IMPACT OF A CUTOFF LOW DEVELOPMENT ON DOWNWARD TRANSPORT OF OZONE IN THE TROPOSPHERE

A study of ozone transfer from the stratosphere to the troposphere has been performed during two phases of the evolution of a cutoff low usi ng both ozone vertical profiles and objective analysis of the European Center for Medium range Weather Forecasting to compute potential vort icity distributions and air mass trajectories. Ozone profiles were mea sured by a ground-based lidar system at the Observatoire de Haute Prov ence (OHP)(43-degrees-55N, 5-degrees-42E) on November 1990. A stratosp heric ozone transport into the troposphere has been observed during a tropopause fold which occurred at the beginning of the cutoff low form ation. On a timescale of a few days the correlation is rather good bet ween the potential vorticity and the ozone time evolution in the upper part of the tropopause fold, and the ozone to potential vorticity rat io is of the order of 30-40 ppb/PVu (1 PVu = 10(-6) K m2s-1 kg-1) in t he tropopause fold. The amount of stratospheric ozone transferred by t he folding process is estimated to be 6.5 +/- 3.5 10(32) Molecules d-1 , using a trajectory analysis, the potential vorticity distribution al ong the trajectory path, and temperature radio sounding profiles, in o rder to estimate the likelihood of the downward transport of ozone. Du ring the erosion phase of the cutoff low, the tropopause definition ha s been changing, and the stratospheric ozone decrease associated with the cutoff low evolution corresponds to an ozone transport of 4.0 +/- 2.0 10(32) molecules d-1, which is of the same order of magnitude as t he impact of the folding process. The mechanism resulting in the downw ard flux of ozone can be twofold: a tropopause lifting associated with convective clouds and small-scale turbulent mixing near the jet strea m. None of them can be discarded, due to the large uncertainties for t heir respective influences, when using the data set available for this study.