Studies of aerosol indirect effects in stratocumulus using a mixed layer model

Anthropogenic aerosols impact the radiative properties of stratocumulus clouds not only by increasing the number of cloud droplets (termed the first aerosol indirect effect, AIE), but by inducing changes in liquid water content resulting from feedbacks involving precipitation and entrainment (the second AIE). Most climate models show that suppression of precipitation by anthropogenic aerosols leads to significant increases in liquid water content which results in a second AIE that is of comparable magnitude to, or even larger than, the first AIE. However, recent observational and detailed model studies have suggested that the relative strength of the second indirect effect is more complex and variable than was previously thought. In this study, I use a simple mixed layer equilibrium model framework to investigate the relative importance of the two AIEs in marine stratocumulus clouds, which is here quantified by the partial derivative of liquid water path with respect to cloud droplet concentration. This framework allows a simplified exploration of some of the important feedback mechanisms involving controlling the strength of the AIEs but is simple enough to be tractable. Control and perturbation simulations are performed in which only the cloud droplet concentration is changed. The mean surface precipitation rate is found to be the key parameter determining the relative strength of the second AIE. I will also present results concerning the sensitivity of the second AIE to different parameterizations and meteorological conditions.