偏东风冷空气与地形相互作用背景下北京局地强降水成因分析

Cause analysis of a local severe precipitation event in Beijing on the background of interaction of easterly cold air and topography

综合使用自动站、卫星TBB、探空、BJ-ANC雷达拼图、雷达变分同化分析系统(VDRAS)风场以及NCEP/GFS0.5°×0.5°分析资料,对漏报的2013年6月4日白天北京局地强降水天气过程及成因进行详细分析。结果表明:(1)该过程是在弱天气尺度背景(即没有明显西来槽和高空冷空气等天气尺度系统强迫)下产生的,且北京地区低层被由渤海湾侵入的偏东风(冷空气)控制,大气层结上暖下冷,有别于通常意义上的夏季强对流天气发生背景,因此造成其预报失误。(2)高空正涡度平流提供了一定的动力条件,但不足以触发强对流,其主要触发机制是低层偏东风冷空气侵入以及暖湿空气在冷空气和地形相互作用下强迫抬升,这一点在今后强对流预报中尤须关注。(3)当夏季早晨850 h Pa以下出现逆温层时,若当日白天太阳辐射升温状况良好,08时探空资料在使用时应进行订正,T-logp图订正后显示北京具有显著对流有效位能(CAPE);此外,较大的中低层垂直风切变和边界层内较大的湿度层结均为雷暴下山后明显增强并造成局地强降水提供了有利的环境条件。(4)雷暴前侧出流与东南风形成的辐合线造成了中尺度对流单体不断东移。

The local severe precipitation event missed by forecasters in Beijing during daytime on 4 June 2013 and its cause of formation were discussed in detail by using AWS data, satellite TBB, regional mosaics from BJ-Auto Now-Casting system, VDRAS wind field and NCEP/GFS analysis data （0.5°× 0.5°）. The results are as follows. （1） When this event occurred, there were no clear synoptic systems in the level 500 hPa, such as cold trough, wind shear, etc. However, we could see a cold air flow from the Bohai Gulf in the low level, above which there was warm air. This is different from the environment typical for a convective weather event in summer, thus leads to the missed forecast of this event. （2） The most important factors leading to local severe precipitation over Beijing area are easterly cold air intrusion, topographic effect and the forced uplifting of warm air, instead of the vorticity advection in the northwest. （3） Typically, the T-logp chart should be revised when there was an inversion layer below 850 hPa in the summer daytime. In this case, a large CAPE is resulted after this revision additionally, a large vertical wind shear in the mid- and low-level and humidity stratification in boundary layer provide a beneficial environment condition for the thunderstorm to strengthen and trigger local severe precipitation after it moved down hills. （4） The convergence line formed between the outflow at front side of the thunderstorm and the inflow of the southeast wind causes mesoscale convective cells continuously moving eastwards.