Mid-tropospheric Frontogenesis and Potential Vorticity

Mid-tropospheric frontogenesis is investigated for a case study in which special three-hourly rawinsonde data are available. Isentropic analyses are generated by objective means. Detailed machine isentropic trajectories lead to the three-dimensional description of the velocity field in space and time accompanying the frontogenesis. Ertel potential vorticity calculations serve as a check on trajectories and as a means of delineating stratospheric from tropospheric air. Miller's frontogenesis equation is applied in two and three dimensions to an intensifying mid-tropospheric baroclinic zone. Frontogenetical and frontolytical regions from the twisting, horizontal confluence, vertical shear of vertical velocity and ageostrophic contributions are delineated. Typical frontogenesis magnitudes are 2C (100 km)-l(3 hrs) -1 and 2C (km)-l(3 hrs)-l in two and three dimensions respectively. Potential vorticity behavior is such to confirm previous observations of the extrusion of stratospheric air into the troposphere. Frictional rather than radiative effects are shown to be more important to the non-conservation of potential vorticity. A synoptic sequence common to many occurrences of midtropospheric frontogenesis is described. Quasi-geostrophic reasoning is employed to explain the initial development of a thermally indirect circulation accompanying the early frontogenesis. Calculations suggest that the frontogenesis is playing a significant role in the downward momentum transport in mid latitudes. Mid-tropospheric frontogenesis seems to be a manifestation of the extrusion of stratospheric air into the iii troposphere during which momentum, potential vorticity, ozone and radioactivity are transported downward and southward. Thesis Supervisor: Frederick Sanders Title: Professor of Meteorology