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Inertia-Gravity waves and their role in mixing

Inertia-Gravity waves and their role in mixing. Geraint Vaughan University of Manchester, UK. Motivation for study. Quasi-monochromatic wave-like features ubiquitous in MST radar data Often seem to be associated with patches of turbulence

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Inertia-Gravity waves and their role in mixing

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  1. Inertia-Gravity waves and their role in mixing Geraint Vaughan University of Manchester, UK

  2. Motivation for study • Quasi-monochromatic wave-like features ubiquitous in MST radar data • Often seem to be associated with patches of turbulence • Generated in baroclinic regions of the upper troposphere, where horizontal gradients in tracer concentrations occur.

  3. The UK MST radar • 46.5 MHz coded pulses • Runs continuously • Typical height resolution 300m, time resolution 2 min • Measures echo power, winds, turbulence (spectral width)

  4. Inertia-gravity waves Phase velocity Long-period gravity waves, affected by Earth’s rotation. Frequency ~ f Horizontal Wavelength several 100 km Vertical wavelength ~2 km Wind vector rotates elliptically with time or ht. Wave packet = ? km Group velocity z Path traced by wind vector over time Phase front

  5. Example: the case of July 1999 Eastward wind component measured over 4 days, 7-11 July 1999

  6. Echo power (dB), showing that wave modulates static stability Spectral width, indicating (weak) turbulence EASTWARD WIND MAXIMA

  7. Wave components • u΄, average from 17h to 19h on 8 July • u΄, 14-14.3 km • Hodograph from 13-17 km as a) • Hodograph in time as b) Wave period: 14 h (0.9f) Vertical λ: 2 km Horizontal λ: 330 km

  8. Synoptic charts

  9. Wave sources • Strong deceleration at jet stream level (e.g. jet exits or highly curved jets) • Baroclinic instability • Instability of a horizontal shear layer • Convection • Orographic forcing

  10. Baroclinic instability After Griffiths and Reeder, QJRMS 1996 MST data during passage of cold front

  11. Instability of shear layer Meteosat water vapour images every 12 hrs from 06h 7 March 1997

  12. Potential vorticity at 320 K, 00h 8 March, showing high-PV streamer 700 mb chart, 12h 8 March, showing development of surface low pressure

  13. Radar data, 8-9 March 1997

  14. Statistical studies of IGW occurrence • Use complete MST radar archive 1990-2005 (data sparse before 1996 but continuous thereafter) • Look for quasi-monochromatic long-period disturbances • Evaluate frequency distributions for wave occurrence and wave parameters • Link to synoptic pattern

  15. Algorithm • Median average MST winds over 30 min • Apply 5 km high-pass filter to each 30 min profile • Median average filtered profiles over 3 hours • Use band-pass filter to separate oscillations with periods 4-8 hrs and 12-24 hrs (over 10 days) • Identify rotation of wind vector through 360º • Fit ellipses to rotating wind segments

  16. Occurrence:Long-period, upward propagating dominate Note change in scale between the two panels Clockwise => upward propagation of energy, vice versa

  17. Height from tropopause: source region near tropopause Downward energy propagation Upward energy propagation Tropopause defined from radar echo power profiles

  18. Vertical wavelength stratospheric tropospheric |Defined by altitude range for 360º rotation of wind vector

  19. Amplitude- semi-major axis of ellipse Cut-off of 0.5 ms-1 used in definition of ellipse

  20. θ Alignment:major axis of ellipse Alignment is the same as the direction of propagation of the wave, with ambiguity of ±180º. It is the angle between the semi-major axis and N

  21. Distribution wrt jet stream speed

  22. Distribution wrt ageostrophic wind speed

  23. Turbulence • Defined as spectral width of radar echo > 1 ms-1 (eddy diffusivity ε ~ 0.01 W kg-1) • Inertia-gravity waves associated with layers of turbulence around 1% of the time • This is comparable with the incidence of turbulence due to mountain waves

  24. Mesoscale Model simulation (UKMO) Eastward wind component at 150 mb 24 hours into a simulation starting 15/1/99. Note position at jet exit Time-height cross-section of U at the selected grid point

  25. Conclusions • MST radar at Aberystwyth observes I-g waves very often, due to its location at the end of the storm tracks • Observed waves are consistent with the main source at the tropopause • These waves cause mixing in the lower stratosphere and at the tropopause where tracer gradients are large

  26. Passage of a cold front, 15/1/99

  27. Passage of a tropopause fold Inertia-gravity wave accentuates shear at top of jet stream, inducing turbulence in a region of tracer gradients

  28. Wave-wave interaction Inertia-gravity waves appear to set up critical layers which result in mountain waves breaking

  29. Aberporth radiosonde 17h 8 July 1999. Temperature perturbations Potential temperature

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