Quantification of lower stratospheric mixing processes using aircraft data

摘要: The reduction in scale of filamentary structures stratospheric tracers due to stretching by the large-scale flow must ultimately be halted processes that lead molecular mixing. A quantitative bound on strength such processes, represented an effective diffusivity D, is here inferred from high-resolution tracer data recent airborne campaigns, using simple mathematical models representing and mixing processes. are based hypothesis structure observed along quasi-horizontal aircraft flight tracks corresponds reality intersection track with sloping sheets tracer. analyzed identify features unambiguously correspond isentropic surfaces. Clearly identifiable filaments seen down scales less than 100 km, transitions values at edge over 10 km. Backward trajectories starting neighborhood show convincingly air inside outside has different origin. Integration equation for gradient forward allows orientation corresponding deduced. On assumption small compared variation velocity field, a diffusion may derived governs spatial sheets. coefficient this product actual D time-varying function depends gradient. latter determined integration trajectories. integrated D. If value sufficiently large, implies filament would diffuse away too rapidly consistent observations. Finding maximum acceptable gives estimate upper real atmosphere. results suggest relevant these can no more -2 m 2 s -l some cases considerably less. Comparison made analysis Waugh et al., [1997], who considered could, basis relation between ratios tracers, deduced partially mixed. Applying our method same observations suggests about 1.4 x -1 . This consistent, if typical aspect horizontal vertical taken into account, al.'s equivalent diffusivity.