Emily Shuckburgh: Publications

PCS photo G. Braathen, NILU

Emily Shuckburgh

Centre for Atmospheric Science
Department of Applied Mathematics and Theoretical Physics
Department of Chemistry
University of Cambridge, UK
E.F.Shuckburgh@amtp.cam.ac.uk

Links to papers (abstracts below)

Diagnosing transport and mixing using a tracer-based coordinate system, Emily Shuckburgh and Peter Haynes, submitted to Physics of Fluids. PDF (36 MB), gzipped PS (9 MB).

Effective Diffusivity as a diagnostic of atmospheric transport. Part I: stratosphere, Peter Haynes and Emily Shuckburgh, J. Geophys. Res, 105, D18, 22777-22794, Sep 27 2000. HTML, PDF, PS, Colour plates.

Effective Diffusivity as a diagnostic of atmospheric transport. Part II: troposphere and lower stratosphere, Peter Haynes and Emily Shuckburgh, J. Geophys. Res, 105, D18, 22795-22810, Sep 27 2000. HTML, PDF, PS, Colour plates.

The influence of the quasi-biennial oscillation on isentropic transport and mixing in the tropics and subtropics, Emily Shuckburgh, Warwick Norton, Alan Iwi and Peter Haynes, J. Geophys. Res., 106, D13, 14,327--14,338. HTML, PDF, PS

The impact of the mixing properties within the Antarctic stratospheric vortex on ozone loss in spring, Adrian Lee, Howard Roscoe, Anna Jones, Peter Haynes, Emily Shuckburgh, Martin Morrey and Hugh Pumphrey, J. Geophys. Res, 106, D3, 3,203-3,211, Feb 16 2001. HTML, PDF, PS, Colour plates.

Diagnosing transport and mixing using a tracer-based coordinate system.

The advection-diffusion equation can be transformed into a pure diffusion equation describing the diffusion of tracer across coordinate surfaces, which are surfaces of equal tracer concentration. The corresponding effective diffusivity may be used to quantify the transport and mixing structure of the flow. This effective diffusivity can be calculated from a tracer field that has evolved in the flow. We illustrate this diagnsotic method using a family of simple chaotic advection flows. The results demonstrate how the effective diffusivity captures the location of barrier regions and mixing regions, and quantifies the strength of mixing and the permeability of barriers in the flow. We also show that the effective diffusivity parameterises the transport of particles relative to the tracer-based coordinates.

Effective Diffusivity as a diagnostic of atmospheric transport. Part I: stratosphere

The transport and mixing properties of the isentropic flow in the lower and middle stratosphere are analysed by using observed winds to advect a tracer on isentropic surfaces in the range 400-850K. The effective diffusivity diagnostic introduced by Nakamura and collaborators is applied to the tracer field in order to identify mixing regions and barriers to transport, and to follow their seasonal evolution. Large effective diffusivity corresponds to strong mixing and small effective diffusivity corresponds to weak mixing, i.e. to barriers. The effective diffusivity shows, in the winter stratosphere of each hemisphere, the evolution of the vortex-edge barrier and the mid-latitude surf zone, and also the extent of any mixing within the vortex. At low latitudes in the stratosphere there is a region of low effective diffusivity whose latitudinal width varies with height, broadening substantially from 400K to 550K and then narrowing slightly above that. The low values of effective diffusivity imply little isentropic transport into or out of this `tropical-reservoir' region. There is a strong seasonal cycle to the reservoir, which has different forms at 400K, 450K-600K and above 650K, determined by the relative influences of tropospheric synoptic eddies and stratospheric planetary waves. Comparison of effective diffusivity between the northern hemisphere winters 1996/97 and 1997/98 shows strong differences at low latitudes according to the phase of the quasi-biennial oscillation (QBO). When there are are QBO easterlies there is a broad region of very low effective diffusivity at low latitudes. When there are QBO westerlies there are very low values of effective diffusivity at low latitudes within the westerlies themselves, but larger values at their edges.

Effective Diffusivity as a diagnostic of atmospheric transport. Part II: troposphere and lower stratosphere

The effective diffusivity diagnostic is used to analyse the transport and mixing properties of the isentropic flow in the upper troposphere and the lower stratosphere. The approach described in Part I, of calculating effective diffusivity from a tracer field advected by observed winds, is applied to the flow on isentropic surfaces in the range 300-450K. Local minima in effective diffusivity on isentropic surfaces in the range 330-400K, indicate transport barriers in each hemisphere associated with the extratropical tropopause. The strongest part of these `tropopause barriers' are coincident with the core of the subtropical jet at about 350K. The seasonal evolution of the tropopause barriers are examined. The effective diffusivity shows the barriers to be strongest in winter and considerably weakened by the monsoon circulations in summer. The barrier in the southern hemisphere is seen to be generally stronger than that in the same season in the northern hemisphere. The minimum value of effective diffusivity is proposed as a new definition of the tropopause, more generally applicable than traditional definitions based on potential vorticity (PV) values. The new definition is compared with PV-based definitions and it is found that a value of +/-2PVU seems to correspond most closely to the minimum effective diffusivity at 330K but values +/-2.5PVU and +/-4.5PVU seem more appropriate at 350K and 370K respectively. These PV values are often exceeded during the summer monsoon period. The polar vortex-edge barrier in the lowest part of the stratosphere is also examined. It is demonstrated that the lower limit of the vortex-edge barrier, i.e. the `sub-vortex' transition, varies in altitude throughout the winter. In the Antarctic the transition generally occurs at 380K and is sometimes as low as 350K. In the Arctic the transition is higher, rarely occurring below 400K and frequently occurring above 450K.

The influence of the quasi-biennial oscillation on isentropic transport and mixing in the tropics and subtropics.

The influence of the quasi-biennial oscillation (QBO) on isentropic transport and mixing in the tropical and subtropical stratosphere is investigated over a period of six years. The transport and mixing is quantified by the equivalent length diagnostic, calculated from tracers simulated in chemical transport models using ECMWF analysed winds. A procedure for calculating equivalent length from tracers, such as N₂O, with a tropical maximum or minimum is devised. Results from the different tracers and different chemical transport models demonstrate the robustness of the equivalent length diagnostic. Equivalent length calculated both from an artificial tracer and from simulated N₂O indicates that when the QBO winds are easterly, mixing is inhibited in the tropics throughout the broad region occupied by the easterlies, whilst when the QBO winds are westerly, mixing is strongly inhibited within the narrow region occupied by the westerlies themselves, but is enhanced in the subtropics. Examination of zonal-mean quasi-geostrophic potential vorticity gradients and horizontal EP fluxes (broken down into contributions from different zonal wavenumbers) suggests that, in the ECMWF analyses, barotropic shear instability of the westerly jet, as well as propagation of planetary waves from the extratropics, drives the subtropical mixing seen in the westerly phase.

The impact of mixing properties within the Antaractic stratospheric vortex on ozone loss in spring.

Calculations of equivalent length frm an artificial advected tracer provide new insight into the isentropic transport processes occurring within the Antarctic stratospheric vortex. These calculations show two distinct regions of approximately equal area: a strongly-mixed vortex core; and a broad ring of weakly-mixed air extending out to the vortex boundary. This broad ring of vortex air remains isolated from the core between late winter and mid-spring. Satellite measurements of stratospheric H₂O confirm that the isolation lasts until at least mid-ctober. A three-dimensional chemical transport model simulation of the Antarctic ozone hole quantifies the ozone loss within this ring and demonstrates its isolation. In contrast to the vortex core, ozone loss in the weakly-mixed broad ring is not complete. The reasons are two-fold. First, warmer temperatures in the broad ring prevent continuous PSC formation and the associated chemical processing (i.e., the conversion of unreactive chlorine into reactive forms). Second, the isolation prevents ozone-rich air from the broad ring mixing with chemically-processed air from the vortex core. If the stratosphere continues to cool, this will lead to increased PSC formation and more complete chemical processing in the broad ring. Despite the expected decline in halocarbons, sensitivity studie suggest that this mechanism will lead to enhanced ozone loss in the weakly-mixed region, delaying the future recovery of the ozone hole.

Last update: 20th Feb 2000