Mid‐tropospheric frontogenesis

Mid‐tropospheric frontogenesis is investigated for a case study in which special three‐hourly rawinsonde data are available. Isentropic analyses are made by objective means. Isentropic trajectories lead to the three‐dimensional description of the velocity field in space and time. Potential vorticity is used to check trajectories and to distinguish 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 arising from twisting, horizontal confluence, vertical shear of vertical velocity and ageostrophic contributions are delineated. Typical frontogenesis magnitudes are 2°C (100 km) −1 (3 hr) −1 and 2°C (km) −1 (3 hr) −1 in two and three dimensions respectively. Previous observations of the extrusion of stratospheric air into the troposphere are confirmed. A synoptic sequence common to many occurrences of mid‐tropospheric frontogenesis is described. A simple explanation is given of the initial development of a thermally indirect circulation accompanying the early frontogenesis. Mid‐tropospheric frontogenesis seems to accompany the extrusion of stratospheric air into the troposphere during which momentum, potential vorticity, ozone and radioactivity are transported downward and southward.