Snapshot of the swirling strength (green volume rendering) and the optical surface color-coded with vertical velocity (downflows in red and upflows in blue) in Run C. The size of the box shown is 4.8 × 4.8 × 1.4   Mm³. The optical surface is hidden in the lower right quadrant, uncovering the swirling structure in the subsurface layers. An animated plot that shows the temporal evolution of the swirling strength without the optical surface is available in the electronic edition of the journal.