Identification of Dynamical Processes at the Tropopause during the Decay of a Cutoff Low Using High-Resolution Airborne Lidar Ozone Measurements

Abstract : In June 1996, an airborne ozone lidar was successfully used to observe the decay of a cutoff low over southern Europe. This weather system was tracked during several days and sampled with an 8-km horizontal resolution. Most of the measurements took place in the 4–12-km altitude range and ozone-rich layers with a vertical thickness often less than 500 m were seen. Ozone vertical cross sections were obtained at the edges of the cutoff low or in frontal regions next to it. Using complementary data it is shown that these bidimensional cross sections characterize the ozone spatial field well. Two main features of the ozone distribution are large variability of the ozone tropopause and the presence of numerous ozone-rich layers within the troposphere. This paper focuses on the first feature. Observed ozone tropopauses compare well with potential vorticity ones derived from ECMWF analyses. The magnitude of the ozone to potential vorticity ratio also indicates that no significant diabatic mechanism contributes to the ozone transfer from the stratosphere to the troposphere above 325 K. An analysis of the evolution of the ozone vertical gradient in the upper troposphere (80–120 ppb) and lowermost stratosphere (120–200 ppb) is used to illustrate its usefulness as a diagnostic tool of dynamical processes. Large differences are found between air masses near and within the cutoff low. Vertical stretching induced by the PV anomaly cannot completely account for them. Differential advection in frontal regions and convective erosion on the eastern edge of the cutoff low tend to sharpen the vertical ozone gradient. On the contrary, clear sky turbulent mixing tends to smooth it. Convective erosion is also likely to transfer ozone from the stratosphere to the troposphere. This is corroborated by ozone vertical cross sections sampling tropospheric air masses three days after the decay of the cutoff low.