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

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