Large-eddy simulation of convective clouds with explicit simulation of cloud droplets via Lagrangian particles

This study presents a new approach of combining a large-eddy simulation (LES) model with a Lagrangian model in order to perform high-resolution LES of three-dimensional convective clouds in which hundreds of millions of cloud droplets (Lagrangian particles) are simulated explicitly. Therefore for each particle the equation of motion including gravity and friction is solved. Furthermore the droplets can change their size by evaporation or condensation and even the droplet growth by collisions is considered. Consequently the liquid water within the model is completely transferred via the Lagrangian particles. The droplet growth by collisions is parameterized with the help of a collision kernel whereas the change of its radius by condensation/evaporation is explicitly calculated from the diffusion equation. In order to achieve realistic numbers of droplets in the clouds a weighting factor is introduced, thus every simulated droplet represents a very high number of real cloud droplets. The most important advantage of this kind of simulation is the possibility to analyze the spatial and temporal evolution of the droplet spectrum, the spatial distribution of the droplet concentration and the droplet tracks directly. These information are very useful and will be used later on for investigations on in-cloud turbulence effects and entrainment processes. Prior to that simulations with single cumulus clouds have been performed to validate this method. The results are very promising and show e.g. the broadening of the droplet spectrum and its evolution into a bimodal distribution. Aside from that the results of a simulation based on the BOMEX shallow cumulus case will be presented.