A new three-moment-based framework for parameterizing CCN and droplet activation

Description

Aerosol effects on clouds and precipitation in large-scale models and the subsequent effects on radiative properties and climate remain one of the most uncertain climate forcings. Much of the uncertainty arises from poor understanding and quantification of the relationship of aerosol properties to cloud condensation nuclei (CCN) and further to cloud properties. Most existing parameterizations are based on either one moment (aerosol mass or number concentration) or two moments (aerosol number and mass concentration), and further, on the assumption of known mathematical forms of aerosol size distributions such as the power-law, lognormal, Gamma, and Weibull distributions.

To evaluate parameterization of aerosol-cloud interactions in climate models against long-term ARM measurements poses a further challenge. To address these challenges as part of the FASTER project, this study extends previous works by establishing a new theoretical framework based on three aerosol moments that accounts for not only aerosol number concentration and mass concentration, but also the relative dispersion of aerosol size distribution. The new three-moment framework permits quantification of the differences in CCN spectra and cloud properties resulting from using those commonly assumed forms of aerosol size distributions. Measurements collected at the ARM SGP site will be examined in the context of evaluating the new formulation. The results will shed light on how to best use ARM aerosol-related measurements to evaluate model performance.