Entrainment of Aerosols and Their Activation in a Shallow Cumulus Cloud Studied with a Coupled LCM-LES Approach

This study examines how entrainment affects aerosol activation, i.e., the early growth of wetted aerosol towards cloud droplets, throughout the life-cycle of a shallow cumulus cloud simulated with a Lagrangian cloud model (LCM). In this approach Lagrangian particles represent a certain number of real aerosols growing continuously to droplets according to the flow fields simulated by large-eddy simulations (LES).

By discriminating between (i) aerosols entrained by the central updraft at cloud base, and (ii) aerosols entrained by mixing at the cloud's lateral edge, it is shown that central updraft aerosols are the major source for cloud droplets as long as the central updraft prevails. The largest fraction of these aerosols activates at cloud base. A minor fraction, consisting of small aerosols, activates at greater heights where sufficient supersaturations for their activation are reached. However, the activation of laterally entrained aerosols dominates at these heights; being the only source for newly nucleated droplets in the dissipating stage of the cloud in which the central updraft decays. Since laterally entrained aerosols activate in highly turbulent regions, their activation is limited by the short time span in which these aerosols are exposed to activation permitting conditions. This favors the activation of intermediate sized aerosols, which need the shortest time to activate. On the other hand, this does not affect aerosol activation in the central updraft due to their longer duration in the updraft. Therefore, activation process parameterizations solely based on supersaturations are valid for the central updraft, but fail for laterally entrained aerosols.

Furthermore, it is found that on average 30 % of all activated aerosols have been laterally entrained. This fraction increases to 50 % at the end of the cloud's life-cycle. For increasingly pollute environments, this fraction increases to 77 %. This is caused by increased rates of both aerosol activation and deactivation, which adjust the activated aerosols' composition of entrainment heights to the general composition of entrainment heights of all (activated and inactivated) aerosols inside the cloud. Therefore, the sub-cloud layer becomes less important as a source for cloud condensation nuclei in increasingly pollute environments.