Observational Evidences for a Generalized Parameterization Scheme for Cloud Droplet Relative Dispersion

Abstract

The relative dispersion (ε, ratio of standard deviation to the mean radius of cloud droplet size distribution) is a key parameter to describe the cloud droplet spectral shape. Its variation with the increasing aerosol number (Na) or cloud droplet number concentration (Nc) has potential impacts on cloud radiative properties and precipitation efficiency simulated in global climate models (GCMs). The parameterization schemes in current GCMs all employ positive ε-Nc relationships, while several recent studies exhibit the observational evidences for negative ε-Nc (Na) relationships. Both theoretical and observational studies also indicate the necessity to consider the humidity condition (such as updraft velocity or cloud liquid water content LWC) or investigate the contrasting ε-LWC/Nc relationships for maritime/clean and continental/polluted clouds. In this study, we extract cloud datasets from VOCALS (VAMOS Ocean Cloud-Climate-Atmosphere-Land Regional Experiment) over the North coast of Chile and aircraft campaign in Beijing over the North China Plain, respectively. We find that ε has the most significant dependence on LWC/Nc among multiple aerosol, cloud and meteorological variables. We also propose a generalized scheme for parameterizing ε and representing the contrasting ε-LWC/Nc relationships for maritime/clean and continental/polluted clouds in GCM. In addition, we analyze the physical mechanism for explaining the observed contrasting ε-LWC/Nc relationships in the two campaigns. The abundant/limited humidity condition in VOCALS/Beijing campaign enables different growth rates for small and large droplets, resulting in a narrowing/broadening of cloud droplet size spectrum with LWC/Nc. This work provides the observed evidences for physically explaining the contrasting ε-LWC/Nc relationships and the mechanism is consistent with the theoretical analysis in a previous study (Chen et al., 2016). The proposed scheme for parameterizing ε is more generalized to representing the contrasting ε-LWC/Nc relationships in this study and can replace the current scheme only for positive ε-Nc relationship in GCM.

Keywords: aerosol-cloud interactionsaerosol activationcloud droplet spectral dispersion