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 the stratocumulus-to-cumulus transition (SCT) using a large-eddy simulation (LES) combined with weak-temperature-gradient (WTG) parameterization. The WTG approximates large-scale circulation by inducing domain-mean subsidence to counter buoyancy perturbations relative to a reference thermodynamic profile. A stationary sea-salt sprayer perturbs transitioning clouds over a Lagrangian domain. Results show that the cloud response to aerosol injection differs significantly depending on whether stratified adjustments in the large-scale circulation in response to buoyancy perturbations are considered. Aerosol injection suppresses precipitation and enhances entrainment in both cases. Additionally, reduced surface sensible heat flux by precipitation suppression weakens boundary layer turbulence. Without the WTG, cloud-top height rises without a compensating adjustment in subsidence, delaying drizzle-induced stratocumulus thinning (“drizzle-depletion” feedback) by several days. With the WTG, intensified subsidence restrains cloud-top growth and accelerates stratocumulus thinning, leading to reduced boundary layer turbulence by weakened longwave cloud-top cooling and increased susceptibility to cloud breakup. For lightly precipitating clouds, aerosol injection accelerates the SCT by enhancing cloud thinning through warming driven by increased entrainment (“deepening–warming” mechanism). For heavily precipitating clouds, where the SCT is dominated by drizzle-depletion feedback, aerosol injection delays the SCT marginally as intensified subsidence amplifies the deepening–warming mechanism. These findings suggest that ignoring large-scale circulation adjustments in limited-domain models may overestimate aerosol cooling effects by ∼ 15–30 W m−2