Speaker
Description
The reflectance, lifetime, and spatial extent of atmospheric clouds – and thus their impact on Earth’s climate and weather – are strongly influenced by how turbulence mixes dry and cloudy air at the cloud edge. This mixing generates small-scale heterogeneities in droplet numbers and sizes across multiple length scales. Here we report on theoretical analysis of in-situ observations of shallow cumulus cloud edges on scales from 1 mm to 10 m, performed by G. Bagheri and E. Bodenschatz. We find that there is significant variability in the underlying microscopic processes under similar atmospheric conditions: some cloud edges expand by entrainment, where relatively moist environmental air dilutes the cloud and droplet evaporation is negligible. In other cases, however, droplet evaporation dominates over dilution. The theory demonstrates that this variability arises from significant differences in turbulent mixing times for different observations. The theory also explains how fluctuations and variability depend on the spatial resolution of the observations. State-of-the-art weather and climate models assume that clouds are instantaneously mixed and therefore spatially uniform at small scales. But our results imply that one must consider time-dependent local turbulent mixing and the resulting inhomogeneities to explain droplet populations at the cloud edge. Mean-field models assuming a global equilibrium must fail. This talk is based on work carried out with F. Hoffmann, G. Bagheri, and E. Bodenschatz.