Stratocumulus to Cumulus Transition Capped by a Light-Absorbing Smoke Layer

Abstract:

Biomass burning aerosol emitted from Africa seasonally advects over the eastern Atlantic and forms a layer of light-absorbing smoke above stratocumulus clouds, which influences heating profiles, dynamics, and cloud microphysics. In this study, large-eddy simulation is used to investigate the effect of the absorbing smoke layer on the stratocumulus to cumulus transition (SCT). A prognostic absorbing smoke model incorporates humidity effects on optical properties, and is coupled with a two-moment bulk microphysics scheme and an interactive radiation code. Smoke both absorbs shortwave radiation and acts as cloud condensation nuclei (CCN). Simulations are of three day duration. 

The simulations assess sensitivity of the SCT to distance of the smoke layer from the cloud top, aerosol optical thickness and single scattering albedo, and precipitation. Our simulations show that 1) As a shortwave absorber, the smoke stabilizes the free atmosphere and strengthens the temperature jump at the boundary layer top, which limits entrainment; 2) Smoke helps evaporate cloud during daytime, which amplifies the diurnal cycle of cloud cover; 3) As a source of CCN, the entrained smoke suppresses rain formation, which inhibits precipitation-generated cloud breakup. The net effect of smoke is modification of heating profiles to limit the deepening of the planetary boundary layer, and suppression of precipitation. This leads to enhancement of the diurnal cycle of cloudiness but a delay in the SCT.