mesosphere coupling the role of waves and tides

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MESOSPHERE COUPLING THE ROLE OF WAVES AND TIDES

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MESOSPHERE COUPLING THE 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. - PowerPoint PPT Presentation

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Page 1: MESOSPHERE COUPLING THE ROLE OF WAVES AND TIDES

MESOSPHERE COUPLINGTHE ROLE OF WAVES AND TIDES

Page 2: MESOSPHERE COUPLING THE 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.

Page 3: MESOSPHERE COUPLING THE ROLE OF WAVES AND TIDES

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

Page 4: MESOSPHERE COUPLING THE ROLE OF WAVES AND TIDES

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.

Page 5: MESOSPHERE COUPLING THE ROLE OF WAVES AND TIDES

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.

Page 6: MESOSPHERE COUPLING THE ROLE OF WAVES AND TIDES

Dynamical instability

J Plane

Page 7: MESOSPHERE COUPLING THE ROLE OF WAVES AND TIDES

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

Page 8: MESOSPHERE COUPLING THE ROLE OF WAVES AND TIDES

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

Page 9: MESOSPHERE COUPLING THE ROLE OF WAVES AND TIDES

Diurnal tide over BrazilZonal and meridional winds at Sao Joao do Cariri 7°S, 36° W

Diurnal tide

Page 10: MESOSPHERE COUPLING THE ROLE OF WAVES AND TIDES

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

Page 11: MESOSPHERE COUPLING THE ROLE OF WAVES AND TIDES

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

Page 12: MESOSPHERE COUPLING THE ROLE OF WAVES AND TIDES

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%

Page 13: MESOSPHERE COUPLING THE ROLE OF WAVES AND TIDES

-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

Page 14: MESOSPHERE COUPLING THE ROLE OF WAVES AND TIDES

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

Page 15: MESOSPHERE COUPLING THE ROLE OF WAVES AND TIDES

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

Page 16: MESOSPHERE COUPLING THE ROLE OF WAVES AND TIDES

uuu

Solar Max – Solar Min: Planetary Waves

Neil Arnold

Changes in the reflection from planetary waves from the lower thermosphere

Page 17: MESOSPHERE COUPLING THE ROLE OF WAVES AND TIDES

Sources of gravity waves

Page 18: MESOSPHERE COUPLING THE ROLE OF WAVES AND TIDES

Ern et al. JGR 2004

Gravity wave momentum flux

Observationat 25 km

Model

Scale!

Page 19: MESOSPHERE COUPLING THE ROLE OF WAVES AND TIDES

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