15.01.2018: FB1-Seminar

11:00 Uhr, Hörsaal West, Düsternbrooker Weg 20

Abstract:

Upper tropospheric Rossby wave packets (RWPs) have found increased interest during the past few years, partly because of their potential role for the predictability of surface weather and their possible connection with extreme weather. The current presentation highlights two issues in connection with diagnosing and tracking such upper tropospheric wave packets. First, a common technique to diagnose RWPs reconstructs the envelope of the wave signal in the meridional wind field using an algorithm based on the Hilbert transform. It turns out that this technique often results in undesirable wiggles in the envelope, and this effect goes back to the semigeostrophic nature of Rossby waves. As a remedy it has been suggested to perform the envelope reconstruction in semi-geostrophic space rather than in physical space. Another problem is that most diagnostics based on the meridional wind or the eddy kinetic energy are associated with a spurious decrease in waviness as soon as the RWP enters is nonlinear stage with overturning contours of potential vorticity or wave breaking. A novel technique based on finite amplitude wave activity does not suffer from this drawback and is, therefore, better suited to reliably diagnose RWPs throughout their life cycle.

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