Meso-β scale torrential rain affected by topography and the urban circulation

Some theoretical features of meso-β scale torrential rain, which are caused by joint action of topography and the urban heat island, are gained by mesoscale dynamic meteorology theory and scale analysis. Using observation datasets with high spatial-temporal resolution based on auto-weather station network and wind profile data from two profilers which are located at different positions, most of the theoretical features are confirmed by three cases which occurred in Beijing in the summer of 2006. The results indicate that (1) the temperature gradient in front of mountains, mainly caused by the urban heat island, is able to engender a relatively isolated vertical wind shear near the windward slope, and the shear is much more important to grow, develop and maintain the mesoscale convective system. The closer the mountain is to urban areas, the stronger the temperature gradient in front of mountains is, and the local stronger vertical wind shear is easy to be at the position. On the other hand, the response time of strong vertical wind shear depends on the intensity of temperature gradient. (2) Once stronger convective precipitation begins on the windward slope, the positive feedback between rainfall intensity and horizontal wind velocity toward the windward slope will appear, and the process is an essential condition to form meso-β scale torrential rain. (3) The stronger the terrain grade is, the stronger ascending motion will be forced and the smaller horizontal-scale mesoscale weather system will be stirred; in front of smoother topography, however, the mesoscale system at a relatively larger horizontal scale is easy to be formed. (4) generally, most of the mesoscale torrential rain processes, which are caused by joint influence of topography and thermodynamic urban circulation, should occur in front of mountains in the evening or the early morning.