A Lagrangian study of interfaces at the edges of shallow cumulus clouds

Abstract:

Interfaces at the edge of an actively growing cloud are studied using direct numerical simulation complemented with a Lagrangian particle tracking routine. The actively growing cloud layer has a fixed in-cloud buoyancy and velocity. A subsiding shell generated at the cloud-environment interface due to evaporative cooling acts as a buffer layer between the cloud core and the environment. Once flow reaches a self-similar regime, 3 x 10⁶ tracers were seeded homogeneously along the domain and properties were recorded along individual trajectories. The traditional cloud boundary considered in LES studies lies deep within the shell. The results of this study show that the actual cloud edge is extended to the Turbulent Non-Turbulent Interface (TNTI) which coincides with the outer edge of the shell. Lagrangian tracers crossing this interface exhibit a sharp jump in enstrophy and a smooth increase in buoyancy. The traditional cloud boundary coincides with the location of minimum buoyancy in the shell and tracers crossing this interface do not show a significant increase in enstrophy. The shell pre-mixes the entraining and detraining air and analysis reveals a highly skewed picture of entrainment and detrainment at the traditional cloud boundary. A preferential entrainment of tracers with mean velocity and specific humidity higher than the mean values in the shell is also seen.