PNNL

Abstract

A diverse collection of models are used to simulate the marine boundary layer in the Southeast Pacific region during the period of the October-November 2008 VOCALS REx field campaign. Regional models simulate the period continuously, while global forecast models and GCMs are run in forecast-mode. The models are primarily compared to extensive observations along a line at 20ºS extending westward from the South American coast. Some of the models are able to simulate the regional low cloud cover well Many of the models substantially underestimate MBL depth. Most models can qualitatively represent the observed offshore gradient of SO2, sulfate aerosol, CCN concentration in the MBL, and the related gradient in cloud droplet concentrations. Quantitatively there are large variations among the models’ simulations. Most models are deficient in CCN at 0.1% supersaturation both in the MBL and free troposphere. The GCMs in this study also have difficulty with properly representing coastal gradients in CCN and cloud droplet number concentration. Based on the differences in model results, quantitative estimation of aerosol-cloud interactions and aerosol indirect effects associated with marine boundary layer clouds in regional and global models remains a challenge, but advances have been made towards reducing its uncertainties.