Transport and mixing in the atmosphere: Anomalous diffusion and stratosphere-troposphere mass exchange.

摘要

Particle dispersion in the stratosphere and mass exchange between the stratosphere and troposphere are investigated in a Lagrangian framework using the United Kingdom Meteorological Office assimilated winds.; In the winter stratosphere, isentropic dispersion can be characterized as chaotic advection. Parcel trajectories intermittently alternate between flying in jets and trapping in coherent quasi-stationary vortices. We show that these trajectories are described by a power-law probability density function (PDF) with a decay exponent of less than 3. For these PDFs mean square displacement per flight step is divergent and the trajectories correspond to Lévy flights (random walk processes with divergent second moment). The variance of the zonal displacement grows faster than linearly with time, corresponding to superdiffusive dispersion. The self-similarity in the trajectories is revealed by the convergence of the scaled PDFs of displacement and the saturation of the normalized third and fourth moments with time.; Stratosphere-troposphere exchange (STE), calculated by a Lagrangian method, exhibits a pronounced annual cycle. The northern hemisphere net flux of stratospheric air into the troposphere across the extratropical tropopause has a primary peak in late spring or early summer, which is consistent with observation. The southern hemisphere net flux of stratospheric air into the troposphere shows a maximum in winter. The preferred locations for stratosphere-to-troposphere transport are identified as the regions with the highest frequency of Rossby wave breaking events and with the storm tracks. This suggests that tropopause folding and baroclinic eddy activity are a primary mechanism for STE. The greatest diabatic injection of tropospheric air into the tropical stratosphere takes place following the annual variation of the Hadley circulation. The tropical upward transport occurs in association with the monsoon anticyclones. The interannual change of total column ozone in the northern hemisphere midlatitudes winter/spring can be explained partly by the variation of STE. The annual net stratospheric mass flux into the troposphere in the northern hemisphere extratropics is estimated as ∼3.5–3.8 × 1017 kg yr−1, which is much larger than previous estimates.