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Analysis of Astex-Stratocumulus Observational Data Using a Mass-Flux Approach

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Abstract

A mass-flux approach is applied to observational data obtained in a convective boundary layer topped with stratocumulus clouds. The observational data were obtained from aircraft measurements during the Atlantic Stratocumulus Transition Experiment (ASTEX). A conditional sampling method is used to calculate average updraft and downdraft values. The vertical fluxes calculated with the mass-flux approach are found to be proportional to the real (measured) fluxes, with a proportionality factor being about 0.6. This value is predicted by theory for two variables having a joint Gaussian distribution function; proportionality factor = 2π-1 ≈ 0.637. The horizontal fractional entrainment and detrainment rates calculated from the data (ε ≈ 1–2 × 10-2 m-1) are an order of magnitude higher than the rates obtained by large eddy simulations for cumulus convection (ε ≈ 2–3 × 10-3 m-1) and two orders of magnitude higher than those used in modelling cumulus convection with a mass-flux scheme in an operational weather forecast model (ε ≈ 3 × 10-4 m-1). A numerical mass-flux model for the thermodynamics was developed and showed that results are in good agreement when compared with measured profiles of the liquid water content.

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Authors and Affiliations

  1. Institute for Marine and Atmospheric Research Utrecht (IMAU), Utrecht University, Princetonplein 5, 3584 CC, Utrecht, The Netherlands

    A. T. J. De Laat & Peter G. Duynkerke

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  1. A. T. J. De Laat
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  2. Peter G. Duynkerke
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De Laat, A.T.J., Duynkerke, P.G. Analysis of Astex-Stratocumulus Observational Data Using a Mass-Flux Approach. Boundary-Layer Meteorology 86, 63–87 (1998). https://doi.org/10.1023/A:1000624004907

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