AAAR 35th Annual Conference October 17 - October 21, 2016 Oregon Convention Center Portland, Oregon, USA
Abstract View
Sensitivity of Cloud-Albedo Aerosol Indirect Effect on Assumed Aerosol Size Distribution Shape
JOHN KODROS, Jeffrey R. Pierce, Colorado State University
Abstract Number: 21 Working Group: Aerosols, Clouds, and Climate
Abstract Aerosol-cloud interactions are among the most uncertain climate forcings partly due to the strong sensitivity of cloud droplet number concentration (CDNC) to size-resolved aerosol number; however, many climate and chemical-transport models do not include explicit treatment of the aerosol size distribution. In order to estimate CDNC and thus the cloud-albedo aerosol indirect effect (AIE), these bulk aerosol models scale particle mass into a size-resolved number distribution by assuming a fixed size-distribution shape (commonly a lognormal distribution). This method fails to capture nonlinear microphysical processes at the regional level. We use the global chemical-transport model, GEOS-Chem, to estimate the AIE due to anthropogenic emissions when using several fixed size-distribution shapes and compare this to estimates using particle number distributions calculated online with the TwO Moment Aerosol Sectional (TOMAS) microphysics scheme. For a given supersaturation, the fixed size-distribution models overestimate CCN in clean conditions and underpredict CCN in polluted regions, leading to a net overprediction of CCN in the simulation with anthropogenic emissions turned off. We show that a fixed size distribution can underestimate the anthropogenic AIE in the global-mean by 0.1-0.3 W m$^(-2) and regionally by more than 2.0 W m$^(-2). Much of this variability is caused by failure to capture changes in nucleation and condensational growth due to altering primary emission mass.