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Richard Swinbank and Alan O'Neill

Abstract

A data assimilation system has been developed at the UK Meteorological Office to analyze the mix of observations available in the troposphere and stratosphere. The data assimilation system is based on the analysis correction scheme used at the UK Meteorological Office for operational weather forecasting.

The assimilation system is currently being used to supply near real-time analyses of meteorological fields from the troposphere and stratosphere to the Upper Atmosphere Research Satellite (UARS) Science Team. At this stage, these analyses are based on a similar set of observations to the operational analyses, so they provide an independent check of the UARS observations. In the stratosphere they are largely based on soundings from the National Oceanic and Atmospheric Administration polar orbiters.

Some results from the assimilation system are presented for periods in January and August 1992. They are compared with equivalent products from the National Meteorological Center. A particular study is made of the Arctic polar vortex in late January.

Future developments of the assimilation system are also described. First, the vertical domain of the model will be extended into the middle mesosphere, with some improvement in vertical resolution. Second, the system will he extended to assimilate data from UARS instruments in addition to the other observation types; these UARS data will include observations of temperatures, winds, and long-lived chemical species.

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Andrew J. Charlton-Perez and Alan O’Neill

Abstract

Long decorrelation time scales of the annular mode are observed in the lower stratosphere. This study uses a simple dynamical model, which has been used extensively to study stratosphere–troposphere coupling to investigate the origin of the long dynamical time scales. Several long runs of the model are completed, with different imposed thermal damping time scales in the stratosphere. The dynamical time scales of the annular mode are found to be largely insensitive to the input thermal damping time scales, producing similar dynamical time scales in all cases below 50 hPa. This result suggests that the hypothesis that long time scales in the lower stratosphere are due to long radiative time scales in this region is false.

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Ping Chen, James R. Holton, Alan O'Neill, and Richard Swinbank

Abstract

The isentropic mass exchange between the Tropics and extratropics in the stratosphere is investigated with a semi-Lagrangian transport model for the periods from 1 June to 31 October 1992 and from 1 December 1992 to 30 April 1993 using winds from the U.K. Meteorological Office data assimilation system. Calculations with an idealized, initially zonally symmetric tracer show that in the middle and upper stratosphere the bulk of tropical air is transported into the midlatitudes of the winter hemisphere although there exist quasi-permeable barriers in the subtropics. The transport takes place in the form of planetary-scale “tongues” of material that are drawn poleward in association with the episodic amplification of planetary-scale waves in high latitudes of the winter hemisphere. Once air of tropical origin is transported to the midlatitudes it is irreversibly mixed with the midlatitude air in the “surf zone.” Air of tropical origin can, however, hardly penetrate into the interior of the winter polar vortex until the breakdown of the vortex. Transport of tropical air into the midlatitudes of the summer hemisphere is strongly inhibited.

In the lower stratosphere, tropical air is transported into the northern and southern midlatitudes. During the period from 1 June to 31 October 1992, the amount of tropical air transported into the Northern Hemisphere is, however, much smaller than that transported into the Southern Hemisphere, and there exist strong gradients in the tracer field in the equatorial region, indicating that there is a quasi-permeable barrier to cross-equator mass exchange. During the period from 1 December 1992 to 30 April 1993, on the other hand, roughly the same amounts of tropical air are transported into the northern midlatitudes and into the southern midlatitudes, and there exist no significant transport barriers in the equatorial area.

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John D. Farrara, Michael Fisher, Carlos R. Mechoso, and Alan O'Neill

Abstract

The early winter (mid-April to mid-July) circulation in the Southern Hemisphere stratosphere is studied. Emphasis is placed on the evolution of strong disturbances with structures dominated by the zonal wavenumber-1 component of the flow (wave 1). The approach to this investigation is based on analysis of 12 years (1979–90) of observational data and comparative analyses of control and hypothesis-testing simulations with a three-dimensional primitive equation model of the stratosphere and mesosphere.Considerable interannual variability is found in both the intensity and timing of wave-1 amplification during early winter. Though usually quasi-stationary, there are six extended periods in the dataset when wave 1 travels steadily eastward and is of large amplitude. Two of these periods (June 1980 and June 1985) are examined in detail. The evolution of the circulation in these two cases resembles that during Canadian warmings in the Northern Hemisphere in several ways. First, there is a large, eastward-moving disturbance with a nearly equivalent barotropic structure, with the largest amplitude in the lower and middle stratosphere. Second, temperature increases are smaller than those observed during final warmings in the Southern Hemisphere. Third, irreversible buckling of contours of Ertel's potential vorticity takes place in a region well away from the zero-wind line. Owing to their geographical preference for development over the South Pacific, wave-1 events in the southern stratosphere during early winter are referred to as South Pacific warmings.The hypothesis-testing simulations suggest that the development of South Pacific warmings is connected with the amplification of wave 1 at 100 mb and that the eastward propagation of the disturbances requires eastward propagation of wave 1 at 100 mb. In addition, the results suggest that development of stratospheric disturbances in the southern stratosphere during early winter depends more on the intensity of wave 1 at 100 mb than on the structure of the zonal-mean flow in the stratosphere.

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Alan O'Neill, William L. Grose, Victoria D. Pope, Hector Maclean, and Richard Swinbank

Abstract

Meteorological analyses, produced at the U.K. Meteorological Office by data assimilation, are used to study the circulation of the stratosphere in the Northern Hemisphere during winter 1991/92. The analyses are supplemented by Lagrangian visualizations of the circulation. The main features discussed are 1) the changes in vertical structure of the circulation, 2) the merger of anticyclones that precipitated a strong stratospheric warming, 3) vortex roll up in the upper stratosphere, 4) the entrainment of air into the polar vortex in the middle and upper stratosphere, and 5) the influence of tropospheric blocking on the lower stratosphere. The study provides a meteorological basis for the interpretation of data from the Upper Atmosphere Research Satellite.

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Andrew J. Charlton, Alan O’Neill, William A. Lahoz, and Paul Berrisford

Abstract

The polar vortex of the Southern Hemisphere (SH) split dramatically during September 2002. The large-scale dynamical effects were manifest throughout the stratosphere and upper troposphere, corresponding to two distinct cyclonic centers in the upper troposphere–stratosphere system. High-resolution (T511) ECMWF analyses, supplemented by analyses from the Met Office, are used to present a detailed dynamical analysis of the event. First, the anomalous evolution of the SH polar vortex is placed in the context of the evolution that is usually witnessed during spring. Then high-resolution fields of potential vorticity (PV) from ECMWF are used to reveal several dynamical features of the split. Vortex fragments are rapidly sheared out into sheets of high (modulus) PV, which subsequently roll up into distinct synoptic-scale vortices. It is proposed that the stratospheric circulation becomes hydrodynamically unstable through a significant depth of the troposphere–stratosphere system as the polar vortex elongates.

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Rowan T. Sutton, Hector Maclean, Richard Swinbank, Alan O'Neill, and F. W. Taylor

Abstract

A technique is introduced by which high-resolution tracer fields may be constructed from low-resolution satellite observations. The technique relies upon the continual cascade of tracer variance from large to small scales and makes use of wind fields generated by a data assimilation scheme. To demonstrate its usefulness, the technique has been applied in a study of isentropic distributions of nitrous oxide in the winter midstratosphere, using measurements made by the Improved Stratospheric and Mesospheric Sounder instrument on the Upper Atmosphere Research Satellite. The results show that the high-resolution fields significantly increase the amount of information that is available from the satellite observations. The fields give insights into the characteristic structure and evolution of tracer distributions at scales that are normally obscured from view. Two results are particularly noteworthy. First, at the interface between low and middle latitudes there is evidence of active mixing. This mixing occurs on the eastern, equatorward side of air that is being drawn toward high latitudes around the polar vortex. Second, in the anticyclone, a complex pattern of transport is revealed. Air drawn in from low latitudes spirals together with ambient midlatitude air. Small scales are generated relatively slowly in the organized flow, and persistent filamentary structures, with transverse scales of hundreds of kilometers or greater, are seen.

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William J. Randel, Fei Wu, Richard Swinbank, John Nash, and Alan O’Neill

Abstract

Global circulation anomalies associated with the stratospheric quasi-biennial oscillation (QBO) are analyzed based on U.K. Meteorological Office (UKMO) assimilated wind and temperature fields. Zonal winds and temperatures from the assimilation are compared with Singapore rawinsonde data (the standard QBO reference time series), showing reasonable agreement but an underestimate of maxima in the UKMO analyses. Global structure of the QBO in zonal wind, temperature, and residual mean meridional circulation (derived from thermodynamic balance and mass continuity) is isolated, showing coherent tropical and midlatitude components. Important aspects of the QBO revealed in these data include 1) out of phase maxima in temperature (and vertical velocity) between the lower and upper stratosphere, and 2) strong seasonal synchronization of midlatitude anomalies. These characteristics are also evident in long records of satellite radiance measurements.

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R. Bradley Pierce, T. Duncan Fairlie, William L. Grose, Richard Swinbank, and Alan O'Neill

Abstract

Lagrangian material line simulations are performed using U.K. Meteorological Office assimilated winds and temperatures to examine mixing processes in the middle- and lower-stratospheric polar night jet during the 1992 Southern Hemisphere spring and Northern Hemisphere winter. The Lagrangian simulations are undertaken to provide insight into the effects of mixing within the polar night jet on observations of the polar vortex made by instruments onboard the Upper Atmosphere Research Satellite during these periods. A moderate to strong kinematic barrier to large-scale isentropic exchange, similar to the barrier identified in GCM simulations, is identified during both of these periods. Characteristic timescales for mixing by large-scale isentropic motions within the polar night jet range from 20 days in the Southern Hemisphere lower stratosphere to years in the Northern Hemisphere middle stratosphere. The long mixing timescales found in the Northern Hemisphere polar night jet do not persist. Instead, the Northern Hemisphere kinematic barriers are broken down as part of the large-scale stratospheric response to a strong tropospheric blocking event. A series of Lagrangian experiments are conducted to investigate the sensitivity of the kinematic barrier to diabatic effects and to small-scale inertial gravity wave motions. Differential diabatic descent is found to have a significant impact on mixing processes within the Southern Hemisphere middle-stratospheric jet core. The interaction between small-scale displacements by idealized, inertial gravity waves and the large-scale flow is found to have a significant impact on mixing within the polar night jet in both hemispheres. These sensitivity experiments suggest that scales of motion that are unresolved in global assimilated datasets may contribute to mass exchange across the kinematic barrier to large-scale isentropic motion.

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R. Bradley Pierce, William L. Grose, James M. Russell III, Adrian F. Tuck, Richard Swinbank, and Alan O'Neill

Abstract

The distribution of dehydrated air in the middle and lower stratosphere during the 1992 Southern Hemisphere spring is investigated using Halogen Occultation Experiment (HALOE) observations and trajectory techniques. Comparisons between previously published Version 9 and the improved Version 16 retrievals on the 700-K isentropic surface show very slight (0.05 ppmv) increases in Version 16 CH4 relative to Version 9 within the polar vortex. Version 16 H2O mixing ratios show a reduction of 0.5 ppmv relative to Version 9 within the polar night jet and a reduction of nearly 1.0 ppmv in middle latitudes when compared to Version 9. The Version 16 HALOE retrievals show low mixing ratios of total hydrogen (2CH4 + H2O) within the polar vortex on both 700 and 425 K isentropic surfaces relative to typical middle-stratospheric 2CH4 + H2O mixing ratios. The low 2CH4 + H2O mixing ratios are associated with dehydration. Slight reductions in total hydrogen, relative to typical middle-stratospheric values, are found at these levels throughout the Southern Hemisphere during this period. Trajectory calculations show that middle-latitude air masses are composed of a mixture of air from within the polar night jet and air from middle latitudes. A strong kinematic barrier to large-scale exchange is found on the poleward flank of the polar night jet at 700 K. A much weaker kinematic barrier is found at 425 K. The impact of the finite tangent pathlength of the HALOE measurements is investigated using an idealized tracer distribution. This experiment suggests that HALOE should be able to resolve the kinematic barrier, if it exists.

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