Investigating the initiation and propagation processes of convection in heavy precipitation over the western Sichuan Basin

摘要

Heavy precipitation events occur often over the western Sichuan Basin in summer, near the transition zone between the Sichuan Basin and the steep terrain of the Tibetan Plateau. One such event — a heavy precipitation process that occurred on 18–20 August 2010, with clear nocturnal peaks — is chosen as a case to tentatively explore how the convection associated with convective-scale precipitation is initiated and propagated. By utilizing the vertical momentum equation from the viewpoint of separating perturbation pressure into dynamic and thermal parts, it is demonstrated that the vertical momentum is induced by the imbalance of several forces, including the dynamic/buoyant part of the perturbation pressure gradient force and the buoyancy force, with the latter dominating during the nocturnal-peak period. Although a negative value of the dynamic perturbation pressure gradient force partly offsets the positive buoyant forcing inside the strong updraft, the pattern of vertical motion tendency is largely attributable to its buoyancy because of its larger magnitude. Relative to the buoyancy component, the dynamic part of the vertical perturbation pressure gradient is also examined, revealing a smaller order of magnitude. Thus, it is the thermal effect that should be responsible for the initiation and propagation of convection. As for the convective-scale precipitation, it always presents a trailing morphology relative to the strong leading-side updraft. Furthermore, overlapping strong signals of vertical motion and its tendency point towards strong precipitation in the future.摘要青藏高原大地形和四川盆地西侧的陡峭地形过渡带，夏季暴雨频发，易带来泥石流等次生地质灾害。本文选定该地区2010年8月的一次暴雨过程，研究了伴随对流尺度降水的强对流的激发和传播。具体借助于垂直动量方程，并将扰动气压分离为动力和浮力分量，通过动力扰动气压梯度、浮力扰动气压梯度、以及浮力等几种强迫作用的不平衡，探索对流激发和传播的主导因子。经分析发现，在本次暴雨过程中，较之动力过程，热力作用对对流的激发和传播发挥着更为重要的作用，垂直运动及倾向指向未来雨区。