Study On Intensification And Initiation Mechanism Of Thunderstorms Under Weak Synoptic-scale Force Background

It demonstrates that thunderstorms coming from Hebei Province into the north‐west mountainous areas account for 50 percents of the thunderstorms in Beijing area, based on the multiyear observational analysis and now‐casting operation experiences. Among those thunderstorms, many may weaken when they get down the hills, while others may intensify. It is very difficult to forecast the evolution trend of those thunderstorms. Especially, it depends on many local minor changes in physical conditions that whether the thunderstorms can get down the hills and whether they can intensify after getting down. This makes it a great challenge in now‐casting operation. On the other hand, local heavy rain happens frequently in Beijing area, and the initiation site and time of the thunderstorms resulting in the local heavy rain are also very hard to get fine forecast or nowcast. By a preliminary analysis of the thunderstorms in Beijing during 2008‐2009, we found that: under weak synoptic‐scale background, it is very hard to capture the favorable conditions and initiation mechanisms on the conventional weather map of different levels. By this time, research and analysis of the local atmospheric physical conditions, meso‐scale systems in low troposphere and complex terrain forcing are particularly important and critical.Scientific and technological achievements( many dense observation data and fine rapid updating analysis system ) made through the Beijing Olympic meteorological support, provide a massive foundation for conducing in‐depth study and solving the above problems. The purpose of this paper is to raise the knowledge about the enhancing mechanisms of the thunderstorms when they get down the hills and the emerging mechanisms in Beijing area, and provide valuable scientific information for the severe convective weather forecasting,nowcasting and early warning.Based on the Variational Doppler Radar Analysis System (VDRAS), combined with a variety of dense observational data, this paper studied the enhancing and emerging mechanisms of two thunderstorm cases developed under weak synoptic background environment in July 2009. One (referred as "722" thunderstorm case) of these two thunderstorms intensified when it went down the hills and another (referred as "729" thunderstorm case) initiated locally. The results show as follows:(1) Characteristics of the process. The "722" thunderstorm, which initiated in the north part of Hebei province and moved to the north‐west mountain area of Beijing, experienced two significant stages of intensifying, one of this happening when it went down the hills and another happening when it reached Chaoyang district of Beijing under plain terrain. The "729" thunderstorm was a typical local emerging thunderstorm, which developed along the west side of piedmont in Haidian district, resulting in 105mm rainfall in two hours. (2) Characteristics of weak synoptic‐scale forcing. Both the "722" thunderstorm and the "729" thunderstorm showed common features that the impact of large‐scale weather systems above 850hPa level was not obvious and there existed a tongue with a warm wet southerly air flow. In addition, a weak surface cold front moved close to Beijing before the "722" thunderstorm moving into Beijing. Obviously, both of them were typical thunderstorms developing under weak synoptic‐scale forcing environment.(3) Favorable atmospheric conditions. These conditions, maintaining of high‐level cold advection, low‐level warm and moist southerly flow, and gathering of the convective unstable energy were necessary for the enhancment of the "722" thunderstorms. Moist air environment on all levels, lower lifting condensation level (LCL) and free convection level (FCL) thunderstorms were key factors for the initiation of "729" thunderstorms.(4) Thermodynamic conditions of the "722" thunderstorms in low troposphere. On the first intensifying phase, terrain forcing played a dominant role. It included: a) uplift of south warm moist air flow at the south of piedmont, resulting in a strong upward motion; b) acceleration of cold pool outflow whose kinetic energy derived from potential energy, leading to strong down flow and resulting in strong convergence; c) lifting of the thunderstorm cold pool outflow, which formed strong low‐level vertical wind shear between north outflow at the low level and south air flow on the surface, and enhanced the dynamic instability. The second intensifying phase of thunderstorms in the eastern urban area, is mainly due to the presence of the strong meso‐γfrontal zone in the boundary layer and the balance between positive and negative vorticity. a) the confrontation in the Chaoyang district of the cold pool outflows of the well‐organized thunderstorm and the low‐level warm tongue, resulting in a strong perturbation temperature gradient; b) surface pressure increasing in the cold pool area, resulting in steep pressure gradient zone; c) strong cold pool outflows and considerable forces in the Chaoyang district with southerly warm air collision produced a strong horizontal wind shear. The above conditions constituted a strong meso‐r frontal zone in the boundary layer which was conducive to the second intensification of the thunderstorm in Chaoyang district. In addition, the coexistence of the updraft in front of the thunderstorm and downdraft at the rear of the thunderstorm in a long time, and the blance between the negative vorticity which was generated by the outflow in the surface layer and the positive vorticity which was generated by low‐level wind shear, were also very favorable for the maintenance and development of the thunderstorms.(5) Main causes for the emergence of the "729" thunderstorms. a) the maintenance of the southerly warm air near the ground and the advancing of easterly to the west, provided a source of water vapor for the emergence of the thunderstorm; b) the intrusion of cold air above the boundary layer, was favorable for the development of convective instability. c) the meso‐r convergence system formed on the west side of piedmont near the surface, was the trigger mechanism for the formation of new thunderstorm cells.