| Severe convective weather has been the research focus in the field of mesoscale meteorology. Storms, tornadoes, strong winds, etc have caused serious harm on human life during the developing of severe convective weather system. Therefore, to understand the physical mechanism and master the active rule of severe convective weather have social scientific and practical significance for the sustainable development of society. In this paper, the severe convection event that occurred in July 21,2012 in North China/Beijing area were analyzed by using observational data of high spatial and temporal resolution and mesoscale numerical simulation, focusing on the characteristics of the rainstorm under a variety of detection methods. And through numerical simulation experiments to investigate structure characteristics and mechanisms for development and maintenance of the mesoscale convective system(MCS) that caused this storm.First, synoptic analysis is conducted about the rainstorm by using the NCEP reanalysis data (1°×1°). The results show that this severe convection event happened under a typical large circulation pattern of North China torrential rains. The northward jump of low level vortex and surface inversion trough are the direct causes of the rainstorm, and abundant moisture transport provided favorable conditions. The rainstorm can be divided into two stages:warm area precipitation dominated by warm moisture air in the first stage(before about 12UTC), frontal precipitation resulting from intersection of cold and warm air in the second stage(after 12UTC).Using the observational data with high spatial and temporal resolution reveal that there are two stages of the rainstorm process, respectively, warm sector precipitation and frontal precipitation. The echoes distribution of warm sector precipitation are scattered, while the echoes of frontal precipitation have obvious form of organization, which main type is similar to PS MCS. The radial velocity analysis shows that low layer winds are southerly, no sight of cold air. LLJ is always maintained in front of the cold front. WPR results is consistent with radial velocity. Southerly wind turn westerly wind after cold front. Meanwhile, the velocity in low-troposphere increases significantly before rainstorm. The strengthening of low-level disturbances, wind direction shear and the ultra low-level jet are likely to be the causes of heavy precipitation. The automatic weather stations observations reveal that Beijing is dominated by the east warm moist air flow before precipitation, and the warm moist air flow make variable pressure per hour negative. Precipitation area and positive variable pressure per hour has a good corresponding relationship in the frontal precipitation stage. Warm area precipitation maintained for a long time and cumulative rainfall is large, and a relatively short time in frontal precipitation, bur the rainfall intensity is strong. The detection of cloud radar show that the echo tops up to 15km, and the echo body located 5-9km high. The strong echo is composed of low broken cloud before precipitation. And the bright band can be identified the distribution of ice particles and water droplets.Due to the limitations of observational data, we further investigate the development and maintenance mechanisms of MCS by the mesoscale numerical simulation. Dynamic and thermodynamic structure shows that divergence flow in upper level and convergence in lower level are in favor of synoptic scale upward motion, and this configuration provides favorable conditions to the development of MCS. The differences of dynamic structures between warm area precipitation and frontal precipitation are obvious, and the dynamic action is the dominant factor in frontal precipitation. Water vapor source analysis showed that the water source in different levels come from diffenent channels. Water vapor mainly come from the bay of Bengal and the South China Sea, and the Vincent make a great contribution to the water vapor transport in later rainstorms. Long time water vapor transport is in favor of the maintenance of the MCS, and the simulation also showed that water vapor transport are mainly from the lower troposphere. Beijing is located in the right rear of the upper level jet and in the left front of the low-level jet. The direction of ULJ and LLJ is vertical, and the precipitation area basically formed in the vicinity of the intersection of ULJ and LLJ. Ascending branch of the vertical cross frontal circulation is connected with the upper level wind field, and strengthened ULJ is in favor of maintaining symmetric instability of upper level. Northern cold air entering rainfall areas through the frontal secondary circulation lead to enhance temperature and humidity gradients which are conductive to the development and maintenance of precipitation. Analysis of dry cold air show that upper troposphere over precipitation is dry area. The presence of the dry area is conductive to the accumulation of energy in lower troposphere. Convective instability energy driven by a certain power is in favor of the occurrence of precipitation. Latent heat release caused by precipitation leads to high potential vorticity(PV) value over the rainfall region. The interaction of convergence of cold and warm air, diabatic heating and low level relative vorticity increases rapidly caused by northward jump of the low level vortex results the PV value greater than 2PVU in the lower troposphere. Humidity analysis show that dry intrusion in upper levels overruns that in low levels may easily lead to the development of convective instability, which is conductive to the enhancement of the precipitation. The rainfall area occurred in front of the relative humidity gradient large value area. Dry intrusion is one of the important mechanisms of the formation of the North China rainstorms. |
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