Articles | Volume 8, issue 3
Atmos. Chem. Phys., 8, 697–707, 2008

Special issue: HIBISCUS - investigating the impact of tropical convection...

Atmos. Chem. Phys., 8, 697–707, 2008

  12 Feb 2008

12 Feb 2008

Turbulent vertical diffusivity in the sub-tropical stratosphere

I. Pisso and B. Legras I. Pisso and B. Legras
  • Laboratoire de Météorologie Dynamique UMR 8539, Paris, France

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 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 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 HIBISCUS 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 dispersion is studied by estimating Lyapunov exponents and studying their variation according to the presence of active dynamical structures.

Final-revised paper