Interpreting the dominant signature of inhomogeneous mixing resulting from dry-air entrainment in clouds.

Abstract

How cloud droplets evaporate when mixed with the dry surrounding air is fundamental to cloud optical properties and lifetime. We find from observations in cumulus clouds made during the ESCAPE field campaign that this mixing process appears strongly inhomogeneous-like, where a subset of droplets evaporate completely as mixing proceeds, rather than all droplets partially evaporating. We visualize the microphysical properties in a two-dimensional evaporation-phase-relaxation space and find that a diffusive turbulent-evaporation model is able to capture the dynamic evolution of the entrainment process. The results indicate that the first evaporating droplets humidify the region around the cloud so that the unmixed dry air rarely reaches the core, explaining why most mixing events appear inhomogeneous. A mixing slope parameter also confirms the nature of the mixing process. On the basis of the inhomogeneous mixing model, we propose a simple parameterization of cloud optical properties suitable for coarse-resolution models.