Results from three case study investigations of upper-level jet stream systems document the existence of stratospheric mesoscale cyclonic wind shear in the layer of maximum wind. Anomalously high values of potential vorticity are shown to coincide with the mesoscale cyclonic shear zone. The high values of potential vorticity within an upper level frontal zone were shown to result from shearing vorticity in the mesoscale high potential vorticity region of the stratosphere which is transported downward into the tropospheric frontal zone and becomes transformed into curvature vorticity with little change in thermal stability. The vertical gradient of diabatic temperature change resulting from vertical shear-induced turbulent heat flux, in layers of CAT, is proposed as the generation mechanism responsible for large values of potential vorticity on the mesoscale. It is proposed that turbulent-scale mixing processes are of fast order importance in the evolution of jet stream frontal zone systems.