Resolved Entrainment in Thermals using Direct Numerical Simulations

Recent work suggests thermals may be the basic unit of atmospheric convection. Various authors have implemented algorithms for identifying thermals in simulations, and studying their aggregate statistics, including the net entrainment rate. However, it is numerically infeasible to simulate systems large enough to self-consistently generate many simultaneous thermals, while also capturing turbulent entrainment due to the thermals. We thus present direct numerical simulations of idealized dry thermals at a range of Reynolds numbers. At low Reynolds numbers, the thermals entrain due to diffusion, and resemble the Hill's vortices commonly found in previous simulations. At higher Reynolds numbers, a turbulent cascade mediates entrainment, leading to much higher entrainment rates. We analyze the bulk entrainment properties of turbulent thermals as a way to extrapolate existing convection simulations to the more realistic regime of turbulent entrainment.