Understanding Mesoscale Organization of Closed-Cell Marine Stratocumulus Using Large-Eddy Simulation

This study uses large-eddy simulation (LES) over an approximately 60x60 km² periodic domain to investigate the mechanisms that promote mesoscale closed cellular organization in well-mixed non-drizzling stratocumulus-topped marine boundary layers. For the case studied, closed cells develop and grow within a simulated day to a 30:1 typical aspect ratio, consistent with satellite observations. Horizontally homogenizing the radiative cooling impedes this evolution.

A composite mesoscale circulation is built by sorting model grid columns from low to high low-pass filtered total water paths. A well-defined mesoscale circulation pattern is found in the interior of the boundary layer with buoyant mesoscale updrafts, thicker cloud, and a slightly higher capping inversion in the moister columns. Thus we interpret the closed cells as a buoyancy-driven mesoscale convective instability.

To better understand this instability, we focus on processes at the cloud top. There is a mesoscale flow of air down the slightly sloping capping inversion from the moist to the dry regions, reinforced by cloud-top radiative cooling. This flow within the sloping inversion fluxes radiatively cooled dry air horizontally from the moist regions to the dry regions, where it is entrained and spreads vertically throughout the boundary layer. The result is to reinforce the mesoscale anomalies that make the closed cells. The sloping inversion flow is not driven as efficiently if the radiative cooling is artificially horizontally homogenized, partly disrupting this positive feedback and the resulting closed cell development.