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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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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 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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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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Edmund K. M. Chang, Sukyoung Lee, and Kyle L. Swanson

Abstract

This paper reviews the current state of observational, theoretical, and modeling knowledge of the midlatitude storm tracks of the Northern Hemisphere cool season.

Observed storm track structures and variations form the first part of the review. The climatological storm track structure is described, and the seasonal, interannual, and interdecadal storm track variations are discussed. In particular, the observation that the Pacific storm track exhibits a marked minimum during midwinter when the background baroclinicity is strongest, and a new finding that storm tracks exhibit notable variations in their intensity on decadal timescales, are highlighted as challenges that any comprehensive storm track theory or model has to be able to address.

Physical processes important to storm track dynamics make up the second part of the review. The roles played by baroclinic processes, linear instability, downstream development, barotropic modulation, and diabatic heating are discussed. Understanding of these processes forms the core of our current theoretical knowledge of storm track dynamics, and provides a context within which both observational and modeling results can be interpreted. The eddy energy budget is presented to show that all of these processes are important in the maintenance of the storm tracks.

The final part of the review deals with the ability to model storm tracks. The success as well as remaining problems in idealized storm track modeling, which is based on a linearized dynamical system, are discussed. Perhaps on a more pragmatic side, it is pointed out that while the current generation of atmospheric general circulation models faithfully reproduce the climatological storm track structure, and to a certain extent, the seasonal and ENSO-related interannual variations of storm tracks, in-depth comparisons between observed and modeled storm track variations are still lacking.

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Paul J. Roebber, Kyle L. Swanson, and Jugal K. Ghorai

Abstract

This research examines whether an adequate representation of flow features on the synoptic scale allows for the skillful inference of mesoscale precipitating systems. The focus is on the specific problem of landfalling systems on the west coast of the United States for a variety of synoptic types that lead to significant rainfall. The methodology emphasizes rigorous hypothesis testing within a controlled hindcast setting to quantify the significance of the results. The role of lateral boundary conditions is explicitly accounted for by the study.

The hypotheses that (a) uncertainty in the large-scale analysis and (b) upstream buffer size have no impact on the skill of precipitation simulations are each rejected at a high level of confidence, with the results showing that mean precipitation skill is higher where low analysis uncertainty exists and for small nested grids. This indicates that an important connection exists between the quality of the synoptic information and predictability at the mesoscale in this environment, despite the absence of such information in the initialization or boundary conditions. Further, the flow-through of synoptic information strongly constrains the evolution of the mesoscale such that a small upstream buffer produces superior results consistent with the higher quality of the information crossing the boundary. Some preliminary evidence that synoptic type has an influence on precipitation skill is also found. The implications of these results for data assimilation, forecasting, and climate modeling are discussed.

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Kyle L. Swanson, Paul J. Kushner, and Isaac M. Held

Abstract

Longitudinal variations in the upper-tropospheric time-mean flow strongly modulate the structure and amplitude of upper-tropospheric eddies. This barotropic modulation is studied using simple models of wave propagation through zonally varying basic states that consist of contours separating regions of uniform barotropic potential vorticity. Such basic states represent in a simple manner the potential vorticity distribution in the upper troposphere. Predictions of the effect of basic-state zonal variations on the amplitude and spatial structure of eddies and their associated particle displacements are made using conservation of wave action or, equivalently, the linearized “pseudoenergy” wave activity. The predictions are confirmed using WKB theory and linear numerical calculations. The interaction of finite-amplitude disturbances with the basic flow is also analyzed numerically using nonlinear contour-dynamical simulations. It is found that breaking nonlinear contour waves undergo irreversible amplitude attenuation, scale lengthening, and frequency lowering upon passing through a region of weak basic-state flow.

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Anastasios A. Tsonis, Kyle L. Swanson, and Paul J. Roebber

The study of networks has recently exploded into a major research tool in many areas of science. The discovery of “small world” and scale-free networks has led to many new insights about the collective behavior of a large number of interacting agents and complex systems. Here we introduce the basic ideas behind networks, as well as some initial applications of networks to the climate system. Our results suggest that the climate system exhibits aspects of small-world networks as well as scale-free networks, with supernodes corresponding to major teleconnection patterns. This result suggests that the organization of teleconnections may play a role in the stability of the climate system. In addition, preliminary work suggests that temporal changes in the network's architecture may be used to identify signatures of global change. These and other applications suggest that networks provide a new tool for investigating and reconstructing climate dynamics from both models and observations.

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Anastasios A. Tsonis, Kyle L. Swanson, and Geli Wang

Abstract

In a recent application of networks to 500-hPa data, it was found that supernodes in the network correspond to major teleconnection. More specifically, in the Northern Hemisphere a set of supernodes coincides with the North Atlantic Oscillation (NAO) and another set is located in the area where the Pacific–North American (PNA) and the tropical Northern Hemisphere (TNH) patterns are found. It was subsequently suggested that the presence of atmospheric teleconnections make climate more stable and more efficient in transferring information. Here this hypothesis is tested by examining the topology of the complete network as well as of the networks without teleconnections. It is found that indeed without teleconnections the network becomes less stable and less efficient in transferring information. It was also found that the pattern chiefly responsible for this mechanism in the extratropics is the NAO. The other patterns are simply a linear response of the activity in the tropics and their role in this mechanism is inconsequential.

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