Measurement of Convective Entrainment Using Lagrangian Particles

Abstract

agrangian particle tracking is used in a large-eddy simulation to study an individual cumulus congestus. This allows for the direct measurement of the convective entrainment rate and of the residence times of entrained parcels within the cloud. The entrainment rate obtained by Lagrangian direct measurement is found to be higher than that obtained using the recently introduced method of Eulerian direct measurement. This discrepancy is explained by the fast recirculation of air in and out of cloudy updrafts, which Eulerian direct measurement is unable to resolve. By filtering these fast recirculations, the Lagrangian calculation produces a result in very good agreement with the Eulerian calculation.The Lagrangian method can also quantify some aspects of entrainment that cannot be probed with Eulerian methods. For instance, it is found that more than half of the air that is entrained by the cloud during its lifetime is air that was previously detrained by the cloud. Nevertheless, the cloud is highly diluted by entrained air: for cloudy air above 2 km, its mean height of origin is well above the cloud base. This paints a picture of a cloud that rapidly entrains both environmental air and its own detritus.