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Weather and Climate Dynamics An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/wcd-2020-3
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/wcd-2020-3
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 17 Jan 2020

Submitted as: research article | 17 Jan 2020

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This preprint is currently under review for the journal WCD.

Waveguidability of idealized midlatitude jets and the limitations of ray tracing theory

Volkmar Wirth Volkmar Wirth
  • Institute for Atmospheric Physics, Johannes Gutenberg University Mainz, Becherweg 21, 55128 Mainz, Germany

Abstract. Ray paths of stationary Rossby waves emanating from a local mid-latitude source are usually refracted equatorward. However, this general tendency for equatorward propagation is mitigated by the presence of a midlatitude jet which acts as a zonal waveguide. This opens the possibility for circum-global teleconnections and quasi-resonance, which suggests that the ability to guide a wave in the zonal direction is an important jet property. This paper investigates waveguidability of idealized midlatitude jets in a barotropic model on the sphere. A forced-dissipative model configuration with a local source for Rossby waves is used in order to quantify waveguidability by diagnosing the latitudinal distribution of waviness in a longitudinal sector far downstream of the forcing. Systematic sensitivity experiments show that waveguidability increases smoothly with increasing jet amplitude and with decreasing jet width. This result is contrasted with the predictions from two idealized theoretical concepts based on (1) ray tracing as derived from Wenztel-Kramers-Brillouin (short: WKB) theory and (2) a sharp jet with a zonally oriented front of potential vorticity. The existence of two so-called turning latitudes, which is the key diagnostic for a zonal waveguide according to ray tracing theory, turns out to be a poor predictor for the dependence of waveguidability on jet amplitude and jet width obtained in the numerical simulations. By contrast, the meridional gradient of potential vorticity correlates fairly well with the diagnosed waveguidability. The poor predictions from ray tracing are not surprising, because the underlying WKB assumptions are not satisfied in the current context. The failure of WKB is traced back to the properties of the underlying equations, and a heuristic argument is presented to elucidate the potential of the PV gradient to act as a proxy for waveguidability.

Volkmar Wirth

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Volkmar Wirth

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Latest update: 26 Feb 2020
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Short summary
This paper investigates the waveguidability of midlatitude jets, i.e., their ability to duct Rossby waves in the zonal direction. Waveguidability is quantified in the framework of an idealized numerical model and compared with the predictions from two complementary theoretical concepts. It is found that the concept of ray tracing, which has been used extensively in the past, does not provide a satisfactory explanation for the numerical results. Further analysis uncovers the underlying reasons.
This paper investigates the waveguidability of midlatitude jets, i.e., their ability to duct...
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