A modeling study of convection initiation prior to the merger of a sea-breeze front and a gust front

Abstract

Convection initiation (CI) prior to the merger of a sea-breeze front (SBF) with a gust front (GF) in North China is investigated using a real-data Weather Research and Forecasting (WRF) simulation with a high resolution of 444.4m. The overall evolution of the GF and SBF is well reproduced by the simulation. The GF was produced by the decaying convective storm over northern Beijing, while the SBF came from the Bohai Sea. Several convective cells were generated between the two fronts even though they were still about 25–30km far away from each other. During the development of these cells, the low-level convergence and conditional instability averaged within the intermediate area between the two fronts were enhanced significantly, both of which favored the initiation of convection.Vertical momentum budgets were conducted in the intermediate area as well as along the backward trajectories of parcels within a selected convective cell. The vertical acceleration was decomposed into dynamic and buoyant components, respectively. The diagnostic results showed that the dynamic acceleration dominated in the low level, while buoyant acceleration became evident only when the parcel reached a high altitude above 2km. Therefore the dynamic forcing appeared to be more relevant to CI. The dynamic acceleration was further decomposed into four terms based on anelastic approximation. The positive dynamic acceleration was mainly caused by fluid extension associated with the low-level convergence, while fluid twisting in the vertical contributed negatively to the dynamic acceleration. The other two terms related to horizontal curvature and height variation of density were negligibly small.