Scallops

Kelvin-Helmholtz waves do not necessarily appear in vertical wind shear alone as is the case for transverse cloud bands. In fact, they can also be observed in WV imagery as horizontal waves (scallops) near the jet axis at the border of the cirrus cloud band accompanying the subtropical jet streak.

Figure 4: Schematic showing the relative position of turbulence to transverse bands and scallops in a jet cirrus cloud band. Dashed lines enclose regions where moderate turbulence is likely.

Scallops are formed by horizontal perturbations of a laminar air flow in regions of strong horizontal wind shear. These horizontal Kelvin-Helmholtz waves are visible because perturbations are generated at the intersection of the cirrus cloud shield and clear skies (figures 4 and 5). The edge of the cloud often appears ragged, sometimes with thicker, lumpy cloud parts.

Figure 5: MSG WV 6.2 Ám satellite image from 11 April 2013 at 10:30 UTC. The red arrows show the scallop pattern over Spain and the Bay of Biscay.