Dynamics of the Cloud-Environment Interface and turbulence effects in a LES of a growing cumulus congestus

Abstract

Abstract A giga-large-eddy simulation of a cumulus congestus has been performed with a 5-m resolution in order to examine the fine-scale dynamics and mixing on its edges. At 5-m resolution, the dynamical production of subgrid turbulence clearly dominates over the thermal production, while the situation is reversed for resolved turbulence, the tipping-point occurring near the 250-m scale. Concerning cloud dynamics, the toroïdal circulation already obtained in previous observational and numerical studies remains, with a strong signature on the resolved turbulent fluxes, the most important feature for the exchanges between the cloud and its environment even though numerous smaller eddies are also well resolved. The environment compensates for the upward mass flux through a large-scale compensating subsidence and the so-called “subsiding shell” composed of cloud-edge downdrafts, both having a significant contribution. A partition is used to characterize the dynamics, buoyancy and turbulence of the inner and outer edges of the cloud, the cloud interior and the far environment. On the edges of the cloud, downdrafts caused by the eddies and by evaporative cooling effects coexist with a buoyancy reversal while the cloud interior is mostly rising and positively buoyant. An alternative simulation, where evaporative cooling is suppressed, indicates that this process reinforces the downdrafts near the edges of the cloud and causes a general decrease of the convective circulation. Evaporative cooling has also an impact on the buoyancy reversal and on the fate of the engulfed air inside the cloud.