Study of the dynamic effects of the upper-level jet stream on the Beijing rainstorm of 21 July 2012

Based on the data from the conventional meteorological observation, including the surface hourly precipitation and the NCEP reanalysis data, using the weather diagnosis and analysis methods, the dynamic processes of the upper-level jet stream for the torrential rainfalls in Beijing, both increased suddenly at the time of 14:00 BT and of the biggest amount at 19:00 BT 21 July 2012 (named simply as 7.21 hereafter) are studied. The results show that the upper-level jet stream moved from the west to over Beijing is related to the Beijing 7.21 rainstorm, and the upper-level jet stream and its divergence field, and the secondary circulation associated with the upper-level jet play an important role in the Beijing 7.21 rainstorm. At 14:00 BT 21, along with the upper-level jet stream northward spreading, the range of the upper-level divergence, which is situated at the right side of the entrance region of the upper level jet stream, expanded significantly. The Beijing region is under the influence of the deep convective motion and affected by the favorable moisture conditions, which was associated by the southeast flow, and thus caused the precipitation of the Beijing area to increase abruptly. At 20:00 BT 21, the upper-level jet stream further northward spreads, and the low-level convergence generated by the northwest vortex, resulted in the strong upper-level divergence over Beijing. The vertical motion enhanced by the strong upper-level divergence over Beijing, which combined with the impact of warm air from the south-east, leading the precipitation of Beijing region to reaching the maximum at 19:00 BT 21. The ascending motions appeared in the right side of entrance region of the upper-level jet, while the descending motions occurred in the southeastern part of Beijing, at either the precipitation-increased suddenly time or the precipitation maximum time of Beijing 7.21 rainstorm. This is an important feature of the secondary circulation of the 7.21 Beijing rainstorm.