Influence of Vortex Couplets Following Cold Fronts Related to the Lateral Friction by Mountains on Elevated Thunderstorm Initiation

Abstract

Convective initiation (CI) over complex terrain poses a difficult problem. By conducting numerical simulations and surface friction experiments, this study investigated the dynamic mechanism of elevated thunderstorm initiation on the eastern side of Dalou Mountain. The evolution characteristics of vortices before and after CI, as well as the impact of lateral friction of the mountain on the formation of vortices, were assessed. Simulation results showed that a pair of vortex couplets following the cold front (CFVs) were associated with lateral friction of the terrain, where the northern and southern sides experienced anticyclonic and cyclonic vortices, respectively. Easterly winds between CFVs could be an indication for CI. CFVs had an unsymmetrical structure in the vertical direction, which had a high depth in the south and low depth in the north, and exhibited nocturnal intensification. In the sensitivity experiments, lateral friction showed the positive impact on the low-level easterlies from afternoon to evening, following the rule that the greater the surface lateral friction, the stronger the easterly. The promotion of positive vorticity in the southern vortex by lateral friction was obviously above 1,500 m, while the enhancement of negative vorticity in the northern vortex was mainly reflected in the low-level layer. The variation of the latter was not as large as that of the former due to the difference of the south and the north terrain. These results provide an important perspective on the qualitative relationship between the lateral friction, vortex couplets, and airflow intensity.