Analysis of an enhanced mountain-to-plain convective storm based on potential divergence and the advection of potential instability

The enhanced mountain-to-plain convective storms in Beijing on 22 May 2021 were simulated using the high-resolution Weather Research and Forecasting model, enabling detailed analyses of convective instability characteristics and underlying causes of stability variations. Generalized potential temperature outperformed traditional potential temperature and equivalent potential temperature in capturing instability variations associated with mid-level latent heating and near-surface evaporative cooling. Local instability variance was primarily governed by potential divergence and the advection of potential instability, with these factors exhibiting out-of-phase distributions. Prior to the onset of heavy precipitation, intense downdrafts transported unstable air from higher levels into more stable regions at lower levels, increasing local near-surface instability, which contributed to the formation of heavy precipitation. During the heavy precipitation stage, vertical divergence between slantwise updrafts and downdrafts in the low–middle stable layers led to destabilization, supporting sustained convective development within the precipitation area. At the leading edge of the heavy precipitation, instability enhancement was primarily driven by vertical advection, and less stable air in the lower levels was transported upward, enhancing instability at higher levels. 针对2021年5月22日发生在北京地区的一次雷暴下山增强的过程, 利用WRF模式进行高分辨率数值模拟.在成功模拟下山对流发生,增强和移动的基础上, 对该过程中位势不稳定特征以及引起位势稳定度变化的原因进行诊断分析.研究结果表明, 广义位温在识别由潜热加热与蒸发冷却引起的不稳定性变化方面, 优于传统的位温和相当位温.通过对位势稳定度趋势方程的诊断分析发现, 局地位势不稳定的变化主要受位势散度和位势不稳定平流的主导, 两者在空间呈反位相分布.在强降水开始前, 强烈的下沉气流将高层的不稳定空气输送到较为稳定的低层, 导致近地面不稳定性增加, 为强降水的形成提供了有利条件.在强降水阶段, 低中层大气层结稳定, 同时倾斜上升气流和下沉气流之间的垂直辐散导致了大气向不稳定变化, 维持了对流的持续发展.在强降水的前沿, 主要由垂直平流驱动不稳定性增加, 低层不稳定空气被向上输送, 增强了高层的不稳定性.