Constraining Processes Involved with Aerosol Effects on Cloud Radiative Forcing

Thursday, December 17, 2015 - 13:40
Add to Calendar

A large number of processes are involved in the chain from emissions of aerosol precursor gases and primary particles to impacts on cloud radiative forcing. Those processes are manifest in a number of relationships that can be expressed as sensitivity factors dlnX/dlnY driving aerosol effects on cloud radiative forcing. These factors include the sensitivity of cloud condensation nuclei (CCN) concentration to emissions, droplet number to CCN concentration, cloud optical depth to droplet number, and cloud radiative forcing to cloud optical depth.  The relationship between cloud optical depth and droplet number can be further decomposed into the sum of two terms involving the sensitivity of droplet effective radius and cloud liquid water path to droplet number. These relationships can be constrained using observations of recent spatial and temporal variability of CCN concentration, droplet number concentration, droplet effective radius, cloud liquid water path, cloud optical depth, and cloud radiative forcing. However, analysis of AeroCom model simulations suggests the relevance of such constraints to anthropogenic aerosol impacts on cloud radiative forcing is doubtful for some terms. Proxies connecting recent spatial/temporal variability to anthropogenic change, or sustained measurements in regions where emissions have changed, are needed to constrain estimates of anthropogenic aerosol impacts on cloud radiative forcing.