Entrainment in Resolved, Dry Thermals

Entrainment in cumulus convection remains ill-understood and difficult to quantify. Entrainment has been postulated to obey a 1/r scaling, but this scaling has not been firmly established. Here, we study the classic case of dry thermals in a neutrally stratified environment using fully resolved direct numerical simulation. We combine this with a thermal tracking algorithm which defines a control volume for the thermal at each time, allowing us to directly measure entrainment. We directly verify the 1/r scaling law. We also vary the Reynolds number (Re) of our thermals between laminar (Re~600) and turbulent (Re~6000) regimes, finding only a 20% variation in entrainment rate, suggesting that turbulence is not necessary for entrainment.