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 using both ozone vertical profiles and objective analysis of the European Center for Medium range Weather Forecasting to compute potential vorticity distributions and air mass trajectories. Ozone profiles were measured by a ground‐based lidar system at the Observatoire de Haute Provence (OHP)(43°55N, 5°42E) on November 1990. A stratospheric ozone transport into the troposphere has been observed during a tropopause fold which occurred at the beginning of the cutoff low formation. On a timescale of a few days the correlation is rather good between the potential vorticity and the ozone time evolution in the upper part of the tropopause fold, and the ozone to potential vorticity ratio is of the order of 30–40 ppb/PVu (1 PVu = 10 −6 K m 2 s −1 kg −1 ) in the tropopause fold. The amount of stratospheric ozone transferred by the folding process is estimated to be 6.5±3.5 10 32 molecules d −1 , using a trajectory analysis, the potential vorticity distribution along the trajectory path, and temperature radio sounding profiles, in order to estimate the likelihood of the downward transport of ozone. During the erosion phase of the cutoff low, the tropopause definition has 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 the impact of the folding process. The mechanism resulting in the downward flux of ozone can be twofold: a tropopause lifting associated with convective clouds and small‐scale turbulent mixing near the jet stream. None of them can be discarded, due to the large uncertainties for their respective influences, when using the data set available for this study.