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The Impact of Stratocumulus Cloud Radiative Properties on Surface Heat Fluxes Simulated with a General Circulation Model

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  • 1 Research Data Corporation, Data Assimilation Office, NASA, Greenbelt, Maryland
  • | 2 European Centre for Medium-Range Weather Forecasts, Reading, United Kingdom
  • | 3 Department of Atmospheric Sciences, University of California, Los Angeles, Los Angeles, California
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Abstract

When sea surface temperatures are prescribed at its lower boundary, the University of California, Los Angeles (UCLA) atmospheric general circulation model (AGCM) produces a realistic simulation of planetary boundary layer (PBL) stratocumulus cloud incidence. Despite this success, net surface solar fluxes are generally overpredicted in comparison to Earth Radiation Budget Experiment (ERBE) derived data in regions characterized by persistent stratocumulus cloud decks. It is suggested that this deficiency is due to the highly simplified formulation of the PBL cloud optical properties. A new formulation of PBL cloud optical properties is developed based on an estimate of the stratocumulus cloud liquid water path. The January and July mean net surface solar fluxes simulated by the revised AGCM are closer to ERBE-derived values in regions where stratocumulus clouds are frequently observed. The area-averaged estimated error reductions range from 24 (Peru region) to 53 W m−2 (South Pacific storm track region). The results emphasize that surface heat fluxes are very sensitive to the radiative properties of stratocumulus clouds and that a realistic simulation of both the geographical distribution of stratocumulus clouds and their optical properties is crucial.

Corresponding author address: Dr. John D. Farrara, Dept. of Atmospheric Sciences, University of California, Los Angeles, 405 Hilgard Ave., Los Angeles, CA 90095-1565. Email: farrara@atmos.ucla.edu

Abstract

When sea surface temperatures are prescribed at its lower boundary, the University of California, Los Angeles (UCLA) atmospheric general circulation model (AGCM) produces a realistic simulation of planetary boundary layer (PBL) stratocumulus cloud incidence. Despite this success, net surface solar fluxes are generally overpredicted in comparison to Earth Radiation Budget Experiment (ERBE) derived data in regions characterized by persistent stratocumulus cloud decks. It is suggested that this deficiency is due to the highly simplified formulation of the PBL cloud optical properties. A new formulation of PBL cloud optical properties is developed based on an estimate of the stratocumulus cloud liquid water path. The January and July mean net surface solar fluxes simulated by the revised AGCM are closer to ERBE-derived values in regions where stratocumulus clouds are frequently observed. The area-averaged estimated error reductions range from 24 (Peru region) to 53 W m−2 (South Pacific storm track region). The results emphasize that surface heat fluxes are very sensitive to the radiative properties of stratocumulus clouds and that a realistic simulation of both the geographical distribution of stratocumulus clouds and their optical properties is crucial.

Corresponding author address: Dr. John D. Farrara, Dept. of Atmospheric Sciences, University of California, Los Angeles, 405 Hilgard Ave., Los Angeles, CA 90095-1565. Email: farrara@atmos.ucla.edu

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