Closed Mesoscale Cellular Convection Driven by Cloud-Top Radiative Cooling

Closed mesoscale cellular convection (MCC) consists of mesoscale cloud patches separated by narrow clear regions. Strong radiative cooling occurs at the cloud top. In this study a dry two-dimensional Boussinesq model is used to study the effects of cloud-top cooling on convection. Wide updrafts and narrow downdrafts are used to indicate the asymmetric circulations associated with the mesoscale cloud patches. Linear analysis of the model indicates only that the longest waves are most unstable and gives no indication of asymmetric convection cells in the linear convective regime. A weakly nonlinear analysis suggests the presence of downdrafts that are narrower than the updrafts, but this effect is not very pronounced for reasonable values of parameters. Fully nonlinear numerical simulations show that strong cloud-top cooling can generate highly asymmetric mesoscale cells corresponding to closed MCC. Nonlinear processes play essential roles in generating and maintaining closed MCC. The effects of cloud-top radiative cooling on the model dynamics can only be fully represented in a fully nonlinear model. Based on the numerical results, a conceptual model is constructed to suggest a mechanism for the formation of closed MCC over cool ocean surfaces.