Analysis of the cloud top mixing layer using DNS

Abstract

It has been recognized during the last decades that cloud effects remain one of the largest sources of uncertainty in model-based estimates of climate sensitivity, and in particular entrainment rates in stratocumulus-topped mixed layers need urgently better models (Stevens, 2002) in order to improve predictions obtained with large-scale approaches, like large-eddy simulations. Cloud-top mixed layers, and in particular the possibility of an entrainment instability appearing at the top of the cloud deck due to buoyancy reversal created by evaporative cooling when the upper subsaturated and hot subsiding air mixes with the lower moist and colder layer, have received considerable attention (Lilly, 1968, Randall, 1980, Kuo and Schubert, 1988, Yamaguchi and Randall, 2008), and even small-scale analysis via laboratory experiments and numerical simulations have been presented (Shy and Breidenthal, 1990, Siems et al., 1990, Siems and Bretherton, 1992). This work continues the study of the cloud-top mixing layer on a small-scale using direct numerical simulation (DNS) in order to improve the understanding of the phenomena taking place at this region, and ultimately derive better entrainment models. The idealized configuration considered here is a two-layer system, the upper layer hot and subsaturated and the lower cold and saturated, with or without shear.