Entrainment in dry and moist thermals

TL;DR

This study uses direct numerical simulations to analyze entrainment in dry and moist thermals, revealing how buoyancy and turbulence influence entrainment rates and vortex structures in different stratification conditions.

Contribution

It demonstrates the effects of buoyancy and turbulence on entrainment and vortex formation in thermals, highlighting the need for high Reynolds number simulations for realistic cloud modeling.

Findings

Turbulence has a minor role in dry thermals at Re < 10^4.

Entrainment rate increases with buoyancy from condensation or unstable stratification.

Condensation and turbulence together create small-scale vorticity, disrupting vortex rings.

Abstract

We study entrainment in dry thermals in neutrally and unstably stratified ambients, and moist thermals in dry-neutrally stratified ambients using direct numerical simulations (DNS). We find, in agreement with results of Lecoanet and Jeevanjee \tb{[J. Atmos. Sci. 76(12), 3785-3801, (2019)]} that turbulence plays a minor role in entrainment in dry thermals in a neutral ambient for Reynolds numbers $Re \lesssim 10^4$. We then show that the net entrainment rate increases when the buoyancy of the thermals increases, either by condensation heating or because of an unstably stratified ambient. This is in contrast with the findings of Morrison et al. [J. Atmos. Sci. 78(3), 797-816, (2021)]. We also show that the role of turbulence is greater in these cases than in dry thermals and, significantly, that the combined action of condensation heating and turbulence creates intense small scale vorticity, destroying the {coherent} vortex ring that is seen in dry and moist laminar thermals. These findings suggest that fully resolved simulations at Reynolds numbers significantly larger than the mixing transition Reynolds number $Re=10^4$ are necessary to understand the role of turbulence in the entrainment in growing cumulus clouds, which consist of a series of thermals rising and decaying in succession.