Abstract
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 (VAMOS Ocean Cloud Atmosphere Land Study Regional Experiment) field campaign. Regional models simulate the period continuously in boundary-forced free-running mode, while global forecast models and GCMs (general circulation models) are run in forecast mode. The models are compared to extensive observations along a line at 20° S extending westward from the South American coast. Most of the models simulate cloud and aerosol characteristics and gradients across the region that are recognizably similar to observations, despite the complex interaction of processes involved in the problem, many of which are parameterized or poorly resolved. Some models simulate the regional low cloud cover well, though many models underestimate MBL (marine boundary layer) depth near the coast. Most models qualitatively simulate the observed offshore gradients of SO2, sulfate aerosol, CCN (cloud condensation nuclei) concentration in the MBL as well as differences in concentration between the MBL and the free troposphere. Most models also qualitatively capture the decrease in cloud droplet number away from the coast. However, there are large quantitative intermodel differences in both means and gradients of these quantities. Many models are able to represent episodic offshore increases in cloud droplet number and aerosol concentrations associated with periods of offshore flow. Most models underestimate CCN (at 0.1% supersaturation) in the MBL and free troposphere. The GCMs also have difficulty simulating coastal gradients in CCN and cloud droplet number concentration near the coast. The overall performance of the models demonstrates their potential utility in simulating aerosol–cloud interactions in the MBL, though quantitative estimation of aerosol–cloud interactions and aerosol indirect effects of MBL clouds with these models remains uncertain.
Highlights
The southeast Pacific (SEP) region has an unusually extensive and persistent low-cloud cover supported by relatively low sea-surface temperatures (SSTs) due to coastalPublished by Copernicus Publications on behalf of the European Geosciences Union.M
This paper addresses several questions: Can the models represent the geographical contrasts in cloud microphysical properties in the SEP? How well do the geographical and vertical concentrations of aerosols agree? How well do the models represent the impacts of these aerosols in the clouds? What problems are common to many models? Do these observations provide a good benchmark for aerosol–cloud interaction?
Our analysis in this paper has been limited to the subset of nine models participating in VOCALS Assessment (VOCA) that have some representation of aerosol processes, which in some cases interacts with cloud microphysics
Summary
The southeast Pacific (SEP) region has an unusually extensive and persistent low-cloud cover supported by relatively low sea-surface temperatures (SSTs) due to coastal. Wyant et al.: Modeling of clouds and aerosols during VOCALS upwelling, strong subsidence, and high static stability in the lower troposphere. There are typically strong east–west aerosol gradients in this marine boundary layer (MBL) between relatively pristine conditions in air masses advecting from the South Pacific Ocean and more polluted air near the west coast of South America (e.g., Bretherton et al, 2010; Allen et al, 2011). Anthropogenic aerosol and aerosol precursor emissions from industrial, agricultural, and transportation sources are incorporated into the MBL directly or through intermittent free-tropospheric flow over the ocean and subsequent entrainment into the MBL (e.g., Clarke et al, 2010; George et al, 2013)
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