Quantifying Global Aerosol Effects on Convection Using the Convective Cloud Field Model (CCFM).

Abstract:

Considerable advances have been made in the last decade or so in the representation of aerosol indirect effects on resolved-scale cloud and precipitation in global climate models. However, the representation of effects on the convective scale is severely limited by the nature of the bulk mass-flux parameterizations used in most global models, contributing to the uncertainty in model estimates of the total effective radiative forcing from aerosol–cloud interactions (ERFaci).

In this study, we investigate aerosol effects on convection at the global scale using the Convective Cloud Field Model (CCFM) within the ECHAM6–HAM2 global aerosol–climate model. This parameterization simulates a spectrum of cloud types interacting through their shared environment, each modeled by an entraining parcel model with explicit updraft velocity and embedded parameterizations of aerosol activation and cloud microphysics. This allows us to analyse the distributions of updraft velocity and cloud droplet number concentration (CDNC) in unresolved convective cloud, as well as the cloud field structure in terms of cloud number, area and depth. We then consider the effect of an aerosol perturbation on the system, from the change in CDNC, through modulation of precipitation and latent heating, to impacts on the cloud field structure which may be indicative of one or more of the proposed mechanisms for aerosol-induced changes on the single cloud or cloud field scale. From simulations in present-day and pre-industrial scenarios, we present an estimate of the convective contribution to global ERFaci, and consider how this might be further constrained.