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Gravity-wave breaking beyond traditional instability concepts

Gravity-wave breaking beyond traditional instability concepts. U. Achatz Goethe-Universität Frankfurt am Main, Germany 6.10.2008. Gravity Waves in the Middle Atmosphere. Becker und Schmitz (2003). Gravity Waves in the Middle Atmosphere. Becker und Schmitz (2003).

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Gravity-wave breaking beyond traditional instability concepts

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  1. Gravity-wave breaking beyond traditional instability concepts U. Achatz Goethe-Universität Frankfurt am Main, Germany6.10.2008

  2. Gravity Waves in the Middle Atmosphere Becker und Schmitz (2003)

  3. Gravity Waves in the Middle Atmosphere Becker und Schmitz (2003)

  4. Gravity Waves and Clear-Air Turbulence Koch et al (2008)

  5. GW Parameterizations • multitude of parameterization approaches(Lindzen 1981, Medvedev und Klaassen 1995, Hines 1997, Alexander and Dunkerton 1999, Warner and McIntyre 2001) • too many free parameters • reasons : • … • insufficent knowledge: conditions of wave breaking • basic paradigm: breaking of a single wave • Stability analyses (NMs, SVs) • Direct numerical simulations (DNS)

  6. Basic Types of GWs Dispersionsrelation: Trägheitsschwerewellen (TSW): Hochfrequente Schwerewellen (HSW):

  7. Basic Types of GWs Dispersion relation: Trägheitsschwerewellen (TSW): Hochfrequente Schwerewellen (HSW):

  8. Basic Types of GWs Dispersion relation : Inertia-gravity waves (IGW): Hochfrequente Schwerewellen (HSW):

  9. Basic Types of GWs Dispersion relation : Inertia-gravity waves (IGW): High-frequency gravity waves (HGW):

  10. Traditional Instability Concepts • static (convective) instability: • amplitude reference (a>1) • dynamic instability • limited applicability to HGW breaking

  11. Traditional Instability Concepts • static (convective) instability: • amplitude reference (a>1) • dynamic instability • limited applicability to HGW breaking

  12. Traditional Instability Concepts • static (convective) instability: • amplitude reference (a>1) • dynamic instability • BUT: limited applicability to GW breaking

  13. GW Instabilities Beyond Traditional Concepts IGW: • NM: Exponential energy growthsimple shear layer: • Static instability if • Dynamic instability if • SV: optimal energy growth over a finite time t(Farrell 1988a,b, Trefethen et al. 1993) HGW: • Significant horizontal gradientsshear-layer concepts fail

  14. GW Instabilities Beyond Traditional Concepts IGW: • NM: Exponential energy growthsimple shear layer: • Static instability if • Dynamic instability if • SV: optimal energy growth over a finite time t(Farrell 1988a,b, Trefethen et al. 1993) HGW: • Significant horizontal gradientsshear-layer concepts fail(e.g. Lombard and Riley 1996, Achatz 2005)

  15. Stability analysis, 2.5D-DNS

  16. Turbulent Dissipation Rates in the Mesosphere

  17. Breaking of an IGW by an SV (Ri > ¼): Measured dissipation rates 1…1000 mW/kg (Lübken 1997, Müllemann et al. 2003)

  18. NMs of an HGW: Growth Rates (Q = 70°)

  19. HGW amplitude after a perturbation by a NM (DNS):

  20. Energetics not the gradients in z matter,but those in f! instantaneous growth rate:

  21. Energetics not the gradients in z matter,but those in f! instantaneous growth rate:

  22. Energetics not the gradients in z matter,but those in f! instantaneous growth rate:

  23. Energetics of a breaking HGW with a0 > 1 Strong growth perturbation energy:shear instability

  24. Energetics of a Breaking HGW with a0 < 1 Strong growth perturbation energy:buoyancy instability

  25. Breaking HGW with a0 < 1: Dissipation rate Dissipation rate (mW/kg) at ist maximum (t = 5P):

  26. Summary • the Richardson number is not a good criterion for turbulence from gravity waves! • IGWs: Breaking by SVs • HGWs:Instabilities due to horizontal gradients • References: • Achatz (2005, Phys. Fluids) • Achatz and Schmitz (2006a,b, J.Atmos. Sci.) • Achatz (2007a,b, J. Atmos. Sci.) • Achatz (2007c, Adv. Space Res.) • all PDFs available from my web page (http://www.geo.uni-frankfurt.de/iau/ThMet/english/Publications/index.html) • see also Fritts et al. (2003, 2006, 2008a,b)

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