Factors determining the effect of aerosols on cloud mass and the dependence of these factors on liquid-water path

Abstract

Increasing aerosols decreases the size of droplets and thus their collection efficiencies, leading to an inefficient conversion of droplets to precipitable raindrops. This, in turn, increases the mass of droplets suspended in the air by decreasing the removal of cloud mass by sedimentation and has been known to be a main mechanism which determines the effect of aerosols on cloud mass. However, a recent study showed that this mechanism played a negligible role in the determination of the cloud mass as compared to aerosol-induced feedbacks between microphysics and dynamics in thin stratocumulus clouds with LWP of ~50 g m or less. This is contrary to studies which have shown that the mechanism associated with the aerosol-induced inefficient conversion plays an important role in the determination of the effect of aerosols on cloud mass. These studies are generally based on clouds with LWP >50 g m −2. Hence, it is important to understand whether the role of aerosol-induced feedbacks in the effect of aerosols on cloud mass depends on the level of LWP. This study examines the dependence of the role of the conversion of droplets to raindrops and their sedimentation in the determination of the effect of aerosols on cloud mass on the level of LWP. Pairs of numerical experiments for high and low aerosol cases are run for four cases of stratiform clouds with different LWPs. Comparisons among these cases show that the role of the conversion and sedimentation becomes less important as the level of LWP decreases. Instead, the role of the feedbacks between microphysics and dynamics become more important with the lowering level of LWP. The results of this study indicate that the traditional approach to the understanding of the aerosol-cloud interactions and its application to the parameterization of these interactions in climate models can be misleading. The understanding of feedbacks between microphysics and dynamics induced by aerosol changes and their parameterization can be critical to the correct assessment of the effect of aerosols on clouds and climate.