Decay of a cut-off low and contribution to stratosphere-troposphere exchange

We present a case study of the decay of a cut-off low over north-west Europ e in June 1996, to establish how the stratospheric air initially contained within it was transferred to the troposphere. Two mechanisms for stratosphe re-troposphere exchange are examined: direct convective erosion of the base of the low, and filamentation of the outer layers of the low along the fla nk of the polar jet stream. The approach taken relies on a combination of i n-situ ozone and humidity measurements by MOZAIC (Measurement of Ozone and water Vapour by Airbus In-service airCraft) aircraft and ozonesondes, and t he European Centre for Medium-Range Weather Forecasts analyses. MOZAIC ozon e is used to choose two analyses eight days apart at the genesis (14 June 1 996) and decay (22 June 1996) of the low which have a consistent ozone/pote ntial-vorticity relationship. Trajectories (both isentropic and three dimen sional (3D)) between these two analyses reveal a consistent pattern; at the base of the low (310 K, 450 mb) all the trajectories attain tropospheric P V values whereas, at 320 K, those trajectories that leave the low experienc e a decrease in PV and those that do not leave the low retain their initial PV. We propose that air parcels leaving the low were stretched into thin f ilaments along the flank of the jet stream, which made them vulnerable to 3 D mixing. A MOZAIC flight on 21 June 1996 provides direct evidence for this process. Up to 22 June 1996 (by which time the low had lost its closed circulation) the satellite images showed very little convection beneath the correspondin g PV anomaly. Mixing was only effective at the very base of the stratospher ic air at 310 K. On 22 June the remaining remnant of high PV was advected i nto a region of deep convection over central and eastern Europe, mixing the remaining stratospheric air into the troposphere. Of the initial mass of 1 0(15) kg of stratospheric air contained in the low, 6 x 10(14) kg was strip ped into filaments along the jet and 4 x 10(14) kg, remained to be mixed by convection during the period 22-23 June 1996.