Influence of Cloud Condensation Nuclei Number Concentration on the Development of a Squall Line Process in East China

The weather research and forecasting (WRF) model was used to simulate a squall line that occurred in East China on 2 May 2016. Through numerical experiments involving five different initial cloud condensation nuclei (CCN) concentrations, the influence of pollution on the development of the squall line was studied, and the Q vector divergence was used. The results show that under mild pollution, the linear structure of the squall line was the most complete and the strong convective structure was the most compact. Excessively heavy or too light pollution led to the dispersion and even rupture of the strong convective structure of the squall line. Moreover, light and moderate pollution levels corresponded with the strongest precipitation, while too heavy or too light pollution had a limiting effect on precipitation. With increase in the pollution level, from a clean atmosphere to light pollution, the width of the strong echo region increased, the height of the strong echo increased, and the echo intensity increased. However, with further increase in the pollution level, the width of the strong echo region decreased, the height of the strong echo decreased, and the echo intensity decreased. In the case of mild pollution, the vertical velocity, vorticity, and divergence were stronger. The intensity was the strongest under pollution; the high pressure of thunderstorm, low pressure of wake, and strength of cold pool gradually weakened as the pollution deepened, but under heavy pollution, the high pressure of thunderstorm, low pressure of the wake, and strength of cold pool were enhanced. As the pollution deepened, the instability of the flow cloud area first increased and then weakened. Under mild pollution, the instability was the strongest. Except for the snow particles, the mixing ratio of other water condensate particles reached the maximum under mild pollution, while the mixing ratio of the snow particles reached the maximum under severe pollution. The Q vector divergence equation of non-geostatic non-static equilibrium was derived. The theoretical analysis and calculation results show that the Q vector convergence area corresponds to the ascending motion, and the divergent area corresponds to the sinking motion. In the case of relatively clean and light pollution, the Q vector divergence is the strongest.