As a laboratory model of downburst, the statistics of a turbulent velocity field of a vertical gravitational flow is investigated. By mechanically breaking a thin film fixed at the bottom of a cylindrical container, a finite mass of a high-density liquid begins to fall into a stationary low-density liquid, forming a vertical thermal. It impinges onto the horizontal ground and then diverges radially outward. By employing particle image velocimetry, the ensemble-averaged maps of velocity vectors, azimuthal vorticity, and turbulent stresses in a meridian plane are obtained. The statistical characteristics in the downdraft stage, impinging stage and diverging stage are examined. The nature and the role of the circulatory flow are demonstrated. The results show reasonable agreement with the actual downbursts observed in the atmosphere. Based on these results, the windshear hazard index for aircraft encountering a downburst is evaluated.
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