On the Roles of Precipitation and Entrainment in Stratocumulus Transitions between Mesoscale States

Abstract

Stratocumulus occur in closed- or open-cell states, which tend to be
associated with high or low cloud cover and the absence or presence of
precipitation, respectively. Thus, the transition between these states
has substantial implications for the role of this cloud type in Earth’s
radiation budget. In this study, we analyze transitions between these
states using an ensemble of 127 large-eddy simulations, covering a wide
range of conditions. Our analysis is focused on the behavior of these
clouds in a cloud fraction (f_c) scene albedo (A)
phase space, which has been shown in previous studies to be a useful
framework for interpreting system behavior. For the transition from
closed to open cells, we find that precipitation creates narrower clouds
and scavenges cloud droplets for all f_c. However, precipitation decreases the cloud depth for f_c > 0.8 only, causing a rapid decrease in A. For f_c
< 0.8, the cloud depth actually increases due to mesoscale
organization of the cloud field. As the cloud deepening balances the
effects of cloud droplet scavenging in terms of influence on A, changes in A are determined by the decreasing f_c only, causing a linear decrease in A for f_c
< 0.8. For the transition from open to closed cells, we find that
longwave radiative cooling drives the cloud development, with cloud
widening dominating for f_c < 0.5. For f_c
> 0.5, clouds begin to deepen gradually due to the decreasing
efficiency of lateral expansion. The smooth switch between cloud
widening and deepening leads to a more gentle change in A compared to the transitions under precipitating conditions.