FIGURE 7

A schematic of the helical motion of air parcels imposed by the radiation of mesoscale atmospheric gravity waves originating from a rotating cluster of snowsqualls. As the parcels approach the mesolow (L in Figure 6), progressively stronger ageostrophic components induce the parcels to become elements of a roll cloud. Within the roll cloud, anticyclonic shear arising from curl exactly counterbalances the cyclonic curl to yield purely horizontal components of spin.