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 Generated in baroclinic regions of the upper troposphere, where horizontal gradients in tracer concentrations occur.

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)

Inertia-gravity waves 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 Phase velocity Phase front Path traced by wind vector over time z

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

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

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

Synoptic charts

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

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

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

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

Radar data, 8-9 March 1997

Statistical studies of IGW occurrence Use complete MST radar archive (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

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 hrs (over 10 days) Identify rotation of wind vector through 360º Fit ellipses to rotating wind segments

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

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

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

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

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 θ

Distribution wrt jet stream speed

Distribution wrt ageostrophic wind speed

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

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

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

Passage of a cold front, 15/1/99

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

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

Aberporth radiosonde 17h 8 July Temperature perturbations Potential temperature