Preprints
https://doi.org/10.5194/wcd-2022-30
https://doi.org/10.5194/wcd-2022-30
 
09 Jun 2022
09 Jun 2022
Status: this preprint is currently under review for the journal WCD.

Robust poleward jet shifts in idealised baroclinic-wave life-cycle experiments with noisy initial conditions

Felix Jäger1,a, Philip Rupp1, and Thomas Birner1 Felix Jäger et al.
  • 1Meteorological Institute Munich, Ludwig-Maximilians-University, Munich, Germany
  • anow at: Institute for Atmospheric and Climate Science, ETH Zurich, Zurich, Switzerland

Abstract. Idealised baroclinic-wave life-cycle experiments are a widely used tool to study fundamental characteristics of mid-latitude baroclinic instability. A typical life-cycle evolves from an initialised baroclinically unstable jet through an exponential growth phase of a particular unstable wave mode, followed by wave breaking during the mature phase, and wave-mean flow interaction driving a jet shift during the decay phase. Many authors distinguish between life-cycles with predominantly anticyclonic (LC1) and cyclonic (LC2) wave breaking and the transition between the two flavours is typically controlled via the strength of cyclonic meridional wind shear in the initial conditions. While baroclinic wave growth has traditionally been triggered via a specified initial perturbation with fixed zonal wave number, this study extends the concept of baroclinic-wave life-cycles by analysing the influence of random initial perturbations without any preferred zonal dependency on the life-cycle evolution. We find that the growth phase shows a robust LC1-LC2 distinction as a function of initialised meridional shear, while a preference for LC1-like characteristics is observed during the decay phase for all life-cycles with non-monochromatic initial perturbations. In particular, the persistent cut-off cyclones that typically form for LC2 initialisations are found to eventually become unstable – the earlier during the life-cycle the stronger the initial noise perturbations. All non-monochromatic life-cycles result in a poleward jet shift in their final state, regardless of the strength of the initial shear. Consistently, anticyclonic wave breaking tends to be predominant during the mature and decay phases, even for LC2 initialisations. Equatorward jet shifts associated with cyclonic wave breaking still exist, although purely as a transient interim state. We show that wave-wave interactions resulting from the initialised random wave spectrum play an important role during all phases of the life-cycle.

Felix Jäger et al.

Status: open (until 26 Jul 2022)

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  • RC1: 'Comment on wcd-2022-30', Dennis Hartmann, 18 Jun 2022 reply

Felix Jäger et al.

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Assets to Jäger et al.: Robust poleward jet shifts in idealised baroclinic-wave life-cycle experiments with noisy initial conditions Felix Jäger, Philip Rupp and Thomas Birner https://doi.org/10.5282/ubm/data.301

Felix Jäger et al.

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Short summary
Mid-latitude weather is dominated by the formation and decay of large cyclonic and anticyclonic eddies, mostly formed during the growth and breaking of baroclinic waves. One way to study these phenomena is via idealised numerical life-cycle simulations, which are typically characterised by cyclonic or anticyclonic wave breaking, depending on the details of the system. We show that all systems exhibit predominantly anticyclonic character if a range of wave modes is allowed to grow simultaneously.