Turbulent vertical diffusivity in the sub-tropical stratosphere

[摘要] Vertical (cross-isentropic) mixing is produced by small-scaleturbulent processes which are still poorly understood andparamaterized in numerical models. In this work we provide estimatesof local equivalent diffusion in the lower stratosphere by comparingballoon borne high-resolution measurements of chemicaltracers with reconstructed mixing ratio from large ensembles ofrandom Lagrangian backward trajectories using European Centre forMedium-range Weather Forecasts analysed windsand a chemistry-transport model (REPROBUS).We focus on a case study in subtropicallatitudes using data from HIBISCUS campaign.An upper bound on the vertical diffusivity is found in this case studyto be of the orderof 0.5 m² s⁻¹ in the subtropical region, which is larger than theestimates at higher latitudes.The relation between diffusion and dispersion is studiedby estimating Lyapunov exponents and studying their variationaccording to the presence of active dynamical structures.

[学科分类] 大气科学