Analysis of Vertical Motion in the Triggering Process of a Convective Event

This study analyzes the convective process triggered by a convergence line intersection in the middle part of the Taihang Mountains in North China on July 31, 2019, investigating the dynamic processes influencing its vertical acceleration. By analyzing the equation of vertical motion, vertical acceleration can be decomposed into a dynamic term and a buoyancy term. Diagnostic results indicate that the buoyancy term dominates this vertical acceleration motion. Analysis of the temporal evolution of the right-hand side term in the vertical motion equation along the backward trajectory of the selected convective cell reveals that the lower-level buoyancy term gradually weakens with horizontal convergence, but significantly strengthens when the air parcel rises above 3.5 km altitude. According to the buoyancy tendency equation, local buoyancy variations are primarily influenced by the stratification term and the three-dimensional divergence forcing term. The former represents the vertical transport of stable stratification from the ambient atmosphere upward, which is unfavorable for maintaining positive buoyancy. However, the latter exhibits a typical “convergence—divergence—convergence” vertical alternating distribution prior to convective triggering. At lower levels, horizontal convergence dominates, with local mass compensation favoring vertical ascent. Between 2—6 km altitude, vertical divergence prevails at lower levels while horizontal divergence dominates at mid-to-upper levels. This maintains three-dimensional divergence within this height range, triggering vertical redistribution of air masses. This process effectively enhances positive buoyancy upward, promoting accelerated vertical ascent and triggering intense convection.