[1] This paper provides a review of stratosphere-troposphere exchange (STE), with a focus on processes in the extratropics. It also addresses the relevance of STE for tropospheric chemistry, particularly its influence on the oxidative capacity of the troposphere. After summarizing the current state of knowledge, the objectives of the project Influence of Stratosphere-Troposphere Exchange in a Changing Climate on Atmospheric Transport and Oxidation Capacity (STACCATO), recently funded by the European Union, are outlined. Several papers in this Journal of Geophysical ResearchAtmospheres special section present the results of this project, of which this paper gives an overview. STACCATO developed a new concept of STE in the extratropics, explored the capacities of different types of methods and models to diagnose STE, and identified their various strengths and shortcomings. Extensive measurements were made in central Europe, including the first monitoring over an extended period of time of beryllium-10 ( 10 Be), to provide a suitable database for case studies of stratospheric intrusions and for model validation. Photochemical models were used to examine the impact of STE on tropospheric ozone and the oxidizing capacity of the troposphere. Studies of the present interannual variability of STE and projections into the future were made using reanalysis data and climate models.

[1] During the EU-project Influence of Stratosphere-Troposphere exchange in a Changing Climate on Atmospheric Transport and Oxidation Capacity (STACCATO), a combined approach of a measurement network and numerical simulations was used to estimate the strength and frequency of stratosphere-to-troposphere transport (STT) events and their influence on tropospheric chemistry. Measurements of surface ozone, beryllium-7, and beryllium-10 concentrations and meteorological parameters at four European high mountain stations, as well as atmospheric profiles obtained by ozone soundings and a high-resolution lidar, were carried out. In order to simulate STT events, seven different models have been applied by the STACCATO partners. These are two trajectory models (LAGRANTO and FLEXTRA), a Lagrangian transport model (FLEXPART), a Lagrangian chemistry-transport model (STOCHEM), a Eulerian transport model (TM3), and two general circulation models (ECHAM4 and MA-ECHAM4). In order to investigate the strengths and weaknesses of each of these models and to identify the reasons for their discrepancies, a detailed comparison with measured data is presented in this paper. These models provided fluxes and concentrations of a stratospheric tracer, as well as the vertical profiles of ozone and radionuclides for a stratospheric intrusion case study that occurred over Europe in the year 1996. The comparison of the model results with the measurement data and the satellite observations revealed that all the models captured the general behavior of the event. However, great differences were found in the intensity and spatial development of the simulated intrusion event.

[1] This paper presents one of the first extensive intercomparisons of models and methods used for estimating stratosphere-troposphere exchange (STE). The study is part of the European Union project Influence of Stratosphere Troposphere Exchange in a Changing Climate on Atmospheric Transport and Oxidation Capacity (STACCATO). Nine different models and methods, including three trajectory methods, one Eulerian method, two Lagrangian and one Eulerian transport model, and two general circulation models applied the same initialization. Stratospheric and tropospheric tracers have been simulated, and the tracer mass fluxes have been calculated through the tropopause and the 700 hPa surface. For a 12-day case study over Europe and the northeast Atlantic the simulated tracer mass fluxes have been intercompared. For this case the STE simulations show the same temporal evolution and the same geographical pattern of STE for most models and methods, but with generally different amplitudes (up to a factor of 4). On the other hand, for some simulations also the amplitudes are very similar.

Abstract. A Lagrangian technique is developed and applied to calculate stratosphere-troposphere exchange in an extratropical cyclone. This exchange is computed from the potential vorticity or PV along trajectories, calculated from ECMWF circulation data. Special emphasis is put on the statistical signi®cance of the results. The computed ®eld of the cross-tropopause¯ux is dominated by elongated patterns of statistically signi®cant large downward and small upward¯uxes. The downward¯uxes mainly occur in the lower part of the considered tropopause folds. The upward¯uxes are found near the entrance of the folds, in the tropopause ridges. The ratio between the area averaged downward and upward cross-tropopause¯uxes increases with increasing strength of the cyclone. Since the largest uxes are shown to occur in the regions with the largest wind shear, where PV-mixing is thought to cause large cross-tropopause¯uxes, the results are expected to be reliable, at least in a qualitative sense. The position of a tropopause fold along the northwest coast of Africa is con®rmed by total ozone observations. The results indicate that the applied Lagrangian technique is an appropriate tool for diagnosing stratosphere-troposphere exchange.

Abstract. A Lagrangian technique is developed and applied to calculate stratosphere-troposphere exchange in an extratropical cyclone. This exchange is computed from the potential vorticity or PV along trajectories, calculated from ECMWF circulation data. Special emphasis is put on the statistical significance of the results. The computed field of the cross-tropopause flux is dominated by elongated patterns of statistically significant large downward and small upward fluxes. The downward fluxes mainly occur in the lower part of the considered tropopause folds. The upward fluxes are found near the entrance of the folds, in the tropopause ridges. The ratio between the area averaged downward and upward cross-tropopause fluxes increases with increasing strength of the cyclone. Since the largest fluxes are shown to occur in the regions with the largest wind shear, where PV-mixing is thought to cause large cross-tropopause fluxes, the results are expected to be reliable, at least in a qualitative sense. The position of a tropopause fold along the northwest coast of Africa is confirmed by total ozone observations. The results indicate that the applied Lagrangian technique is an appropriate tool for diagnosing stratosphere-troposphere exchange.Key words: Meteorology and atmospheric dynamics (general circulation; mesoscale meteorology; middle atmosphere dynamics)