Distinct Mixing Regimes in Shallow Cumulus Clouds

Understanding the nature of mixing between cloudy air and its surroundings is an important and yet, open question. In this research, we use high-resolution (10 m) bin-microphysics Large Eddy Simulation of a cumulus cloud, together with a Lagrangian passive tracer tracking method, to study mixing. We analyze the passive tracers as a function of their trajectories and the thermodynamic conditions they undergo inside and outside the cloud. Three main mixing regimes (core, periphery, and skin) are identified, each determining a subset of tracers with similar trajectories. These mixing regimes can be observed throughout the cloud's lifetime and they provide evidence for the presence of an undiluted core in shallow cumulus clouds. At the dissipation stage, a fourth regime is identified: cloud-top entrainment followed by downdrafts.