Processes Controlling the Entrainment and Liquid Water Response to Aerosol Perturbations in Non-Precipitating Stratocumulus Clouds

It has been widely reported that an increase in aerosol concentration in non-precipitating clouds leads to a decrease in their liquid water path. Here we examine the physical mechanisms that drive this response in both subtropical and Arctic stratocumulus clouds using large eddy simulations and mechanism suppression tests. Three processes have been previously identified to contribute to the decrease, namely, the size-dependency of evaporation, sedimentation, and radiation and all act to modulate the rate of entrainment of warm, dry air at the boundary layer top. We find that the liquid water path decrease is correlated with the increased entrainment, as expected, but that the decrease is enhanced by a reduction in cloud radiative cooling. The reduced cloud radiative cooling can occur even though locally at cloud top the radiative cooling rates are stronger and helping to enhance entrainment. We find that slower droplet sedimentation contributes to the increased entrainment and decreased liquid water in both cases. Faster evaporation caused directly by smaller, more numerous droplets decreases the liquid water path but does not necessarily increase the entrainment rate. On the other hand, stronger radiative cloud top cooling caused directly by smaller droplets increases the entrainment as much as slower sedimentation does but does not decrease the liquid water path as much. In general, processes that either directly or indirectly increase radiative cooling at cloud top are more important in the Arctic case and processes that increase the evaporation rate are more important in the subtropical case.