Sensitivity of warm clouds to large particles in measured marine aerosol size distributions – a theoretical study

Aerosol size distribution has major effects on warm cloud processes. Here, we use newly acquired marine aerosol size distributions (MSD), measured in-situ over the open ocean during the Tara Pacific expedition (2016–2018), to examine how the total aerosol concentration (Ntot) and the shape of the MSD change warm cloud's properties. For this, we used a toy-model with detailed bin-microphysics. The changes in the MSDs affected the clouds' total mass and surface precipitation. In general, the clouds showed higher sensitivity to changes in Ntot than to changes in the MSD's shape, except for the case where the MSD contained giant and ultragiant cloud condensation nuclei (GCCN, UGCCN). For increased Ntot, most of the MSDs drove an expected non-monotonic trend of mass and precipitation. However, the addition of GCCN and UGCCN drastically changed this trend, such that surface rain saturated and the mass monotonically increased with Ntot. GCCN and UGCCN changed the interplay between the microphysical processes by triggering early initiation of collision-coalescence. The early fall-out of drizzle in those cases enhanced the evaporation below the cloud base. Testing the sensitivity of rain yield to GCCN and UGCCN revealed an enhancement of surface rain upon the addition of larger particles to the MSD, up to a certain particle size, when the addition of larger particles resulted in rain suppression. This finding suggests a physical lower bound can be defined for the size ranges of GCCN and UGCCN.气溶胶粒径分布对暖云过程具有显著影响。本研究利用2016–2018年塔拉太平洋科考航次期间在开阔洋面原位测得的新获取海洋气溶胶粒径分布（marine aerosol size distribution，MSD），探究气溶胶总浓度（total aerosol concentration，Ntot）与海洋气溶胶粒径分布形态如何调控暖云特性。为此，本研究采用了搭载精细分档微物理过程的toy模型。海洋气溶胶粒径分布的变化会影响云总质量与地面降水量。总体而言，云系对气溶胶总浓度变化的敏感性高于对海洋气溶胶粒径分布形态变化的敏感性，但当粒径分布中包含巨云凝结核（giant cloud condensation nuclei，GCCN）与超巨云凝结核（ultragiant cloud condensation nuclei，UGCCN）时除外。当气溶胶总浓度升高时，多数海洋气溶胶粒径分布会呈现出预期的云质量与降水的非单调变化趋势。但加入GCCN与UGCCN后，这一趋势发生了显著改变：地面降水趋于饱和，且云总质量随气溶胶总浓度升高呈单调递增态势。GCCN与UGCCN通过触发碰撞-合并过程的提前启动，改变了微物理过程间的相互作用格局。此类情形下毛毛雨的提前沉降增强了云底下方的蒸发过程。通过探究降水产率对GCCN与UGCCN的敏感性，研究发现：向海洋气溶胶粒径分布中加入更大粒径的粒子时，地面降水会得到增强，直至达到某一临界粒径；若继续加入更大粒径的粒子，则会抑制降水。该研究结果表明，可针对GCCN与UGCCN的粒径范围定义一个物理下限阈值。