Further Evidence of the Mesoscale and Turbulent Structure of Upper Level Jet Stream–Frontal Zone Systems

Abstract Recent aircraft observations of the mesoscale and turbulent structure of upper level frontal zone-jet stream systems provide further evidence of stratosperic mesoscale cyclonic wind shear and associated anomalously high values of potential vorticity in the layer of maximum wind. Measurements of turbulent heat flux in regions of clear air turbulence above and below the layer of maximum wind (LMW) document the first-order importance of turbulent-scale processes in the generation and dissipation of potential vorticity. Ozone concentration measurements illustrate the intrusion of stratospheric air into the troposphere and give evidence of the effect of turbulent mixing processes in the LMW. It is proposed that the nonconservative property o@ potential vorticity permits air parcels to enter the stratosphere by direct transport across the potential vorticity discontinuity in the LMW, in agreement with earlier isentropic trajectory calculations.