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

Shallow convective clouds play a crucial role in Earth's energy budget, as they modulate the radiative transfer in the atmosphere and participate in the vertical transport of aerosols, energy, and humidity. The parameterizations representing these complex, vital players in weather and climate models are mostly based on a description of steady-state plumes and are a source of major uncertainty. Recently, several studies have shown that buoyant thermals are inherent in atmospheric convection and contain a toroidal (ring) vortex. This work studies those vortices in growing shallow cumulus (Cu) clouds using high-resolution (10 m) Large Eddy Simulations that resolve these vortices in much detail. Recent analysis of such data showed that small-scale turbulent diffusion is unable to explain the large diluted portion of the cloud. Here we advocate for the important role of the Cu toroidal vortex (TV) in cloud dilution and present the complex dynamics and structure of a Cu TV. Nevertheless, since the vortex dominates the cloud's dilution, simplicity emerges when considering the cloud's lateral mass flux profile. The cloud mixing is quantified using direct flux calculations and Eulerian tracers. In addition, Lagrangian tracers are used to identify the origin of the entrained air and its thermodynamic properties. It shows that most of the air entrained by the vortex is not recycled by the vortex, yet is significantly more humid than the environment. We suggest that the development of new models describing thermals, together with their toroidal vortices, might improve cloud parameterizations in weather and climate models. Shallow convective clouds play a crucial role in climate as they transfer heat in the vertical dimension and affect radiation transfer in the atmosphere. These clouds are much smaller than climate models' resolution, and so they are represented by simplified equations in weather and climate models (parameterizations). This simple representation of such important processes is one of the largest sources of uncertainty in climate models. In particular, the process of clouds mixing with their dry surroundings is known to have a large contribution to the uncertainty reflected by clouds in climate prediction. In this work, we use high-resolution simulations of cumulus clouds to investigate the toroidal (ring) vortex that is located at the top of a rising thermal (a known property of cumulus dynamics). We show that cloud-scale vortices dominate cloud dilution and are at least as important as stochastic turbulent motions which are often considered in mixing parameterizations. These ideas can serve for future parameterizations of shallow cumulus clouds in coarse-resolution models. Coherent and turbulent toroidal (ring) vortices are presented in 10 m resolution LES of shallow cumulus clouds The toroidal vortex dominates the cloud's dilution, as shown by the cloud's lateral mass flux profile and Eulerian tracers analysis Lagrangian tracers reveal that most entrained air is environmental (not recycled by the vortex) with high relative humidity