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Large-Eddy Simulations of a Drizzling, Stratocumulus-Topped Marine Boundary Layer

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  • 1 NASA Goddard Institute for Space Studies, New York, New York
  • | 2 Royal Netherlands Meteorological Institute, De Bilt, Netherlands
  • | 3 Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, Los Angeles, California
  • | 4 Department of Atmospheric Sciences, University of Washington, Seattle, Washington
  • | 5 Max Planck Institute for Meteorology, Hamburg, Germany
  • | 6 UCAR Visiting Scientist Program, NOAA/Geophysical Fluid Dynamics Laboratory, Princeton, New Jersey
  • | 7 NOAA/Earth System Research Laboratory, Boulder, Colorado
  • | 8 School of Marine and Atmospheric Sciences, Stony Brook University, Stony Brook, New York
  • | 9 Department of Meteorology, University of Utah, Salt Lake City, Utah
  • | 10 MAE Department, West Virginia University, Morgantown, West Virginia
  • | 11 Met Office, Exeter, United Kingdom
  • | 12 National Center for Atmospheric Research, Boulder, Colorado
  • | 13 Frontier Research Center for Global Change, Japan Agency for Marine–Earth Science and Technology, Yokahama, Japan
  • | 14 Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado
  • | 15 Department of Atmospheric Science, University of Wyoming, Laramie, Wyoming
  • | 16 Department of Meteorology, University of Reading, Reading, United Kingdom
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Abstract

Cloud water sedimentation and drizzle in a stratocumulus-topped boundary layer are the focus of an intercomparison of large-eddy simulations. The context is an idealized case study of nocturnal stratocumulus under a dry inversion, with embedded pockets of heavily drizzling open cellular convection. Results from 11 groups are used. Two models resolve the size distributions of cloud particles, and the others parameterize cloud water sedimentation and drizzle. For the ensemble of simulations with drizzle and cloud water sedimentation, the mean liquid water path (LWP) is remarkably steady and consistent with the measurements, the mean entrainment rate is at the low end of the measured range, and the ensemble-average maximum vertical wind variance is roughly half that measured. On average, precipitation at the surface and at cloud base is smaller, and the rate of precipitation evaporation greater, than measured. Including drizzle in the simulations reduces convective intensity, increases boundary layer stratification, and decreases LWP for nearly all models. Including cloud water sedimentation substantially decreases entrainment, decreases convective intensity, and increases LWP for most models. In nearly all cases, LWP responds more strongly to cloud water sedimentation than to drizzle. The omission of cloud water sedimentation in simulations is strongly discouraged, regardless of whether or not precipitation is present below cloud base.

Corresponding author address: Andrew S. Ackerman, NASA Goddard Institute for Space Studies, New York, NY 10025. Email: andrew.ackerman@nasa.gov

Abstract

Cloud water sedimentation and drizzle in a stratocumulus-topped boundary layer are the focus of an intercomparison of large-eddy simulations. The context is an idealized case study of nocturnal stratocumulus under a dry inversion, with embedded pockets of heavily drizzling open cellular convection. Results from 11 groups are used. Two models resolve the size distributions of cloud particles, and the others parameterize cloud water sedimentation and drizzle. For the ensemble of simulations with drizzle and cloud water sedimentation, the mean liquid water path (LWP) is remarkably steady and consistent with the measurements, the mean entrainment rate is at the low end of the measured range, and the ensemble-average maximum vertical wind variance is roughly half that measured. On average, precipitation at the surface and at cloud base is smaller, and the rate of precipitation evaporation greater, than measured. Including drizzle in the simulations reduces convective intensity, increases boundary layer stratification, and decreases LWP for nearly all models. Including cloud water sedimentation substantially decreases entrainment, decreases convective intensity, and increases LWP for most models. In nearly all cases, LWP responds more strongly to cloud water sedimentation than to drizzle. The omission of cloud water sedimentation in simulations is strongly discouraged, regardless of whether or not precipitation is present below cloud base.

Corresponding author address: Andrew S. Ackerman, NASA Goddard Institute for Space Studies, New York, NY 10025. Email: andrew.ackerman@nasa.gov

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