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William L. Grose and Kenneth V. Haggard

Abstract

The present study describes the analysis of a sudden warming which developed spontaneously during one winter of a 33-month simulation using a three-dimensional, spectral, quasi-geostrophic model. The changes that occurred in the circulation and thermal structure of the winter polar stratosphere during the warming are shown to be in close agreement with observed behavior. That is, characteristic features of stratospheric warmings such as 1) enhanced vertical flux of eddy energy into the stratosphere, 2) rapid temperature increase in high latitudes with a reversal of the zonal mean temperature gradient between midlatitude and pole, 3) destruction of the circumpolar cyclonic vortex, and 4) marked deceleration of the westerly jet and the appearance of zonal mean easterlies are reproduced by the model. The energetics of the warming are also consistent with features characteristic of observed events. In addition, certain aspects of the dynamical development of the present model simulation are shown to be in agreement with results from other models, but certain contrasting aspects will be discussed.

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William L. Grose and Brian J. Hoskins

Abstract

An investigation has been conducted of the steady response to orography as described by the linearized shallow-water equations on the sphere. Results have been obtained for both idealized and realistic climatological mean zonal flows when perturbed by simple isolated mountains. The response is interpreted in terms of the Rossby wavetrains and the energy dispersion ideas developed in Hoskins et al. (1977). Subsequent results from perturbing the several zonal flows by the earth orography are then readily understood. The surprisingly good comparison with observation suggests that the qualitative insight gained from the simple model is useful. The main difference from previous work is the emphasis on the two-dimensional nature of the horizontal propagation on the sphere. In particular, for a jet at 30° impinging on a mountain at the same latitude, there is a tendency to produce two wavetrains—one to the north and one to the south. At 60°–80° downstream these wavetrains are out of phase, giving a “blocking” region with a ridge to the north and a trough to the south. The southern train produces enhanced equatorial easterlies centered 30°–40° downstream.

The results give interesting indications of the regions of influence of mountains and suggest that quantitative theories of the stationary waves must contain a full representation of the spherical domain.

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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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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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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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