Radiation Impacts on Conditionally Unstable Moist Convection

Abstract

Abstract The present work analyzes the impacts of radiative cooling in three-dimensional high-resolution direct numerical simulations of moist Rayleigh–Bénard convection. An atmospheric slab is destabilized by imposing a warm, moist lower boundary and a colder, dryer upper boundary. These boundary conditions are chosen such that the atmosphere is relaxed toward a conditionally unstable state in which unsaturated air parcels experience a stable stratification and unsaturated parcels experience an unstable one. Conditionally unstable moist Rayleigh–Bénard convection in the absence of radiative transfer produces self-aggregated convectively active cloudy regions separated by a quiescent unsaturated environment. Such convection is strongly limited by diffusion and is unable to transport much energy. As radiative cooling partially compensates for the adiabatic warming in the unsaturated environment and destabilizes the lower unsaturated boundary, its inclusion results in a significant enhancement of convective activity and cloud cover. A dry convectively unstable region develops at the lower boundary in a way that is reminiscent of the planetary boundary layer. Convective transport increases through the entire layer, leading to a significant enhancement of the upward transport of energy and water.