贵港地区发生的高温事件的时间变化规律及持续性高温天气过程对应的大气环流场演变进行了统计诊断分析。(1)广西东南部贵港地区的高温日主要出现在6—9月，其中7—8月是高温日频发的月份；3天以上的持续高温日数占总高温日数的6成；8—9月的高温日数具有明显的增加趋势。(2)桂东南在6—9月的持续性高温天气与一些特定的天气系统的存在有紧密联系。持续的下沉运动是造成广西东南部升温的主要原因，而导致垂直下沉运动的主要原因又与大陆高压、副热带高压或南海-菲律宾海出现的热带低压系统的北移路径偏东有关，持续时间较长的高温过程还与热带气旋的活动时间较长和强度偏强有关。(3)高压控制的晴空少云天气可导致地表接收更多的太阳辐射，使得地表温度的升高，从而导致地表向上的长波辐射及感热通量增强，加热近地面空气，这些有利于近地面升温的热力过程以及垂直下沉绝热加热过程的增强，使得高温天气过程得以维持。 

TEMPORAL VARIATIONS OF EXTREME HEAT DAYS IN GUIGANG OF GUANGXI AND RELATED CIRCULATION BACKGROUND

Extreme heat events are the frequent weather disasters in summer in southern China. The circulation background of persistent extreme heat events and its influence mechanism deserve further study. This study statistically analyzed the temporal variation of extreme heat events in Guigang in southeastern Guangxi in different months from 1961 to 2010, and examined the evolution of circulation in the weather process of extreme heat events with different duration, using the observed station data and ECMWF ERA-Interim Reanalysis data. Climatologically, extreme heat days in Guigang mainly appear from June to September, especially in July and August. 60% of the extreme heat days are contributed by persistent heat events with lifetime of three days or more. The extreme heat days in August and September have an increasing trend. The phenomena of extreme heat are closely associated with some special circulation systems. Continuous vertical sinking motion is a major forcing to lead to extreme heat in Guigang, and the causes giving rise to sinking motion are mainly continental high pressure systems or western Pacific subtropical highs, and the eastward northward-moving paths of tropical cyclones originating in the South China Sea or the Philippine Sea. The longer durations of the extreme heat events are also associated with the longer duration and stronger intensity of tropical cyclones. The sunny days associated with high pressure systems lead to the enhancement of downward solar radiation, which is conducive to a warmer surface, and then the intensification of upward long wave radiation and sensible heat flux, raising the near-surface temperature. The near-surface thermal processes and the adiabatic process associated with descending motion work together to cause long-lasting extreme heat events in Guigang.