Entrainment of aerosols and their activation in shallow cumulus clouds - large-eddy simulations with an embedded Lagrangian particle model
Responsible: Fabian Hoffmann?
Project type: ​DFG research project
Duration: 01/04/2014-31/03/2017

Shallow cumulus clouds are affecting the climate in many ways. One main impact is the reflection of extraterrestrial shortwave radiation (albedo), which is regulated by the amount and spectral distribution of cloud droplets and, therefore, the amount of aerosols. However, only activated aerosols can serve as condensation nuclei for cloud droplets. Primary activation of aerosols takes place at the cloud base by the adiabatic cooling of rising air parcels. For the activation above cloud base (secondary activation) two explanations exist: (i) the activation of aerosols, which are too small to be activated at cloud base, and (ii) the lateral entrainment of aerosols into the cloud and their subsequent activation in the supersaturated environment of the cloud. The general objective of the current project is to obtain an improved understanding of the processes which lead to the activation of aerosols in shallow cumulus by applying large eddy simulations (LES) and a Lagrangian particle model (LPM) to parameterize cloud physics, in which Lagrangian particles (so-called super-droplets) represent a certain number of real droplets/aerosols of the same features (e.g., droplet size, mass of aerosol). This Lagrangian perspective of cloud physics allows tracking of individual droplets/aerosols and the direct calculation of entrainment rates. This is required and will be used to quantify how the different processes of activation contribute to the amount and spectral distribution of cloud droplets during the life-cycle of an individual cloud, in a cloud ensemble (i.e., dependencies on cloud features like cloud height) or under different background aerosol conditions.