Stirring and mixing in two-dimensional divergent flow

While stirring and mixing properties in the stratosphere are reasonably well understood in the context of balanced (slow) dynamics, as is evidenced in numerous studies of chaotic advection, the strongly enhanced presence of high-frequency gravity waves in the mesosphere gives rise to a significant unbalanced (fast) component to the flow. The present investigation analyses result from two idealized shallow-water numerical simulations representative of stratospheric and mesospheric dynamics on a quasi-horizontal isentropic surface. A generalization of the Hua–Klein Eulerian diagnostic to divergent flow reveals that velocity gradients are strongly influenced by the unbalanced component of the flow. The Lagrangian diagnostic of patchiness nevertheless demonstrates the persistence of coherent features in the zonal component of the flow, in contrast to the destruction of coherent features in the meridional component. Single-particle statistics demonstrate t2 scaling for both the stratospheric and mesospheric regimes in the case of zonal dispersion, and distinctive scaling laws for the two regimes in the case of meridional dispersion. This is in contrast to two-particle statistics, which in the mesospheric (unbalanced) regime demonstrate a more rapid approach to Richardson’s t3 law in the case of zonal dispersion and is evidence of enhanced meridional dispersion

Large-scale mixing in the middle atmosphere

grantor: University of TorontoLarge-scale mixing in the Earth's middle atmosphere (stratosphere and mesosphere) is governed by various fluid-dynamical phenomena. Sought in the present investigation is a quantification of the notion that the stratosphere, dominated by planetary-scale Rossby waves, is characterized by "stirring", while the mesosphere, dominated by internal gravity waves, is characterized by "mixing". The concepts of balance versus imbalance and spectral nonlocality versus locality are used to assess results from two shallow-water numerical experiments representative of stratospheric and mesospheric dynamics on a quasi-horizontal isentropic surface. Topological properties of the flow are examined using various geometrical diagnostics. Assessment of the Okubo-Weiss and Hua-Klein criteria, which characterize strain- and vorticity-dominated regions, reveals that velocity gradients are strongly influenced by the presence of an unbalanced component to the flow. Patchiness, wherein the average Lagrangian velocity is monitored, demonstrates persistence of coherent spatial structure in the zonal direction, owing to the presence of a zonal shear flow, and the destruction of spatial structure in the meridional direction. Spatial distributions of Liapunov exponents, which monitor the stretching properties of the flow, illustrate the erosion of large-scale strain-dominated regions (characteristic of stratospheric dynamics) with the inclusion of an unbalanced component. The results from each of these diagnostics suggest that stirring provides an appropriate description for stratospheric dynamics, and mixing, as characterized by turbulent diffusion, for mesospheric dynamics. Statistical analyses of absolute (single-particle) and relative (two-particle) dispersion further elucidate these connections. Single-particle statistics are governed by the Lagrangian velocity, and relative dispersion by the Lagrangian velocity gradients. The former behaviour is demonstrated in the non-Fickian ('t'2) zonal dispersion evident for both the stratospheric and mesospheric cases. Differences between the stratospheric and mesospheric regimes are however manifested in distinctive meridional dispersion statistics at long times. By contrast, relative dispersion is shown to effectively capture the distinction between spectrally nonlocal and local dynamics, and in so doing, to reflect the unbalanced dynamics characterized by shallow slopes of the kinetic energy spectra. Explored also in the present work is the determination of an appropriate description for passive tracer dynamics in spatially and temporally irregular flow.Ph.D

Addressing human security in the Arctic in the context of climate change through science and technology

Human security, Climate change, Science and technology policy, Quality-of-life indicators,