Environmental & Climate Sciences Department Seminar

Although Lagrangian methods for the representation of cloud microphysics date back more than half a century, they have regained considerable attention in the last decade due to the advent of so-called Lagrangian cloud models (LCMs). Today, LCMs are not only considered a valuable alternative to commonly applied Eulerian cloud models (ECMs), but also the future of cloud microphysical modeling. The main difference between LCMs and ECMs is the representation of cloud microphysics, e.g., the cloud droplet size distribution (DSD): ECMs discretize the DSD by several bins or only predict a few statistical moments of it. LCMs, on the other hand, model the DSD by Lagrangian particles, each representing an ensemble of identical droplets. This talk will cover the fundamentals of LCMs, covering the implementation of warm-phase microphysical processes, the differences and advantages to ECMs, as well as novel analysis techniques only possible in LCMs. Subsequently, I will focus on a recent approach for the modeling of unresolved supersaturation fluctuations to be applied in LCMs. Since LCMs are typically coupled to a large-eddy simulation (LES) model for predicting dynamics and thermodynamics, supersaturation fluctuations are constrained by the LES resolution. Using the new approach, supersaturation fluctuations down to the Kolmogorov length scale may be considered, including their specific effects on cloud microphysics, most importantly typically unresolved inhomogeneous mixing. Results from warm-phase boundary layer cloud simulations will be presented, focusing on the entrainment process in stratocumulus clouds. Finally, an initial view toward the modeling of entrainment and mixing in mixed-phase clouds will be given, assessing the importance of inhomogeneous microphysical processes in these clouds.