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K. L. Swanson

Abstract

The barotropic instability of a forced, axisymmetric vortex patch is examined and linked to prior work on the destabilization of a single Rossby wave via triad interactions. Both oscillatory instabilities, which share certain qualitative aspects with Rossby wave triads, as well as stationary instabilities are studied. It is shown that instability is confined to quite limited regions of the parameter space described by the magnitude of flow variation along the vortex edge and the zonally averaged flow along the vortex edge. However, more realistic flow variations along the vortex edge expand the region of parameter space where instability is found. The nonlinear manifestation of these instabilities is examined, and it is found that the leading oscillatory branch of instabilities can lead to large perturbations of the vortex patch, and its ultimate filamentation. Extension of these results to observed structures in the troposphere and stratosphere is discussed.

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K. L. Swanson

Abstract

A potent mechanism for the generation of low-frequency atmospheric variability on vortex basic states consisting of a single potential vorticity jump, or contour, separating two regions of uniform equivalent barotropic potential vorticity is described. Such basic states represent in a simple manner the potential vorticity distribution of the extratropical upper troposphere. It is shown that the group velocity for stationary waves propagating on such states can vanish for realistic zonal variations in the basic-state flow along the vortex edge, leading to local exponential disturbance growth due to the accumulation of wave action. Further, pseudo-energy stability criteria are derived that suggest that exponentially growing global disturbances are possible for sufficiently strong zonal variations in the flow along the vortex edge.

These predictions are examined using linear and nonlinear initial value problem calculations. For wavenumber-1 flow variations in the basic-state zonal flow along the vortex edge, no global instability occurs. However, strong local disturbance growth in response to weak stationary forcing does occur and can lead to irreversible deformation of the vortex. For wavenumber-2 and higher variations in the basic-state zonal flow along the vortex edge, global instability occurs if the stability criteria is violated. These instabilities have peak dimensional e-folding times on the order of one week, with faster growth rates corresponding to stronger zonal variations in the flow along the vortex edge. Quantization of the zonal scale of amplifying disturbances occurs, indicating disturbance resonance with the underlying zonal variations in the basic-state flow along the vortex edge. In the nonlinear regime, longer wavelength disturbances lead to large amplitude periodic fluctuations of the vortex. Intermediate wavelength disturbances are shown to yield suprisingly realistic blocking events, while short wavelength disturbances saturate at amplitudes too small to change the overall structure of the vortex.

The pervasiveness of instability in this simple system suggests similar processes may be important for blocking transitions and the generation of low-frequency variability in the extratropical atmosphere. Preliminary results from the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis show that the climatological 330-K isentropic potential vorticity is accurately characterized as the time average of a fluctuating single-contour vortex. Wave action conservation on basic states constructed using dynamical fields on the 330-K isentropic surface reproduces observed shifts in low-frequency variability that occur during the El Niño cycle. Further, these shifts lead to transient-driven time mean flow anomalies that have a teleconnection pattern-like structure, despite the fact that meridional propagation of waves is forbidden in this system. The ability of this system to accurately simulate diverse atmospheric phenomena as well as explain certain aspects of upper-tropospheric dynamics suggests that it may provide a powerful new paradigm with which to view low-frequency dynamics in the climate system.

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K. L. Swanson

Abstract

Analysis of frequency distribution functions of potential vorticity (PV) on middleworld isentropic surfaces that intersect the tropopause reveals that dynamics on these surfaces are dominated by the tropopause separating subtropical and polar PV pools with definitive characteristic values. The dominance of the tropopause on these surfaces suggests that the subseasonal transient variance should be a function solely of the time mean PV. Exploiting this relation, it is shown that knowledge of the annual mean PV distribution allows quite skillful deduction of interannual anomalies in transient variance. This analysis is extended to the dynamic tropopause, where it is shown that polar and subtropical jets clearly emerge in fields of the most frequent tropopause potential temperature. Implications of these results on the current understanding of how patterns of interannual variance arise are discussed.

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Kyle L. Swanson

Abstract

The nature of extratropical tropospheric low-frequency variability remains an important, unresolved problem in the overall dynamics of the climate system. Primarily, this is due to the complexity of dynamics operating on low-frequency timescales of 10–100 days; both synoptic- and planetary-scale dynamical processes are fully active and strongly interactive. This review explores two issues that frequently arise in the study of low-frequency variability, and emphasizes the continuing value of idealized dynamical models in interpreting low-frequency variability. The first issue concerns the extent to which the extratropical atmosphere supports planetary-scale instabilities, and whether a simple picture of such instabilities can be developed. It is argued that under certain circumstances such instabilities do exist, and result from the accumulation of stationary wave energy in local resonant cavities that emerge in zonally varying barotropic flow. The second issue concerns the interaction of synoptic transients with the zonally varying planetary-scale flow. Simple dynamical settings reveal the essence, but also the ambiguity underlying the interaction between these two scales. The implications of these simple model arguments for the current understanding of low-frequency variability in more complicated models as well as nature is discussed, along with the role of such simple models in the overall climate modeling hierarchy.

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E. G. Njoku
and
L. Swanson

Abstract

Satellite microwave measurements of sea surface temperature (SST), sea surface wind speed, atmospheric water vapor and cloud liquid water have been analyzed for the three-month period from July to October 1978. During this period the Scanning Multichannel Microwave Radiometer (SMMR) on the Seasat satellite provided continuous orbital coverage of the world's oceans. Monthly contour maps and zonal averages of the SMMR measurements have been produced to examine the consistency of the Seasat data over longer temporal and spatial scales than has hitherto been investigated. With small (∼0.5°C) bias corrections to the SST estimates, the SMMR appears capable of detecting SST anomalies of ≲1°C, except where radio frequency inteference occurs. The SMMR wind speed and water vapor distributions indicate large-scale atmospheric circulation patterns, and provide complete coverage in regions of sparse ship and radiosonde data. The SMMR cloud liquid water measurements show features similar to those observed in measurements of cloud cover by visible and IR sensors, but indicate the greater SMMR sensitivity to liquid rather than ice content. These SMMR results indicate the potential for future use of microwave radiometry in ocean, weather and climate applicationss.

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K. L. Swanson
and
R. T. Pierrehumbert

Abstract

The general character of stationary Rossby waves in a state with homogenized tropospheric PY as opposed to more conventionally assumed PV distributions is discussed. It is shown that the expected changes in wave pattern are inconsequential.

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M. L. Mooney
and
R. N. Swanson

Abstract

Simultaneous measurements of the standard deviation of the azimuth and vertical wind direction fluctuations from a digital recording system and the range in wind direction fluctuations from an analog recording system have been used to determine the scaling factor required for converting the range data to estimates of standard deviations.

In general, the study shows that constant scaling factors used in estimating the standard deviation from the range of wind direction fluctuations are representative of the median values of the standard deviation. However, the standard deviation is likely to be underestimated during periods of stable atmospheric conditions and slightly overestimated during neutral through unstable atmospheric conditions.

The results also indicate that the conversion of analog readings to standard deviations, using the range in wind direction fluctuations, is sensitive to recorder response, data reduction techniques and scale of turbulence. As a result, the scaling factor value is dependent on measurement, recording and data reduction procedures.

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Iordanka N. Panayotova
and
Kyle L. Swanson

Abstract

Meridional asymmetry arising from the inclusion of meridional variation in the Coriolis parameter is shown to be a fundamental property of the higher-order dynamics of Eady edge waves. This asymmetry may be relevant to structural characteristics of observed atmospheric transients, particularly short waves propagating along the extratropical dynamic tropopause.

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Kyle L. Swanson
and
Raymond T. Pierrehumbert

Abstract

The relative effects of dynamics and surface thermal interactions in determining the heat flux and temperature fluctuations within the lower-tropospheric portion of the Pacific storm track are quantified using the probability distribution functions (PDFs) of the temperature fluctuations and heat flux, Lagrangian passive tracer calculations, and a simple stochastic model. It is found that temperature fluctuations damp to the underlying oceanic temperature with a timescale of approximately 1 day but that dynamics still play the predominant role in determining atmospheric heat flux, due to eddy mixing lengths within the storm track of ≤ 5° latitude. These results are confirmed by the favorable comparison of the PDFs of the model-generated and observed temperature fluctuations and heat flux.

The implications of strong thermal damping in the lower troposphere are discussed and speculations are made regarding the effect of such damping upon baroclinic eddy life cycles and the general circulation.

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Kyle L. Swanson
and
Paul J. Roebber

Abstract

All meteorological analyzed fields contain errors, the magnitude of which ultimately determines the point at which a given forecast will fail. Here, the authors explore the extent to which analysis difference fields capture certain aspects of the actual but unknowable flow-dependent analysis error. The analysis difference fields considered here are obtained by subtracting the NCEP and ECMWF reanalysis 500-hPa height fields. It is shown that the magnitude of this 500-hPa analysis difference averaged over the North Pacific Ocean has a statistically significant impact on forecast skill over the continental United States well into the medium range (5 days). Further, it is shown that the impact of this analysis difference on forecast skill is similar to that of ensemble spread well into the medium range, a measure of forecast uncertainty currently used in the operational setting. Finally, the analysis difference and ensemble spread are shown to be independent; hence, the impact of these two quantities upon forecast skill is additive.

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