The Analysis on A Shallow Beta-meso-scale Convective System Causing Strong Precipitation - Part I: Kinematical and Thermodynamic Structure

Abstract A strong ephemeral precipitation hits the south west of Beijing in the late afternoon on July 30th, 2008. The convection develops in the southeasterly flow which is between a landing Typhoon named "Phoenix" and Pacific High. Besides, there is also a trough, at the same time, approaching to Beijing at 500hPa. The data from AWS and VDRAS (Variational Doppler Radar Analysis System) are used in the analysis of both the three-dimension structure and developmental mechanism of a Beta-meso-scale system giving cause for the precipitation. The results show that even though the convection is shallow comparatively (5 to 6km), its precipitation rate is very high and the structure is complex too. The primary members of this Beta -meso-scale system are: a high in the surface allobaric filed (ie. max positive center of pressure perturbation), a cool heap from surface to 5 to 6km, a convergence line in the BL and a meso-scale front zone. It is also worthy of notice that there is a vertical deformation field, which is an important characteristic of the kinematical structure. The role of this deformation field is to strengthen the convergence further at the lower levels and restrict the convection below the mid troposphere. At the same time, there is no strong synoptic divergence in the upper troposphere. Consequently, the system is strong but not very deep. Moreover, the cool heap is another important member of the system. It is relative to the evaporation cooling of the rain drops and upper cold air pouring down into the system. The cool heap cools the air which flows in and leads to a succedent descent. As result, the surface pressure below the cool heap goes up. The allobaric field brings the change of wind direction in a small area, which is the main mechanism for the formation of the convergence line. Meanwhile, in the cool heap, the air between 800 and 5000m is saturated, which is the main thermodynamic character of the meso-scale system. According to the sounding at 20 BJT, the LCL is only about 500m. Furthermore, there is some moisture convergence in the convection area and specific humidity decreases along with height. Therefore, when the damp air is brought to upper levels by strong anaflow, it will lead to obvious condensation. It is why a shallow convective system could cause strong precipitation. In conclusion, the kinematical and thermodynamic structure of the convective system in Beijing is quite different from that developing near MeiYu front in south China.

Key Words: kinematical and thermodynamic structure shallow convection strong precipitation meso-scale cool heap vertical deformation field