The aerosol indirect effect of cloud droplet sedimentation through stratocumulus entrainment

Large-eddy simulations of stratocumulus clouds and observations of ship and volcano tracks in stratocumulus suggest that enhanced aerosol can significantly reduce the liquid water content of nondrizzling stratocumulus, counteracting the Twomey effect. This appears to be caused by sensitivity of stratocumulus entrainment rate to cloud droplet sedimentation. If cloud droplets are more numerous and hence smaller, they fall slower and tend to remain in the entrainment zone. This allows turbulent mixing of overlying dry air to evaporate more liquid water and produce cooler, denser downdrafts that promote more efficient entrainment. The increased entrainment drying results in a thinner cloud layer.

Bretherton et al. (2007 GRL) proposed an entrainment closure that incorporates this effect. We present single-column and global simulations with version 5 of the NCAR Community Atmosphere Model that show that the single-column model can replicate the sensitivity of cloud liquid water path to droplet concentration found with LES of stratocumulus-capped mixed layers. We also show the impact of sedimentation-entrainment feedback on clouds and radiation balance in global simulations using both pre-industrial and anthropogenically-perturbed aerosols.