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.