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J. D. Mahlman

Abstract

An investigation of the kinetic energy budget of a “minor breakdown” of the stratospheric polar night vortex is performed. The computation covers the period 15 November-15 December 1958, for the 100–50 mb layer north of 40N.

Vertical motions and mean meridional circulations are computed from the thermodynamic equation. The calculations show a two-cell pattern with descending motions in mid-latitudes and ascent over the polar cap. During the period of restabilization after the “minor breakdown,” a small area of mean descent appears over the polar cap.

During the amplification stage, the internal energy conversions are acting to increase the eddy kinetic energy. The mean meridional circulation is direct at higher latitudes, opposite to that occurring during a major breakdown.

The restabilization period is characterized by a reversal in sign of the internal energy conversions and by large boundary fluxes of zonal kinetic energy.

Kinetic energy dissipation values are obtained as computational residuals. The values are large and probably unrealistic. It is shown that spuriously large computed kinetic energy dissipations can result from errors in the radiational estimates.

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J. D. Mahlman

Abstract

A calculation of the mean transverse circulation about the polar front jet stream is performed by using a diagnostic balance, ω-equation method. The results show a thermally-direct mean transverse circulation about the jet stream system for this case study.

An examination of the kinetic energy balance of this jet stream reveals that the direct transverse circulation is probably strong enough to maintain the jet against frictional dissipation but not enough to provide large lateral export of energy. However, significant amounts of energy are transferred upward across the tropopause.

Further considerations are employed to argue that the mean transverse circulation obtained here is compatible with the observed distributions of temperature and potential vorticity about the jet core.

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J. D. MAHLMAN

Abstract

In order to gain a greater understanding of the physical processes acting in the lower stratosphere during a major breakdown of the polar night vortex, a computation of the direction and magnitude of the mean meridional circulation is performed by employing a heat budget method. This computation reveals that the mean cell operates to produce rising motion over the polar regions before, during, and after the breakdown period. The calculations show that horizontal eddy heat flux provides the predominant mechanism for the large temperature increases observed over the polar cap during the time of the vortex breakdown. As a supplement to the above computation, mean vertical velocities were determined with respect to a curvilinear coordinate system oriented along a line of maximum circulation intensity at 50 mb. The result showed that the mean cell operates in the direct sense prior to the major breakdown when measured relative to this curvilinear system.

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Kevin Hamilton and J. D. Mahlman

Abstract

A study has been made of the evolution of the zonal-mean zonal wind and temperature in a multiyear integration of the 40-level, 3° × 3.6° resolution “SKYHI” general circulation model (GCM) that has been developed at GFDL. In the tropical upper stratosphere the mean wind variation is dominated by a strong semiannual oscillation (SAO). The peak SAO amplitude in the model is almost 25 m s−1 and occurs near the 1 mb level. The phase of the SAO near the stratopause is such that maximum westerlies occur shortly after the equinoxes. These features are in good agreement with the available observations. In addition the meridional width of the stratopause SAO in the GCM compares well with observations.

A diagnostic analysis of the zonal-mean momentum balance near the tropical stratopause was performed using the detailed fields archived during the GCM integration. It appears that the easterly accelerations in the model SAO are provided by a combination of (i) divergence of the meridional component of the Eliassen-Palm flux associated with quasi-stationary planetary waves and (ii) mean angular momentum advection by the residual meridional circulation. The effects of the residual circulation dominate in the summer hemisphere, while the eddy contributions are more important in the winter hemisphere. The westerly accelerations in the model SAO result from the convergence of the vertical momentum transport associated with gravity waves that have a broad distribution of space and time scales. Thus, in contrast to some simple theoretical models, large-scale equatorial Kelvin waves appear to play only a very minor role in the dynamics of the SAO in the SKYHI GCM.

A second equatorial SAO amplitude maximum was found in the tropical upper mesosphere of the GCM. This apparently corresponds to the mesopause SAO that has been identified in earlier observational studies. While the observed phase of this oscillation is reproduced in the model, the simulated amplitude is unrealistically small.

The model integration included the computation of the concentration of N2O. The results show a fairly realistic simulation of the semiannual variation of tropical stratospheric N2O mixing ratio seen in satellite observations.

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Syukuro Manabe and J. D. Mahlman

Abstract

This paper describes the stratosphere as simulated by the time integration of a global model of the atmosphere as developed at the Geophysical Fluid Dynamics Laboratory of NOAA.

It is shown that the model is capable of simulating a number of the features of the seasonal variation in the stratosphere. For example, it qualitatively reproduces the seasonal reversals of zonal wind direction in the mid-stratosphere between westerlies in winter and the zonal easterlies prevailing during the summer season. In the mid-latitude region of the lower model stratosphere, zonal mean temperature is highest in the winter when solar radiation is weak. At the cold equatorial tropopause of the model, the seasonal variation of temperature is also quite different from that which would be expected from the seasonal variation of solar radiation. These results are in qualitative agreement with the observed variation.

Attempts are made to identify the factors which are responsible for the various aspects of the seasonal variation of the model stratosphere, based upon detailed budget analyses of angular momentum, heat and eddy kinetic energy. It is found that, with the exception of the high-latitude regions, the seasonal variation of temperature in the lower model stratosphere is essentially controlled by dynamical effects rather than by the seasonal variation of local heating due to solar radiation.

The stratosphere as simulated by the global model has large interhemispheric asymmetries in the shape of the polar westerly vortex, the magnitudes and the distributions of eddy kinetic energy, and the meridional circulation in the winter hemisphere. Interhemispheric asymmetries in orography are apparently responsible for the interhemispheric differences in the quasi-stationary component of energy flux from the troposphere to the stratosphere of the model, and thus account for many of the asymmetries in the stratospheric circulation. In particular, the simulated stratospheric Aleutian anticyclone is shown to be related to the presence of the strong quasi-stationary tropospheric jet stream off the east coast of Asia.

Some of the important shortcomings of the model in simulating the stratosphere include an exaggeration of the magnitudes of the various components of the eddy kinetic energy budget at the top computational level (10 mb) of the model and an overestimation of the intensity of the polar westerly vortex. Also, the model fails to reproduce the mid-winter “sudden stratospheric warming” phenomenon and the quad-biennial wind reversal in the equatorial stratosphere. It is suggested that the performance of the model at the top level suffers from the coarseness in the vertical finite-difference resolution and the lid boundary condition imposed at the top of the model atmosphere.

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J. D. Mahlman and W. J. Moxim

Abstract

The so-called “sigma” coordinate system has seen increasing use in numerical models developed for general circulation and climate simulation, as well as for weather forecasting. Concurrently, there is an increasing demand for accurate analysis of the interactive physical process included in these model integrations. However, because of the necessity to transform the model information from sigma levels back to more conventional coordinate surfaces (such as pressure), significant inaccuracies usually result.

To reduce these inaccuracies, an alternative analysis procedure is introduced which avoids the usual ambiguous evaluation of vertical velocity in the transformed coordinate. Tests of this alternative method show that substantial increases in model analysis accuracy can be obtained.

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S. D. Mahlman and W. J. Moxim

Abstract

An 11-level general circulation model with seasonal variation is used to perform an experiment on the dispersion of passive tracers. Specially constructed time-dependent winds from this model are used as input to a separate tracer model. The methodologies employed to construct the tracer model are described.

The experiment presented is the evolution of a hypothetical instantaneous source of tracer on 1 January with maximum initial concentration at 65 mb, 36°N, 180°E. The tracer is assumed to have no sources or sinks in the stratosphere, but is subject to removal processes in the lower troposphere.

The experimental results reveal a number of similarities to observed tracer behavior, including the average poleward-downward slope of mixing ratio isopleths, strong tracer gradients across the tropopause, intrusion of tracer into the Southern Hemisphere lower stratosphere, and the long-term interhemispheric exchange rate. The model residence times show behavior intermediate to those exhibited for particulate radioactive debris and gaseous C14O2. This suggests that caution should be employed when either radioactive debris or C14O2 data are used to develop empirical models for prediction of gaseous tracers 'Which are efficiently removed in the troposphere.

In this experiment, the tracer mixing ratio and potential vorticity evolve to very high correlations. Mechanisms for this correlation are discussed. The zonal mean tracer balances exhibit complex behavior among the various transport terms. At early stages, the tracer evolution is dominated by eddy effects. Later, a very large degree of self-cancellation between mean cell and eddy effects is observed. During seasonal transitions, however, this self-cancellation diminishes markedly, leading to significant changes in the zonal mean tracer distribution. A possible theoretical explanation is presented.

For this tracer dispersion problem, probably the most significant model shortcoming is the inability of the general circulation model to produce the midwinter stratospheric sudden warming phenomenon.

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S. Miyahara, Y. Hayashi, and J. D. Mahlman

Abstract

In order to study interactions between gravity waves and planetary flow in the middle atmosphere, a 3° latitude by 3.6° longitude version of the 40-level GFDL “SKYHI” general circulation model is analyzed using bihourly sampled output data.

It is shown by a space-time spectral analysis that gravity waves in the mean zonal westerlies (easterlies) mainly consist of westward- (eastward-) moving components, and carry easterly (westerly) momentum upward, and decelerate the mean zonal westerlies (easterlies) in the mesosphere.

Zonal momentum flux convergence due to gravity waves accounts for nearly all of the Eliassen–Palm (E–P) flux divergence in the summer mesosphere. This convergence accounts for 30%–50% of that in the winter upper mesosphere. However, this percentage is probably an underestimate since the convergence is significantly enhanced in a high resolution (1° × 1.2°) model currently being integrated.

Vertical propagation of gravity waves is affected not only by the, mean zonal wind but also by velocity perturbations associated with planetary waves. The drag force due to gravity waves acts to suppress stationary planetary waves in the winter mesosphere.

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R. A. Plumb and J. D. Mahlman

Abstract

The GFDL general circulation/tracer model has been used to generate the transport coefficients required in two-dimensional (zonally averaged) transport formulations. This was done by assuming a flux-gradient relationship and then, given gradient and flux statistics from two independent (and contrived) model tracer experiments, to derive the coefficients by inversion of this relation. Given the mean meridional circulation from the GCM, the antisymmetric and symmetric parts of the coefficients tensor determine the advective and diffusive contributions to the net meridional transport in the model. The effective transport circulation thus defined differs substantially from the Lagrangian mean and residual circulations and is in fact a simpler representation of the model circulation than either of these. The diffusive component is coherently structured, comprising the following components:

(i) Strong quasi-horizontal mixing (Dyy ∼ 1 × 106 m2 s−1) in the midlatitude lower troposphere, apparently associated with fronts and the occlusion of synoptic systems.

(ii) A band of strong quasi-horizontal mixing (Dyy ∼ 2 × 106 m2 s−1) stretching across the tropical upper troposphere and the subtropical winter stratosphere. This band follows the band of weak zonal mean winds and is a manifestation in the model of the “surf zone” recently identified by McIntyre and Palmer as a region of breaking planetary waves. Outside the “surf zone,” Dyy ≲ 5 × 105 m2 s−1 in the stratosphere, consistent with other recent estimates.

(iii) Intense vertical mixing (Dzz ≳ 10 m2 s−1) in the troposphere at and near the latitudes of the intertropical convergence zone.

(iv) Vertical (Dzz ∼ 5−10 m2 s−1) through much of the troposphere, a substantial component of which is associated with subgrid-scale mixing (model convective processes).

The validity of the flux-gradient relation as a parameterization of eddy transport processes was tested by implementing the parameterization in a two-dimensional model, using these derived coefficients. In comparison experiments it was found that at the two-dimensional model could reproduce well the zonally-averaged evolution of tracers in the GCM; the quantitative errors that were found may in part result from the finite model resolution, rather than from errors in formulation. Therefore, although the flux-gradient relation is formally justified only in the small-amplitude limit, it appears to be a useful practical description of large-scale transport by finite-amplitude eddies.

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J. D. Mahlman, L. J. Umscheid, and J. P. Pinto

Abstract

The GFDL “SKYHI” general circulation model has been used to simulate the effect of the Antarctic “ozone hole” phenomenon on the radiative and dynamical environment of the lower stratosphere. Both the polar ozone destruction and photochemical restoration chemistries are calculated by parameterized simplifications of the still somewhat uncertain chemical processes.

The modeled total column ozone depletions are near 25% in spring over Antarctica, with 1% depletion reaching equatorial latitudes by the end of the 4½–year model experiment. In the lower stratosphere, ozone reductions of 5% reach to the equator. Large coolings of about 8 K are simulated in the lower stratosphere over Antarctica in late spring, while a general cooling of about 1–1.5 K is present throughout the Southern Hemisphere lower stratosphere. The model atmosphere experiences a long-term positive temperature-chemical feedback because significant ozone reductions carry over into the next winter.

The overall temperature response to the reduced ozone is essentially radiative in character. However, substantial dynamical changes are induced by the ozone hole effect. The Antarctic middle stratosphere in late spring warms by about 6 K over Antarctica and the lower midlatitude stratosphere warms by approximately 1 K. These warming spots are produced mainly by an increased residual circulation intensity. Also, the Antarctic vortex becomes tighter and more confined as a result of the reduced ozone. These two dynamical effects combine to steepen the meridional slope of quasi-conservative trace constituent isolines. Thus, the entire transport, radiative, and dynamical climatology of the springtime stratosphere is affected to an important degree by the ozone hole phenomenon. Over the entire year, however, these dynamical effects are considerably smaller.

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