对流云对大气气溶胶和相对湿度变化响应的数值模拟

利用二维面对称分档云模式研究了气溶胶颗粒物浓度和尺度谱分布对混合相对流云微物理过程和降水的影响, 并重点讨论了气溶胶效应随环境相对湿度的变化。结果表明, 在初始热力和动力条件相同的情况下, 相对清洁的海洋性云在发展和成熟阶段能更有效地产生雨滴、 冰晶和霰粒, 形成更强的雷达反射率。随着气溶胶浓度增加, 比如在本文模拟的污染大陆性云中, 气溶胶粒子数浓度的增加限制云滴增长, 不利于降水粒子的形成。模拟结果也发现, 环境相对湿度对气溶胶效应有显著影响, 即当地面相对湿度从50%增大到70%时, 所模拟的云从浅对流泡发展为深对流云; 气溶胶对云微物理特性和降水的影响在干空气中较小, 但在湿空气中表现非常显著, 这与前人结果一致。随着相对湿度的增加, 冰相粒子出现的时间提前, 增长加快, 云砧范围扩大, 但相对来说, 降水起始时间对相对湿度的变化比气溶胶更敏感。

The Response of Convective Clouds to Aerosol and Relative Humidity: A Numerical Study

Using a two-dimensional spectral resolving cloud model, the effects of number concentration and size distribution of aerosol particles and relative humidity on mixed-phase convective clouds are investigated, and the changes in aerosol's effects with relative humidity are discussed. The results show that, under the same initial thermodynamic and dynamic conditions, the clean maritime clouds produce raindrops, ice crystals, and graupel particles more effectively in both development and mature stages, resulting in a stronger radar reflectivity. With the increasing in aerosol number concentration, such as in the polluted continental clouds simulated here, excessive aerosol particles limit the growth of cloud droplets and the formation of precipitation particles. It is also found that the aerosol effect on cloud properties and precipitation is strongly dependent on the environmental relative humidity, that is, as relative humidity increases from 50% to 70% at the surface, the cloud changes from shallow cumulus cell to a deep convective cloud. The aerosol effect is not as important in the dry air as in the humid air. This is consistent with the previous studies. The increase of relative humidity makes ice particles form earlier and grow faster, and the cloud anvil expands to a larger area. However, the rain initiating time is more sensitive to relative humidity than to aerosol particle concentration.