a) Application of the segmentation mask to the geometrical depth of the iso-surface of τ = 1 for a single frame. Red indicates the location below the calculated iso-surface of τ = 1 in the convection zone and blue the location above this calculated level. b) Granular diameter vs. the mean geometrical depth where τ = 1 for the entire time series. The trend of the scatter is illustrated by linear fits (solid black lines) and a third-order polynomial fit (dashed dark-gray line).