Inverse Energy Cascade and the Upscale Cellular Growth of Stratocumulus Clouds

Florent Brient , Basile Poujol

Stratocumulus clouds are shallow convective clouds that exhibit mesoscale cellular organisation, with cell sizes of about 25--30 times the boundary-layer depth. Yet the physical processes controlling this large and robust aspect ratio remain unclear. Here we investigate this question using large-eddy simulations (LES) that reproduce the diurnal cycle of marine stratocumulus.

The simulations show an upscale growth of the cellular organisation at a rate of about one boundary-layer depth per hour (+1/h), until an equilibrium consistent with observations is reached. To analyse energy transfers across scales, we apply a turbulent energy cascade methodology to the 3D wind field. We find an inverse cascade of turbulent kinetic energy, transferring energy from small-scale vertical thermals, constrained by the boundary-layer depth, to large-scale horizontal structures. This cross-advection transfer acts mostly at night, when the boundary layer is coupled to the surface; during the day, decoupling gives rise to an isotropic horizontal--horizontal inverse cascade that also contributes to the mesoscale organisation. Additionally, the cells also feed a forward cascade, transferring their energy horizontally from the cell scale to smaller scales.

These results suggest that the upscale growth of cells in the stratocumulus-topped boundary layer is more consistent with a control by kinetic energy transfers and by stratification at the surface and cloud top than by humidity--radiation interactions. These findings are consistent with recent studies of superstructures in Rayleigh--Bénard convection simulations. Future work should now focus on the links between such dry superstructures and mesoscale organisation of atmospheric boundary layers, and on plume-plume and plume-environment interactions.