Waveguidability of idealized midlatitude jets and the limitations of ray tracing theory
- Johannes Gutenberg University Mainz, Institut für Physik der Atmosphäre, Mainz, Germany (vwirth@uni-mainz.de)
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 of a jet to guide a wave in the zonal direction is an important 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 (1) on ray tracing as derived from WKB theory and (2) on 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 prediction from ray tracing is not surprising, because the underlying WKB assumptions are not satisfied in the current context.
How to cite: Wirth, V.: Waveguidability of idealized midlatitude jets and the limitations of ray tracing theory , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2585, https://doi.org/10.5194/egusphere-egu2020-2585, 2020
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Hi Volkmar, very nice presentation of a really fascinating and important paper! You motivate some of your work with the QRA ideas and how in some instances WKB theory is applied despite its assumptions not really being fulfilled. I still try to get my head around what the implications of your work are for the QRA ideas. To what extent do your results confirm/challenge the existence of QRA under suitable conditions? Is the lack of predictive power of the existance of two turning latitudes for a good waveguide somehow challenging the existance of QRA (which is based on the idea that two turning latitudes make a decent waveguide) or can we just not say much about the existance of QRA because the assumptions of the underlying theory are often not fulfilled? Thank you!
Hi Matthias,
you are asking very relevant questions. In the final section of my WCD paper I do acknowledge that the concept of ray tracing and its use to diagnose waveguidabilty may be at least partly jutified. Justified in the sense that background states which have two turning latitudes (as opposed to only one or even zero) tend to have ON AVERAGE larger waveguidability as diagnosed with my simulations. This means that a statistical application would make some (mild) sense.
It is really the more literal application of the ray tracing ideas that I am very skeptical about. I believe that my work shows that separating the dynamical state into "waveguide" versus "no waveguide" is hard to justify, as it would suggest a dichotomy which simply does not exist. Rather, there is a very smooth transition between the two extremes.
Hi Volkmar, very nice study. It's my understanding that ray-tracing / QRA theory is often applied using a time and/or zonal mean state, rather than a true 'background state'. This could muddy the water in terms of cause and effect because if a large amplitude wave is present it will change the mean state. Can you say anything about this with your model? I wondered if using a time or zonal mean state to predict the waveguidability might bring the two predictions closer together?
Hi Volkmar, interesting to investigate this question. Somehow "waveguidability" relates to "Rossby elasticity" but neither has a clear definition? I think the Dritschel and McIntyre argument is that the Rossby elasticity depends on the PV contrast between two regions of more uniform PV. It is not so clear whether it is important that the PV changes over an abrupt step or a smoother transition. To some extent the answer to this must depend on the nature of the disturbances that approach the jet (PV contrast). For example, should the width of the PV gradient zone be narrower than the scale of disturbances approaching, whether they be vortex dipoles or wave packets?