Laboratory observations of turbulent penetrative‐convection planforms

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Abstract
A laboratory model of the convective planetary boundary layer has been used to observe planforms near the surface and the top of the mixed layer. Near the surface the dominant convection pattern consists of irregular open cells with average horizontal dimension of 1.2h, where h is the height of the mixed layer. Smaller‐scale dendritic convergence lines appear near the edges of the open cells and are swept horizontally toward the most energetic plumes. These are often located where several open cell convergence lines intersect. Sometimes a plume is observed to be in a state of rotation, giving the appearance of a dust devil. Viewed in planform near the top of the mixed layer, or in side view, the plumes appear as penetrating, spreading domes of mixed layer fluid. Together, the plumes and their domes constitute closed cells which coexist with the open cells and have about the same horizontal dimension of 1.2h. In planview the closed cells usually appear to be randomly distributed. Over about 4% of the available area, however, the closed cells form the periphery of a large open cell ring with average diameter 3.3h, corresponding to the large rings observed by Hardy and Ottersten [1969] using radar.