(color online) (a) An example demonstrating the configuration among several positive-negative coupled vortex pairs (shaded, 10 m ׁs ), columns of strong vertical velocities (blue solid lines with a spacing of 3 m⋅s in between) distributed between the vortex couple and streamline (thin solid line with vector arrow) along 23.3° N at 06 UTC on 15 June, 2010. The purple arrow refers to the intrusion of downdraft, which is favorable to the generation of near-ground vorticity; (b) Another example illustrating how the low-level convergence makes vorticity develop and vortex line concentrated. In detail, it shows the zonal-vertical cross sections of vertical vorticity (shaded, 10 m⋅s ), the vertical velocity (isolines with a spacing of 3 m⋅s in between) and streamlines (thin solid line with vector arrow) along 29.82° N at 16 UTC on 19 August, 2010. The MPS circulation west of 102.9° E and vertical updraft east of 102.9° E induced by SWV are marked as large hollow curve arrow.