Observational And Model Studies On The Back-Building Processes Of Convective System In Zhejiang And Shanghai

With global warming,the intensity and frequency of heavy rainfall events in Zhejiang and Shanghai have increased.According to statistics,the heavy rainfall events in Zhejiang and Shanghai tends to be short-duration and high-intensity.Short-term torrential rainfall is often related to mesoscale convective system,and the back-building process of mesoscale convective system is an organization which is benefit to the formation of short-term torrential rainfall.The back-building process,a pattern when new cells form repeatedly on the upstream side of the old ones and produce stratiform rain downstream,is very conducive to cause heavy rain.This paper focuses on the theme of the back-building process in Zhejiang and Shanghai.The three short-term torrential rainfall events caused by the back-building processes of the mesoscale convective systems in Zhejiang-Shanghai region from 2013 to 2014 were studied.The formation mechanism of the back-building process in Zhejiang and Shanghai was studied by using observational data and numerical model simulation.And study the back-building processes' role in short-term rainstorms.The main conclusions are as follows:The three torrential rainfall events were short-duration and high-intensity(the 4-hour cumulative rainfall center during the process is over 100 mm).The analysis of the surface element filed shows that the interaction between the surface convergence line and the convectively generated cold pool outflow is an important factor in the initiation and maintenance of the back-building process.In the initial stage of the back-building process,the surface wind streamline showed that there was a vortex-like disturbance at the intersection of the convergence line and the cold pool outflow boundary.The spatial structure of the surface vortex-like disturbance is analyzed by combining the radar retrieved data.It shows that the surface vortex-like disturbance is very shallow and disappears after convective cell generating.The surface vortex-like disturbance is formed by the confluence of different surface airflows,and its presence is beneficial to the convergence enhancement at the triple point,thereby promoting the ascending motion to help the generation of convective cell.The direction of the wind field on the surface convergence line and the direction of the cold pool's motion were analyzed,and it was found that two of the processes were in the same direction and the other was in the opposite direction.The numerical simulation results show that for the same direction case,the cold pool generated by the old convection system outflow and the surface convergence line caused a wind speed convergence which enhances updraft.Besides,a low level of free convection height makes some contribution to the new cells continuously generate with weak ascending motion;and the opposite direction case,the cold pool generated by the old convection system outflow and the surface convergence line caused a wind speed convergence caused a wind direction convergence,which is much stronger than the same direction.The strong convergence can cause strong ascending motion so that the new convection cells can still formed continuously with a high level of convection.The sensitivity test was used to study the contribution of the back-building process to short-term torrential rainfall.The back-building process of the mesoscale convective system was removed and it was found that the intensity of the storm center was weakened.The results of convective cell's precipitation efficiency calculated by the results of the control run indicate that the convective cell has a higher precipitation efficiency,which indicated the high precipitation efficiency of convective cells is another important factor of short-term torrential rainfall.In addition,the surface vortex-like disturbances appearing at the intersection of the surface convergence line and convective generated cold pool are analyzed by the vorticity budget and the circulation budget.The results show that the friction mixing term plays an important role in the early stage of surface vortex-like disturbance generation,and plays a negative role when the development stage of the surface vortex-like disturbance.The tilting term played an important role on the development and mature stage of surface vortex-like disturbance.