Top: mean Doppler downflow number 5 from time 11.4 h to time 15.2 h (left) and from time 15.2 h to 19.7 h (right) relative to the beginning of the sequence, overlapped with the mean horizontal velocities. Bottom: vorticity characterised by the |Γ₁| >  0.4 parameter (downflow number 5) from time 11.4 h to 15.2 h (left) and from time 15.2 h to 19.7 h (right), overlapped with the mean horizontal velocities. White and black represent counter-clockwise Γ₁ and clockwise velocities, respectively.