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Observations of Vertical Velocity Skewness within the Marine Stratocumulus-Topped Boundary Layer

Kerry A. MoyerThe Pennsylvania State University, Department of Meteorology, University Park, Pennsylvania

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George S. YoungThe Pennsylvania State University, Department of Meteorology, University Park, Pennsylvania

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Abstract

During June and July 1987 the First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment (FIRE) was conducted within the marine stratocumulus-topped boundary layer off the coast of California. One of the goals of the experiment was to investigate the structure of turbulent convection within the FIRE boundary layers. The skewness of vertical velocity has proven to be a very useful statistic in the quest to understand this structure. Profiles of vertical velocity skewness from three turbulence flights conducted during FIRE are presented here. These three flights were chosen because each represents a distinctly different pattern of vertical velocity skewness. The observations of vertical velocity skewness from each of these flights are compared to the skewness statistics derived from a large-eddy simulation (LES) of a boundary layer whose profile of buoyancy flux is qualitatively similar to that found on the given FIRE flight. Plausible explanations are put forward in an effort to address some of the differences between the FIRE observations of vertical velocity skewness and those predicted by the LES models. It is suggested that the exclusion of shortwave radiation and/or precipitation from these LES models may be at least partially responsible for some of these deviations.

Abstract

During June and July 1987 the First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment (FIRE) was conducted within the marine stratocumulus-topped boundary layer off the coast of California. One of the goals of the experiment was to investigate the structure of turbulent convection within the FIRE boundary layers. The skewness of vertical velocity has proven to be a very useful statistic in the quest to understand this structure. Profiles of vertical velocity skewness from three turbulence flights conducted during FIRE are presented here. These three flights were chosen because each represents a distinctly different pattern of vertical velocity skewness. The observations of vertical velocity skewness from each of these flights are compared to the skewness statistics derived from a large-eddy simulation (LES) of a boundary layer whose profile of buoyancy flux is qualitatively similar to that found on the given FIRE flight. Plausible explanations are put forward in an effort to address some of the differences between the FIRE observations of vertical velocity skewness and those predicted by the LES models. It is suggested that the exclusion of shortwave radiation and/or precipitation from these LES models may be at least partially responsible for some of these deviations.

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