An Improved Double-Gaussian Closure for the Subgrid Vertical Velocity Probability Distribution Function

Journal Article · Thu Jan 03 00:00:00 EST 2019 · Journal of the Atmospheric Sciences

DOI:https://doi.org/10.1175/JAS-D-18-0149.1· OSTI ID:1489424

Fitch, A. C. ^[1]

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1. Department of Meteorology, Stockholm University, Stockholm, Sweden, and Pacific Northwest National Laboratory, Richland, Washington, and Bolin Centre for Climate Research, Stockholm, Sweden

Abstract The vertical velocity probability distribution function (PDF) is analyzed throughout the depth of the lower atmosphere, including the subcloud and cloud layers, in four large-eddy simulation (LES) cases of shallow cumulus and stratocumulus. Double-Gaussian PDF closures are examined to test their ability to represent a wide range of turbulence statistics, from stratocumulus cloud layers characterized by Gaussian turbulence to shallow cumulus cloud layers displaying strongly non-Gaussian turbulence statistics. While the majority of the model closures are found to perform well in the former case, the latter presents a considerable challenge. A new model closure is suggested that accounts for high skewness and kurtosis seen in shallow cumulus cloud layers. The well-established parabolic relationship between skewness and kurtosis is examined, with results in agreement with previous studies for the subcloud layer. In cumulus cloud layers, however, a modified relationship is necessary to improve performance. The new closure significantly improves the estimation of the vertical velocity PDF for shallow cumulus cloud layers, in addition to performing well for stratocumulus. In particular, the long updraft tail representing the bulk of cloudy points is much better represented and higher-order moments diagnosed from the PDF are also greatly improved. However, some deficiencies remain owing to fundamental limitations of representing highly non-Gaussian turbulence statistics with a double-Gaussian PDF.

• Sponsoring Organization: USDOE Office of Science (SC), Biological and Environmental Research (BER)
• Grant/Contract Number: Climate Model Development and Validation; AC06-76RLO 1830
• OSTI ID: 1489424
• Journal Information: Journal of the Atmospheric Sciences, Journal Name: Journal of the Atmospheric Sciences Vol. 76 Journal Issue: 1; ISSN 0022-4928
• Publisher: American Meteorological Society
• Country of Publication: United States
• Language: English