Distinct Mixing Regimes in Shallow Cumulus Clouds

Abstract

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.

The Role of the Toroidal Vortex in Cumulus Clouds' Entrainment and Mixing

“…The questions it raises are important not only for the cloud physics community that desires to better understand clouds; It is also pivotal for weather and climate prediction, as the parameterizations of Cu clouds and their mixing are one of the main sources of uncertainty in largescale models. Figure 8 gives a schematic picture of the cloud's structure that arises from a compilation of this work with a few others (Arieli et al, 2024;Eytan et al, 2021Eytan et al, , 2022Pinsky et al, 2021Pinsky et al, , 2022.…”

“…Because there was some uncertainty regarding both the exact location of the cloud edge (i.e., sub-grid location) and the particle's position, the entrance or exit to the cloud was defined only when the distance to the cloud edge (DFCE; calculated as in Eytan et al (2022)) was larger than the simulation resolution (10 m). For more details see Arieli et al (2024).…”

“…This zone, whose presence is further corroborated by high-frequency in situ measurements (Gerber, 2000;Katzwinkel et al, 2014) and a theoretical diffusion-evaporation toy model , suggests that turbulent diffusion by mixing can explain only the intense dilution that occurs up to several tens of meters away from the edges and into the cloud. Arieli et al (2024) had recently used Lagrangian tracers to show how the cloud can be separated into three distinct regimes: transition zone (cloud's skin), core, and periphery. This raises the question of what process generates diluted volumes deep in the cloud.…”