The Dynamics of Shallow Continental and Trade-Wind Cumulus as Revealed by Large Eddy Simulation

We consider the dynamics of the evolution of non-precipitating trade-wind cumulus and continental cumulus in the commonly used scene albedo (A) vs. cloud fraction (f_c) framework. Both types of cumulus are shown to trace out a super-linear relationship between A and f_c, related to the fact that cumulus clouds deepen as they widen.

In continental shallow cumulus the relationship between A and f_c exhibits strong hysteresis over the course of a diurnal cycle, with afternoon albedo higher than morning albedo (for the same cloud fraction). We show that this hysteresis results from an increase in the relationship between cloud depth and cloud area (aspect ratio) as convection strengthens and the clouds deepen in response to surface heating. Both the cloud aspect ratio and the hysteresis are shown to be remarkably robust in large eddy simulations (LES) of the LES ARM Symbiotic Simulation and Observation (LASSO) cases.

In the case of LES of trade-wind cumulus, the power-law cloud field size distribution is shown to oscillate between a relative abundance of small clouds (steep slopes on a log-log plot) and a relative abundance of large clouds (flat slopes), with timescales of 15 to 80 min. Analysis of the cloud fields suggests that cumulus clouds grow larger and deeper as their underlying plumes aggregate; this is followed by breakup of large plumes and a tendency to smaller clouds. The cloud and thermal size distributions oscillate back and forth approximately in unison. The implications of this co-evolution of clouds and thermals for the question of whether cloud development is driven by sub-cloud properties (“nature”) or the environmental conditions they encounter (“nurture”) are addressed.