The Semidirect Aerosol Effect: Comparison of a Single-Column Model with Large Eddy Simulation for Marine Stratocumulus

B. T. Johnson Department of Meteorology, University of Reading, Reading, Berkshire, United Kingdom

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Abstract

Large eddy simulations (LES) show that the presence of black carbon aerosols in marine boundary layers leads to a marked reduction of stratocumulus liquid water path (LWP) by heating the cloud layer and suppressing convection in the boundary layer. The reduction of LWP leads to a positive radiative forcing known as the semidirect effect. In this study LES results are compared with results from the National Center for Atmospheric Research (NCAR) Single-Column Community Climate Model (SCCM). The SCCM represents clouds and boundary layer processes through simple parameterization schemes that are typical of general circulation models (GCMs) used for climate experiments. In a case study in which black carbon aerosols were introduced in a stratocumulus-capped boundary layer the SCCM gave a semidirect aerosol radiative forcing that was a factor of 5 smaller than the value obtained from the LES. The cloud response to absorbing aerosols was underestimated because of the way that cloud cover and cloud radiative properties were parameterized in the SCCM. Furthermore, the SCCM gave a poor representation of processes, such as entrainment and boundary layer decoupling, that are crucial to determining stratocumulus LWP. This study shows that GCMs may not include all the physical processes necessary to adequately capture the semidirect aerosol effect. Previous GCM estimates of the semidirect effect that have incorporated simple cloud parameterizations should, therefore, be treated with some caution.

Current affiliation: National Center for Atmospheric Research, Boulder, Colorado

Corresponding author address: B. T. Johnson, National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307. Email: benj@ucar.edu

Abstract

Large eddy simulations (LES) show that the presence of black carbon aerosols in marine boundary layers leads to a marked reduction of stratocumulus liquid water path (LWP) by heating the cloud layer and suppressing convection in the boundary layer. The reduction of LWP leads to a positive radiative forcing known as the semidirect effect. In this study LES results are compared with results from the National Center for Atmospheric Research (NCAR) Single-Column Community Climate Model (SCCM). The SCCM represents clouds and boundary layer processes through simple parameterization schemes that are typical of general circulation models (GCMs) used for climate experiments. In a case study in which black carbon aerosols were introduced in a stratocumulus-capped boundary layer the SCCM gave a semidirect aerosol radiative forcing that was a factor of 5 smaller than the value obtained from the LES. The cloud response to absorbing aerosols was underestimated because of the way that cloud cover and cloud radiative properties were parameterized in the SCCM. Furthermore, the SCCM gave a poor representation of processes, such as entrainment and boundary layer decoupling, that are crucial to determining stratocumulus LWP. This study shows that GCMs may not include all the physical processes necessary to adequately capture the semidirect aerosol effect. Previous GCM estimates of the semidirect effect that have incorporated simple cloud parameterizations should, therefore, be treated with some caution.

Current affiliation: National Center for Atmospheric Research, Boulder, Colorado

Corresponding author address: B. T. Johnson, National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307. Email: benj@ucar.edu

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