The Meso β-Scale Convective System of a Heavy Rain Event on July 10, 2004 in Beijing

An analysis on the meso β-scale convective system of a heavy rain event on July 10, 2004 in Beijing is performed by using the special observational data, including automatic meteorological stations data, radar images and satellite images and NCEP/NCAR reanalysis data on the basis of successful simulation. It is found that the heavy rainfall process is generated by a meso β-convective system which is produced in a large scale warm area. The short wave trough in the mid level of troposphere, the convergence between the southwest air current from west wind trough and the southeast air current from the north part of warm shear line in low troposphere provide a good background condition. The meso β-scale convective system is formed by the mergence of two meso-scale convective clusters, it shows an ellipse shape structure of a horizontal scale of 150 km×100 km and the time scale of about 5 hours. It shows the features of the meso-scale convergence line(or convergence center)in low levels stream fields during its occurrence and development stage, and the strong meso-scale convective clouds echo band and meso-scale convergence line exhibited in radar reflectivity image and in radar velocity fields are often related with the occurrence and the development of the meso β-scale convective system. During the stage of strong development, the meso β-scale convective system shows strong baroclinity perpendicular features and has a similar structure of slantwise updraft current to convective storm. Its occurrence and development are forced by the meso-scale convergence line of low troposphere in strong convective instability condition and a warm tongue below 700 hPa. The convergence between the southern air current and eastern air current and the invading of the cold air in the boundary layer lead to the strengthen of the energy front, which is helpful to induce the generation of the meso β-scale convective system.In addition, the cloud top infrared brightness temperature(TBB)of the meso β-scale convective system that induces the heavy rain on July 10, 2004 in Beijing is at-45 ℃ or so, and the updraft airflow reaches the height of around 300 hPa, which means in this case the convection is only activating in the low level of the troposphere in contrast with the deep convective systems of meso β-scale convective system in the mid and lower reaches of Yangtze River and South China, which symbolize with infrared brightness temperature between-70 and-85 ℃, and the updraft airflow reaches the top of troposphere. Future research is needed on whether this conclusion is characteristic for popular convective heavy rain process in North China.