Previous work (Romps, 2010) found large entrainment rates of ~100% per kilometer for deep convection using a new technique for large-eddy simulations called "Eulerian direct measurement". This technique, however, was unable to pinpoint the reasons for this high entrainment rate. In this study, massless Lagrangian particles are used to perform "Lagrangian direct measurement" of the convective entrainment rate. The results confirm the large entrainment rates found previously, and the particle histories provide an explanation: fast recycling of air into and out of the cloud. These results show that clouds rapidly entrain both environmental air and their own detritus.
Sample trajectories from a large-eddy simulation of a cumulus congestus. Colors correspond to the mass fraction of condensed water.