A quantitative measure of polar vortex strength using the function <i>M</i>

Smith, Madeleine Lindsay; McDonald, A. J.

doi:10.1002/2013jd020572

Cited by 28 publications

(29 citation statements)

References 88 publications

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“…[22] discuss particular examples in which contour plots of a Lagrangian descriptor may not clearly capture the presence of singularities, but the velocity fields in these examples are not representative of the ones found in the geophysical context considered in the present paper. In view of these demonstrated properties, M has already been successfully applied in studies on dynamical and transport processes in the stratosphere ( [24,17,25,26,27,12]). Figure 3 shows stereographic projections of the function M evaluated using τ = 5 days on the 850 K isentropic surface for the same days as Figure 1.…”

Section: The Lagrangian Descriptor Function Mmentioning

confidence: 99%

Lagrangian study of the final warming in the southern stratosphere during 2002: Part I. The vortex splitting at upper levels

et al. 2019

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The present two-part paper provides a Lagrangian perspective of the final southern warming in 2002, during which the stratospheric polar vortex (SPV) experienced a unique splitting. Part I focuses on the understanding of fundamental processes for filamentation and ultimately for vortex splitting on a selected isentropic surface in the middle stratosphere. Part II discusses the three-dimensional evolution of the selected sudden warming event. We approach the subject from a dynamical systems viewpoint and search for Lagrangian coherent structures using a Lagrangian descriptor as a tool. In this Part I we work in the idealized framework of a kinematic model that allows for an understanding of the contributing elements of the flow in late September during the splitting. We introduce a definition of kinematic SPV boundary based on a criterion for binning parcels inside and outside the vortex according to their values of the Lagrangian descriptor and associated PDF. This definition is justified by using arguments that go beyond heuristic considerations based on the potential vorticity. Next, we turn to the filamentation proceses along this kinematic boundary, and the role of Lagrangian structures in the SPV splitting. We determine a criterion for splitting based on the structure of the evolving unstable and unstable manifolds.

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Section: The Lagrangian Descriptor Function Mmentioning

confidence: 99%

Lagrangian study of the final warming in the southern stratosphere during 2002: Part I. The vortex splitting at upper levels

et al. 2019

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“…The Lagrangian trajectory model used in this study was developed at the University of Canterbury and is a modified version of that used and discussed in Alexander et al (2013), McDonald and Smith (2013) and Smith and McDonald (2014). It uses a fourth-order Runge-Kutta algorithm, with a 10 min time-step, with reanalysis wind speeds determined at the trajectory position using the spatial-temporal interpolation scheme detailed above.…”

Section: Methodsmentioning

confidence: 99%

“…the rate of ozone loss in the stratospheric polar vortex (Tuck et al, 2003). Calculated trajectories are also used to infer various metrics of mixing (Nakamura, 1996;Haynes and Shuckburgh, 2000;Smith and McDonald, 2014). Determining trajectories is also central to domain-filling techniques which allow fine-scale structure in chemical constituent fields to be derived from space-based measurements (Sutton et al, 1994;Smith and McDonald, 2014).…”

Section: S Friedrich Et Al: a Comparison Of Loon Balloon Observamentioning

confidence: 99%

A comparison of Loon balloon observations and stratospheric reanalysis products

et al. 2017

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Abstract. Location information from long-duration superpressure balloons flying in the Southern Hemisphere lower stratosphere during 2014 as part of X Project Loon are used to assess the quality of a number of different reanalyses including National Centers for Environmental Prediction Climate Forecast System version 2 (NCEP-CFSv2), European Centre for Medium-Range Weather Forecasts (ERAInterim), NASA Modern Era Retrospective-Analysis for Research and Applications (MERRA), and the recently released MERRA version 2. Balloon GPS location information is used to derive wind speeds which are then compared with values from the reanalyses interpolated to the balloon times and locations. All reanalysis data sets accurately describe the winds, with biases in zonal winds of less than 0.37 m s −1 and meridional biases of less than 0.08 m s −1 . The standard deviation on the differences between Loon and reanalyses zonal winds is latitude-dependent, ranging between 2.5 and 3.5 m s −1 , increasing equatorward.Comparisons between Loon trajectories and those calculated by applying a trajectory model to reanalysis wind fields show that MERRA-2 wind fields result in the most accurate simulated trajectories with a mean 5-day balloonreanalysis trajectory separation of 621 km and median separation of 324 km showing significant improvements over MERRA version 1 and slightly outperforming ERA-Interim. The latitudinal structure of the trajectory statistics for all reanalyses displays marginally lower mean separations between 15 and 35 • S than between 35 and 55 • S, despite standard deviations in the wind differences increasing toward the equator. This is shown to be related to the distance travelled by the balloon playing a role in the separation statistics.

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“…McDonald and Smith, 2013), Lagrangian metrics such as Lyapunov exponents or function M (e.g. Garny et al, 2007;Smith and McDonald, 2014;Serra et al, 2017), the complexity of tracer gradients through effective diffusivity (e.g. Haynes and Shuckburgh, 2000), or a combination of different metrics (e.g.…”

Section: Introductionmentioning

confidence: 99%

Bifurcation of potential vorticity gradients across the Southern Hemisphere stratospheric polar vortex

2018

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Abstract. The wintertime stratospheric westerly winds circling the Antarctic continent, also known as the Southern Hemisphere polar vortex, create a barrier to mixing of air between middle and high latitudes. This dynamical isolation has important consequences for export of ozone-depleted air from the Antarctic stratosphere to lower latitudes. The prevailing view of this dynamical barrier has been an annulus compromising steep gradients of potential vorticity (PV) that create a single semi-permeable barrier to mixing. Analyses presented here show that this barrier often displays a bifurcated structure where a double-walled barrier exists. The bifurcated structure manifests as enhanced gradients of PV at two distinct latitudes -usually on the inside and outside flanks of the region of highest wind speed. Metrics that quantify the bifurcated nature of the vortex have been developed and their variation in space and time has been analysed. At most isentropic levels between 395 and 850 K, bifurcation is strongest in mid-winter and decreases dramatically during spring. From August onwards a distinct structure emerges, where elevated bifurcation remains between 475 and 600 K, and a mostly single-walled barrier occurs at other levels. While bifurcation at a given level evolves from month to month, and does not always persist through a season, interannual variations in the strength of bifurcation display coherence across multiple levels in any given month. Accounting for bifurcation allows the region of reduced mixing to be better characterised. These results suggest that improved understanding of cross-vortex mixing requires consideration of the polar vortex not as a single mixing barrier but as a barrier with internal structure that is likely to manifest as more complex gradients in trace gas concentrations across the vortex barrier region.

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A quantitative measure of polar vortex strength using the function M

Cited by 28 publications

References 88 publications

Lagrangian study of the final warming in the southern stratosphere during 2002: Part I. The vortex splitting at upper levels

Lagrangian study of the final warming in the southern stratosphere during 2002: Part I. The vortex splitting at upper levels

A comparison of Loon balloon observations and stratospheric reanalysis products

Bifurcation of potential vorticity gradients across the Southern Hemisphere stratospheric polar vortex

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