Recognition of Inter‐Cloud Versus Intra‐Cloud Controls on Droplet Dispersion With Applications to Microphysics Parameterization

Abstract

AbstractAerosols in the atmosphere can have a large impact on weather and climate. One of its lesser‐known impacts is the aerosol‐dispersion effect, which refers to the feedback between the aerosol (or cloud droplet) concentration and the relative dispersion of the droplet size distribution. This study attempts to reconcile the inconsistent sign of the correlation between aerosol number and droplet dispersion found in past studies by distinguishing local correlation from cloud‐mean correlation. We find that while the cloud‐mean correlation between aerosol concentration and droplet size relative dispersion is largely nonexistent, in‐cloud regions with high relative dispersion are typically associated with low local droplet concentration. Even stronger correlation is found with respect to the local activation fraction (the ratio of droplet concentration to total droplet and aerosol concentration). Based on this local correlation, we propose a new equation to diagnose the shape parameter in marine stratocumulus clouds for double moment bulk schemes in cloud‐resolving or large‐eddy simulations. This new equation is more physically realistic than many existing ones that are based on the weak cloud‐mean correlation.