Investigation of isobaric mixing as a mechanism for boundary-layer cloud formation

Convective clouds generally form when air parcels rise, expand, cool down, and create supersaturated conditions to activate aerosols as cloud droplets. Hence, the lifting condensation level (LCL) is generally considered to be a predictor of the cloud base height. However, it is common to detect clouds formed below the LCL. These small clouds are characterized by relatively low liquid water content and a few-micron radius cloud drop. Although these clouds significantly contribute to Earth’s radiation budget, it is still unclear as to how they formed and maintained. In situ measurements of such clouds are also very rare. It is therefore still a challenge to explain what mechanism supports such cloud formation. This study reports on an investigation of the formation mechanism for marine clouds observed in horizontal penetrations during one flight of the Aerosol and Cloud Experiments in Eastern North Atlantic (ACE-ENA) campaign.

We analyze a dataset from the aircraft and scanning lidar measurements to investigate how the droplet size distributions and cloud base heights varied at different altitudes. A prominent and nearly-constant peak at 10 μm diameter in the concentration of new cloud droplets is found at various altitudes. This makes bimodal and multiple-peaked cloud droplet size distributions when they are averaged. However, the local cloud droplet size distributions are narrower than averaged distributions. Some local distributions at various altitudes have a monomodal shape with a peak at 10 μm, suggesting that they are newly formed. The cloud base heights estimated from scanning lidar are also varied, which implies that the small cloud formation occurs at various altitudes. A mixing diagram analysis based on the Gaussian mixing model shows that isobaric mixing of air with different temperatures and humidities is one of the important processes generating small cloud formation with similar microphysical properties at multiple altitudes.