Cloud droplet size distribution and optical properties only weakly linked to aerosol size

TL;DR

This study demonstrates that under most conditions, cloud droplet size and optical properties are weakly affected by aerosol size, simplifying climate modeling of aerosol-cloud interactions.

Contribution

The paper provides experimental and simulation evidence that cloud properties are often insensitive to aerosol size distributions, challenging previous assumptions.

Findings

Cloud optical properties are mostly unaffected by aerosol size.

Detectable effects occur only with high aerosol concentrations and weak convection.

Cloud reflectivity can often be predicted from aerosol number alone.

Abstract

Changes in aerosol concentrations can modify cloud brightness, producing a strong but poorly constrained influence on Earth's energy balance. Because cloud reflectivity depends on the size distribution of cloud droplets, and aerosol size strongly governs activation into droplets, one might expect cloud properties to be sensitive to aerosol size distributions. Here we show, through a combination of cloud chamber experiments and high-resolution simulations, that cloud microphysical and optical properties are often insensitive to aerosol size. Detectable impacts on cloud optical properties occur only under weak convective forcing and high aerosol concentrations. These results indicate that, in most conditions, cloud reflectivity can be predicted from aerosol number alone without detailed knowledge of aerosol size distributions, providing new constraints on how aerosol perturbations affect climate.