Impact on the stratocumulus-to-cumulus transition of the interaction of cloud microphysics and macrophysics with large-scale circulation

Abstract. This study examines the impact of the interaction of cloud microphysics and macrophysics with the large-scale circulation on stratocumulus-to-cumulus transition (SCT) by combining large-eddy simulation (LES) with a parameterization of weak temperature gradient (WTG) stratified adjustment. The WTG approximates the interaction with the large-scale circulation by inducing domain-mean subsidence to compensate for buoyancy perturbations with respect to a reference thermodynamic profile. A stationary sea-salt sprayer perturbs the transitioning clouds over the Lagrangian domain moving along the trajectory. It is revealed that the cloud response to aerosol perturbation is markedly different depending on whether stratified adjustments in the large-scale circulation in response to buoyancy perturbations are considered. In both cases, aerosol injection into heavily precipitating clouds suppresses precipitation and enhances entrainment. Without application of WTG, cloud-top height rises without a compensating adjustment in subsidence, and the drizzle-induced thinning of the stratocumulus layer is delayed by several days. When WTG adjustment is applied, intensified large-scale subsidence restrains the growth of cloud top height, and increases warming and drying of the stratocumulus layer leads to cloud thinning. The thinned clouds, characterized by reduced emissivity and weakened longwave (LW) radiative cooling efficiency, become more susceptible to cloud breakup. Simultaneously, the reduced sensible heat flux from the surface by precipitation suppression reduces turbulence within the boundary layer. For lightly precipitating clouds, the transition, mainly driven by the warming effect due to enhanced entrainment by increased sea-surface temperature ('deepening-warming' mechanism), is hastened by aerosol injection due to accelerated cloud thinning. For heavily precipitating stratocumulus, in which the pace of SCT is fast due to the loss of clouds by drizzle ('drizzle-depletion' feedback), aerosol injection delays the transition by only a few hours because the deepening-warming mechanism becomes more important by intensified subsidence. Our results imply that the magnitude of the cooling effects of aerosol may be overestimated by as much as ~15–30 W m^-2 when the adjustment in large-scale circulation is not accounted for in a limited-domain model simulations.