Entrainment of aerosols and their activation in shallow cumulus clouds - Large-Eddy Simulations with an Embedded Lagrangian Particle Model

• Applicant: Professor Dr. Siegfried Raasch
• Subject Area: Atmospheric Science
• Term: from 2014 to 2017
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 252353065

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) with an embedded Lagrangian particle model (LPM) to parameterize cloud physics, in which Lagrangian particles (so-called super-droplets) represent a certain number of real droplets/aerosols with same features (e. g., droplet size, aerosol mass). 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 number of cloud droplets during thelife-cycle of an individual cloud, in a cloud ensemble (i. e., dependencies on cloud features like cloud height) or under different background aerosol conditions (maritime and continental).

• DFG Programme: Research Grants
• Participating Persons: Professor Dr. Jörg Schumacher; Dr. Holger Siebert