太行山地形影响下的极端短时强降水分析

2015年8月2日午夜和2011年8月9日前半夜,在两种不同天气系统背景下太行山东麓都出现了小时雨量超过50 mm的极端短时强降水天气,两次过程都是雷暴先在太行山区触发加强,经过下山2 h先后在丘陵站平山和山前平原站石家庄市区产生极端短时强降水。利用常规探测资料、地面加密观测资料、石家庄SA多普勒天气雷达资料,对不同天气系统背景下太行山特殊地形影响的极端短时强降水成因进行分析。结果表明:偏东气流被南北向的太行山地形强迫抬升,且与下山雷暴出流形成中尺度辐合线触发新的雷暴,雷达回波呈现后向传播特征和列车效应造成局地极端短时强降水。太行山地形通过增强辐合上升运动、增大垂直风切变使雷暴下山加强。不同天气系统强迫下,太行山特殊地形对雷暴发展作用不同。在偏西气流引导下,暖区极端短时强降水由阵风锋触发,具有突发性、降水时间短、伴随风力大的特点,下山雷暴出流加快且与山前偏东风的辐合加强,陆续在丘陵区和山前平原触发对流与下山雷暴合并加强造成极端短时强降水;而在东北气流引导下,回流冷锋和阵风锋共同触发的极端短时强降水具有持续时间较长、降雨总量较大、伴随风力较小的特点,太行山东坡对东北冷湿回流有阻挡积聚作用,东北偏北来的雷暴出流边界西端在迎风坡上强迫抬升使雷暴触发并加强,东北气流遇山后发生气旋性偏转使雷暴出流转向东南下山,与平原的偏东风辐合加强,造成丘陵区和山前平原的总降雨时间更长、降雨总量更大。

Investigation of Extreme Flash Rain Events on the Impact of Taihang Mountain

In the midnight of 2 August 2015 and the early night of 9 August 2011, extreme flash rain events were observed in the Taihang Mountain area under the background of different weather systems. The thunderstorms strengthened in the Taihang Mountain area, and extremely heavy rainfall were observed in Pingshan Station and Shijiazhuang Station in 2 h. Based on routine upper level and surface observation, automatic weather station (AWS) data and Doppler weather radar data, the reasons of the extreme flash rain impacted by Taihang Mountain with different backgrounds are analyzed. The results show that easterly air flow is forced to uplift in the mountains, then encountered with thunderstorm outflow forming mesoscale convergence line to trigger new thunderstorms. Local heavy rainfall is affected by the backward propagation characteristics and train effect of radar echo. Convergence ascending motion and vertical wind shear are enhanced by the Taihang Mountain terrain strengthening thunderstorm in the process of going down the mountain. The Taihang Mountain terrain plays different roles in the development of thunderstorms under different weather systems. Under the guide of westerly flow, extreme heavy rainfall is triggered by a gust front in warm sector, charactered by abruptness, short time and strong winds. Thunderstorm outflow down mountain is accelerated and convergence with piedmont easterly wind is strengthed to trigger new thunderstorms, being combined with the downhill flows and forming extremely heavy rainfall in the hilly region. However, under the guide of northeast flow, extreme short time heavy rainfall triggered by reflux cold front and gust front features longer duration, heavier rainfall, and weaker wind. Northeast cold wet reflux is resisted by east slope of Taihang Mountain. The western end of the thunderstorm outflow boundary from NNE ward is forced to uplift on windward slope to trigger and strengthen thunderstorms. Northeast airflow turns its direction in cyclonic deflection and the thunderstorms flow down to the southeast, so that convergence with the easterly wind in plains is strengthened, making the total time of rainfall longer and precipitation greater in hilly areas and piedmont.