Global and regional modeling of clouds and aerosols in the marine boundary layer during VOCALS: the VOCA intercomparison

摘要

A diverse collection of models are used to simulate the marine boundarylayer in the southeast Pacific region during the period of theOctober–November 2008 VOCALS REx (VAMOS Ocean Cloud Atmosphere Land StudyRegional Experiment) field campaign. Regional models simulatethe period continuously in boundary-forced free-running mode, while globalforecast models and GCMs (general circulation models) are run in forecast mode. The models are comparedto extensive observations along a line at 20° S extendingwestward from the South American coast. Most of the models simulate cloudand aerosol characteristics and gradients across the region that arerecognizably similar to observations, despite the complex interaction ofprocesses involved in the problem, many of which are parameterized or poorlyresolved. Some models simulate the regional low cloud cover well, thoughmany models underestimate MBL (marine boundary layer) depth near the coast. Most modelsqualitatively simulate the observed offshore gradients of SO, sulfateaerosol, CCN (cloud condensation nuclei) concentration in the MBL as well as differences inconcentration between the MBL and the free troposphere. Most models alsoqualitatively capture the decrease in cloud droplet number away from thecoast. However, there are large quantitative intermodel differences in bothmeans and gradients of these quantities. Many models are able to representepisodic offshore increases in cloud droplet number and aerosolconcentrations associated with periods of offshore flow. Most modelsunderestimate CCN (at 0.1% supersaturation) in the MBL and freetroposphere. The GCMs also have difficulty simulating coastal gradients inCCN and cloud droplet number concentration near the coast. The overallperformance of the models demonstrates their potential utility in simulatingaerosol–cloud interactions in the MBL, though quantitative estimation ofaerosol–cloud interactions and aerosol indirect effects of MBL clouds withthese models remains uncertain.