The Roles of Surface Convergence Line and Upper‐Level Forcing on Convection Initiation Ahead of a Gust Front: A Case Study

Abstract

AbstractUsing observation data and the simulations of Variational Doppler Radar Analysis System (VDRAS), the convection initiation (CI) ahead of a gust front over the Yangtze River Delta in Jiangsu province, China on 5 June 2009 was studied. The CI was located in the exit region of the upper level jet with moderate CAPE. It is found that, there was a northeast‐southwest convergence line near the ground ahead of the gust front, which could not trigger the new convection alone. The upper‐level updraft above the surface convergence line could not trigger the new convection alone either. The superposition of the updraft in the upper and low levels promoted the initiation and development of the new convective cell. Finally, the new convective cell developed continuously to the southwest and southeast, and extended to the southeast under the influence of the upper‐level wind. And the sensitivity experiments show that the surface convergence line and the upper‐level convergence were both important for the CI, especially for the strong convection. This discrete CI ahead of the gust front was influenced by a combination of low‐level and upper‐level factors. In the process of the CI, the updraft in the upper level intensified faster than that in the low level gradually. The buoyancy force was greater than the vertical perturbation pressure gradient force (VPPGF) in the low level, while the VPPGF increased faster than buoyancy in the upper level. Therefore, the dynamics of the updraft in the low and upper levels were different.