Using Lagrangian drops to study the raindrop size distribution in shallow cumulus

Despite the ever increasing grid resolution, understanding precipitation remains one of the major challenges in numerical weather prediction as well as climate modelling. To investigate warm rain microphysical processes on a particle-based level, a Lagrangian approach for simulating raindrops in LES is presented, which closely relates to the recently developed super-droplet method, but focuses solely on the raindrop phase. Such a framework explicitly includes all relevant processes such as accretional growth, selfcollection among raindrops, evaporation and sedimentation, while the Eulerian LES provides the highly resolved, time-dependent, thermodynamical background fields.

We will introduce the concept of Lagrangian drops and apply the method to investigate the evolution of raindrops in isolated shallow cumulus clouds. Differences and similarities to two-moment bulk rain microphysics simulations are shown with a focus on the process level of accretion and evaporation rates. In particular, sensitivities of the bulk rain microphysics concerning the assumed shape of the raindrop size distribution are considerably larger than uncertainties of the Lagrangian drop method. Furthermore, we will show results on the development of the shape of the raindrop size distribution derived from Lagrangian drop statistics of individual shallow cumulus clouds. We also hope to present results from a Lagrangian drop simulation of a field of shallow cumulus clouds investigating the role of recirculation and large-eddy hopping of raindrops on the development of precipitation.