Parameterization of Bulk Condensation in Numerical Cloud Models

Abstract

A recent study showed that aerosol-induced feedbacks between microphysics and dynamics predominantly determined the cloud-mass response to aerosols in thin clouds with liquid-water path (LWP) of ∼50 g m-2 or less; in this paper, cloud mass represents the time- and area-averaged LWP and LWP is the column-integrated cloud liquid content (LWC); LWC is the mass of cloud liquid per unit volume of air. This is contrary to studies which have shown that aerosol-induced inefficient conversion and sedimentation play 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. Pairs of numerical experiments for high and low-aerosol cases are run for four cases of stratiform clouds with different LWPs. All of these cases show that the role of condensation or evaporation of cloud liquid in the cloud-mass response to aerosol is more important than that of conversion of cloud liquid to rain and sedimentation (or precipitation). This indicates that focusing only on parameterization of autoconversion and sedimentation to represent aerosol effects on cloud mass in climate models can be misleading. Also, this study finds that the effect of aerosol-induced sedimentation suppression on the cloud-mass response to aerosol becomes less important as LWP lowers. Instead, the effect of aerosol-induced changes in condensation or evaporation on the cloud-mass response becomes more important with the decreasing LWP. These changes in condensation (and associated changes in evaporation) are caused by interactions (or feedbacks) among aerosol, droplet surface area, supersaturation, and instability around cloud base. © 2011 Elsevier Ltd.