Special seminar: Limitations of refractive index diagnostics for Rossby wave propagation along a jet waveguide
The horizontal propagation of Rossby waves over planetary-scale distances has been studied for several decades. In recent years this topic was discussed in connection with the resonant amplification of Rossby waves, which requires a good zonal waveguide for its existence. If resonant amplification were a relevant mechanism in the Earth’s atmosphere, it would have important implications for the occurrence of extreme weather and the changes to be expected in a future climate.
The current contribution presents a critical appraisal of the often-used “refractive index” as a diagnostic for Rossby wave propagation and the existence of waveguides. It is shown that a straightforward application of this diagnostic may give rise to misleading results in relevant situations. In particular, the refractive index diagnostic may suggest the existence of a waveguide where most likely there is none. The reasons are, (1) that the underlying WKB assumption may not be satisfied, and (2) that finite-amplitude effects may produce artifacts. The presentation will discuss and illuminate both aspects with the help of barotropic model simulations. It is suggested that ideas from finite amplitude wave activity may help to circumvent some of the above issues. Finally, the concept of a “leaky channel” is introduced as a means to quantify leakiness in a non-perfect waveguide.
- KlimaCampus 008
- Grindelberg 5
- Nedjeljka Žagar