Mechanistic study of a downhill merging and enhancement of convection in Beijing

Numerical simulation of the merging of a thunderstorm cluster from the mountain area near Beijing and a thunderstorm over the adjacent plains on 23 August 2021, along with a diagnosis and analysis of the cold pool and vertical motion, reveals the following: (1) The thunderstorm cluster in the mountain area moved slowly westward, weakening during its descent, whereas the thunderstorm cluster in the urban area moved rapidly eastward and intensified. Eventually, the two thunderstorm clusters encountered each other at the foot of the mountain and organized into a linear convective system. (2) Prior to merging, the thunderstorm cluster in the mountain area was blocked by warm advection to the east, causing the system to slow down, the cold pool to weaken, and the convergence and ascent associated with the cold pool outflow to diminish. In contrast, the thunderstorm cluster over the adjacent plains was driven by cold advection to the west, accelerating the system's movement, strengthening the cold pool, and enhancing the convergence and ascent driven by the cold pool outflow. After the thunderstorm clusters merged, the convergence of the northwesterly and southeasterly winds, as well as precipitation, led to the rapid accumulation of cold air, strengthening the cold pool and its upward development, which acted similarly to a terrain feature, further enhancing convergence and ascent. (3) The vertical motion reveals that before merging, the thunderstorm cluster in the mountain area was dominated by negative buoyancy at lower levels, which suppressed the development of ascent, whereas the thunderstorm cluster over the adjacent plains was driven by positive disturbances in the vertical pressure gradient force, which increased ascent. After the merging, the positive disturbances in the vertical pressure gradient force dominated below 2 km, and as the vertical motion increased, the positive buoyancy gradually became the dominant driver, further strengthening the ascent. The analysis suggests that the positive potential temperature disturbance and the southeasterly or southerly winds over the adjacent plains had opposing effects on the two approaching thunderstorm clusters, with the thunderstorm cluster over the adjacent plains taking the lead during the merging process.

摘要

以往针对北京平原雷暴群和下山雷暴群合并过程的研究相对较少, 利用WRF模拟数据对2021年8月23日一次此类对流活动过程分析发现： (1) 雷暴群前侧风场和热力条件的差异, 使得山区雷暴群移动和冷池发展受阻, 而平原雷暴群则相反, 最终在山脚处合并, 增强后的冷池起到类似地形作用, 增强辐合和上升运动； (2) 合并前, 山区雷暴群低层负热力浮力抑制上升运动发展, 平原雷暴群低层正扰动垂直气压梯度力加速上升运动, 合并后, 正扰动垂直气压梯度力 (2 km以下) 和正热力浮力 (2 km以上) 共同驱动上升运动发展. 本文主要对冷池和垂直运动分析, 以期为北京地区的临近预报提供一些有用的科学参考.