巨盐核对云滴活化影响的数值模拟研究

利用包含云凝结核(CCN)与巨核(GCCN)的核化,云滴凝结和碰并增长的分档气块模式模拟研究了不同的CCN数浓度、上升气流速度、CCN中值半径以及云底温度等情况下GCCN对CCN活化的影响,结果表明,在水汽供应相对充足的情况下GCCN对云滴活化数浓度的影响并不明显;而当水汽供应相对不充足时,增加GCCN至1 cm-3的量级以上可以有效减少CCN的活化数浓度.在水汽供应不充分且其他条件相同的情况下,增大CCN的平均直径或是增加云底温度都可以使GCCN对云滴活化的抑制作用增强.对比分析不同的GCCN数浓度对清洁大气和污染大气云底以上300 m高度处粒子谱型的影响可以看出,在水汽供应不充分的条件下加入GCCN,初始时刻CCN的数浓度对GCCN产生的大云滴数目及云滴谱宽的影响较小.在水汽供应相对充足的情况下,GCCN对CCN活化基本没有抑制作用,但此时在高过饱和度峰值下生成的大量小云滴争食水汽,反而导致云滴群凝结增长速度小于水汽供应相对不充足的情况,此时加入的GCCN可以先活化形成大云滴.

Numerical Simulation of the Impact of Giant Cloud Nuclei on Cloud Droplet Activation

The effect of giant cloud condensation nuclei (GCCN) on the cloud droplet activation at different initial concentrations, median cloud condensation nuclei (CCN) radii, and updrafts and cloud base temperatures is investigated using a bin model that considers the nucleation of CCN and GCCN, condensation, coalescence, and collection of water droplets. A numerical simulation shows that the total number concentration of active cloud droplets simply increases with increasing GCCN if the water vapor supply is sufficiently strong; in contrast, when the updraft and water vapor supply are weak, it decreases with higher GCCN number concentration. Further, a shift in the media diameters of the CCN distribution to larger sizes or a decrease in the cloud base temperature both increase the relative impact of GCCN on the activation of cloud droplets. A comparison of cloud droplet spectra 300 m above the cloud base in clear and polluted conditions shows that the initial CCN concentration has little effect on GCCN activation, which would produce large cloud droplets and broaden the size spectra. However, at a large updraft, the addition of GCCN has little impact on the number concentration of droplets, but activation of a large number of cloud droplets at high peak supersaturation exhausts the water vapor and decreases the growth rate of water droplets. Thus, added GCCN can first be activated to form big droplets and facilitate the collection process.