Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 7 3 2007 10.5194/acpd-7-6603-2007 http://www.atmos-chem-phys-discuss.net/7/6603/2007/ http://www.atmos-chem-phys-discuss.net/7/6603/2007/acpd-7-6603-2007.html http://www.atmos-chem-phys-discuss.net/7/6603/2007/acpd-7-6603-2007.pdf 6603 6629 2007-05-16 Turbulent vertical diffusivity in the sub-tropical stratosphere I. Pisso ipisso@lmd.ens.fr B. Legras Laboratoire de Meteorologie Dynamique UMR 8539, Paris, France Vertical (cross-isentropic) mixing is produced by small-scale turbulent processes which are still poorly understood and parametrized in numerical models. In this work we provide estimates of local equivalent diffusion in the lower stratosphere by comparing balloon borne high-resolution measurements of chemical tracers with reconstructed mixing ratio from large ensembles of random Lagrangian backward trajectories using European Center for Medium-range Weather Forecasts analysed winds and a chemistry-transport model (REPROBUS). We have investigated cases in subtropical latitudes using data from HIBISCUS campaign. Upper bound on the vertical diffusivity is found 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 dispersion is studied by estimating Lyapunov exponents and studying their variation according to the presence of active dynamical structures. Aliwell, S R., Halsall, J F., Pratt, K. 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