under a Creative Commons License. Atmospheric Chemistry and Physics Turbulent vertical diffusivity in the sub-tropical stratosphere

Abstract. Vertical (cross-isentropic) mixing is produced by small-scale turbulent processes which are still poorly understood and paramaterized in numerical models. In this work we provide estimates of local equivalent diffu-sion in the lower stratosphere by comparing balloon borne high-resolution measurements of chemical tracers with re-constructed mixing ratio from large ensembles of random Lagrangian backward trajectories using European Centre for Medium-range Weather Forecasts analysed winds and a chemistry-transport model (REPROBUS). We focus on a case study in subtropical latitudes using data from HIBIS-CUS campaign. An upper bound on the vertical diffusivity is found in this case study to be of the order of 0.5 m2 s−1 in the subtropical region, which is larger than the estimates at higher latitudes. The relation between diffusion and disper-sion is studied by estimating Lyapunov exponents and study-ing their variation according to the presence of active dynam-ical structures.