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MESOSPHERE COUPLINGTHE ROLE OF WAVES AND TIDES
Spectra show that waves & tides of large amplitude dominate the MLT region
A typical power spectrum of horizontal winds at a height of ~ 90 km.
In this case the data are recorded by a meteor radar over Esrange (68oN). The spectrum is calculated using data for Jan-Dec 2000. (Younger et al., 2002).
3. Planetary waves
Particular frequencies, occurring in the period range ~ 2 – 16 days. Stationary planetary waves possible. All are “natural resonances of the atmosphere”
2. Gravity waves
A continuous spectrum with periods from ~ 5 mins to 12+ hours
1. Tides
Well-defined oscillations occurring at harmonics of a solar day – 24, 12 and 8 hrs (others are very weak). Solar forced.
200
50
400
300
200
0
100
600 800
Alti
tud
e k
m
Temp K
O
O2
O3
H2O Convective
Solar tidal forcing
Tides are thermally driven
Absorption of solar radiation throughout the atmosphere,
Absorption of UV radiation by stratospheric ozone and of infrared by water vapour in the troposphere.
Plus Absorption of shortwave radiation by oxygen molecules and atoms in thermosphere
Plus Interaction between tidal modes
Amplitude Growth with Increasing Height
N Mitchell
HE
IGH
T
Wave source
A wave of amplitude V ms-1 has energy per unit volume, E, Joules per m3 where:
E = ½V2
( = atmospheric density)
If the wave is not dissipating, then E is a conserved quantity.
Now, decreases exponentially with height – a factor of ~ 300,000 from the ground to ~ 90 km.
As the wave ascends, if energy is to be conserved, the amplitude, V, must rise to balance the decrease in density, .
Sources inc. vigorous convection, flow over mountains, ageostrophic adjustment etc.
Breaking Waves Transfer Energy & Momentum to the Background Flow
N Mitchell
HE
IGH
T
Wave source
Wave amplitudes thus grow until a “breaking level” is reached.
Breaking level
• Wave energy is no longer conserved.
• Wave energy turbulent energy
• Momentum carried by the wave is deposited into the mean flow and imposes a force on the flow of the background atmosphere – “wave drag”.
• Momentum deposited by waves provides up to ~ 70% of the momentum of the flow in the MLT.
• The MLT has a wave-driven large-scale circulation.
Dynamical instability
J Plane
Wave Instabilities Constrain Wave Growth
OH airglow images 16:19 – 17:25 UT, at a height of ~ 87 km, over Japan, 23/12/95. The images are spaced by ~ 3 minutes. The centre of each image is the zenith. The horizontal wavelength of the original waves is ~ 27 km and the period was deduced to be ~ 6 minutes
Yamada et al., GRL, 2001
Tidefrequency, ω1
wavenumber, m1
Non-linear interaction
A family of secondary waves, including two waves:
“sum wave”: frequency (ω1 + ω2), wavenumber (m1 + m2)“difference wave”: frequency (ω1 - ω2), wavenumber (m1 - m2)
Sum and difference waves can beat with the tide, causing a modulation of the tide’s amplitude at the frequency of the planetary wave
How much does this process contribute to the observed variability of tides?
Planetary Wavefrequency, ω2
wavenumber, m2
Tidal/Planetary-Wave Non-Linear Coupling - Theory
Diurnal tide over BrazilZonal and meridional winds at Sao Joao do Cariri 7°S, 36° W
Diurnal tide
ZONAL WINDS OVER ESRANGE (68oN, 21oE) , AUGUST 5-20, 1999
Semi-diurnal tide with planetary wave modulation
Horizontal winds calculated from meteor drifts
N. J. Mitchell
Planetary wave modulation
Tidal trends at 130 km from magnetometer data
20%
60°N
52°N
22°N
At mid-latitudes a 20% reduction in the amplitude of the tidal signature at ~ 130 km altitude since the middle of the 20th century
May be linked to ozone depletion worldwide
Ozone and water vapour heating are possible sources
M Jarvis
Modelling of Sq tidal signaturesbased on Ross and Walterscheid, GRL, 1991
Upper stratospheric ozone
1900 1950 2000
Lower thermospheric tide
Upward-propagating tide
Calculations suggest a decrease in the tidal signatures seen in geomagnetic Sq variation of >12%
7% 18%40 km
> 12%
-90 -75 -60 -45 -30 -15 0 15 30 45 60 75 90LATITUDE (degree)
0
5
10
15
20
25
30
35
40
45August
Latitude
Tid
al A
mp
litu
de
m/s
Diurnal tide - zonal wind
Lines model, 90 (solid) & 95 km (dash)Symbols, data (MF & meteor radar, 90km)
Pancheva et al
Latitude
Tid
al A
mpl
itude
m/s
Semidiurnal tide - zonal wind
Lines model, 90 (solid) & 95 km (dash)Symbols, data (MF & meteor radar, 90km)
Pancheva et al
Lerwick, (60ºN, 1ºW)
Wavelet analysis of magnetometer data. Peaks at tidal andplanetary wave periods. Blue dotted line (winter) many planetary wavesRed dotted line (summer) few planetary waves
Semi-diurnal tide
Diurnal tide
16 day wave
5 day wave
uuu
Solar Max – Solar Min: Planetary Waves
Neil Arnold
Changes in the reflection from planetary waves from the lower thermosphere
Sources of gravity waves
Ern et al. JGR 2004
Gravity wave momentum flux
Observationat 25 km
Model
Scale!
Red arrow - direction of gravity waveYellow dot – all sky imager location
Infra-red satellite image
Vadas et al. Ann. Geophys. 2009
• Ray tracing shows deep convective plumes likely to be the source of gravity waves in the OH layer
• Mostly direct propagation but ducted and reflected waves possible