Gravity Waves Yan

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Observations of Gravity Waves inHIRDLS Data

Xiuping Yan, Neil Arnold, John Remedios

EOS & RSPP, Department of Physics & Astronomy

University of Leicester

26/6/2008



1Observations of Gravity Waves in HIRDLS Data

Contents

Motivation

What are gravity waves?

Why are gravity waves important?

Gravity waves in the atmosphere

How can we observe gravity waves?

HIRDLS instrument

Gravity wave temperature perturbations

Climatology of gravity wave amplitude and wavelength

Summary




Motivation

• Generate a global climatology dataset of atmospheric gravitywaves

• Provide constraints for the parameterization of gravity waves inglobal circulation models




What are gravity waves?

• Atmospheric Gravity Waves (GWs): waves created by the action

of gravity on density variations in the stratified atmosphere

• Atmosphere:

– a stably stratified fluidexcept for the planetary

boundary layer (0 to~100-3000 m), whichmeans a quasi-steadyslowly changingbackground

– capable of supportinggravity waves

http://csep10.phys.utk.edu/astr161/lect/earth/atmosphere.html

https://reader009.{domain}/reader009/html5/0512/5af61ccb2715e/5af61c• Restoring force: buoyancy





• Gravity waves modify the atmospheric circulation and influence the

atmospheric thermal structures

– The gravity wave zonal mean forces causereversals of zonal mean jets and drive amean meridional transport circulation that

leads to a warm winter mesopause and acold summer mesopause

– drive the stratospheric tropical quasi-biennial oscillation (QBO) and the semi-annual oscillation (SAO) both in the

stratosphere and mesosphere – interact with tidal and planetary wave

motions

Fritts and Alexander, 2003

westward winds eastward winds

westward forcing westward forcing

eastward forcing westward forcing




• Gravity waves influence the atmospheric compositional structures

– induce turbulence and thus turbulent mixing to change the transportof chemical species

– locally cool the lower stratosphere (−78 ºC) and lead to the formation

of PSCs


Strong wind




Gravity waves in the atmosphere

• The most obvious sources of gravity wave: topography, convection,

and wind shear

• Vertical wavelengths: ~2 km to few tens km

• Horizontal wavelengths: tens to up to a thousand kilometers

• Frequencies of gravity waves: Coriolis parameter to buoyancyfrequency

• The horizontal amplitude of gravity waves: exponentially increaseswith altitude because of the decrease of density




How can we observe gravity waves?

• In-situ measurements:

– instrumented balloon and aircraft • Ground-based remote sensors:

– radar, lidar, sodar and radiosonde

– observations at a fixed location

– time-continuous fine-resolution measurements for smallscale wave features

– spatially discontinuous in the horizontal direction

– MST radar observation for troposphere and lowerstratosphere and upper mesosphere in the summer

– Lidar has similar altitude range as HIRDLS, but requiresclear skies

• Satellite remote sensors:

– consistent global coverage for observing global activities ofgravity waves

– LIMS (Fetzer and Gille, 1994), MLS (Wu and Water, 1996),SABER (Pressue et al., 2006), and HIRDLS (Alexander etal., 2008)

http://www.esa.int/esaLP/LPmetop.html

http://en.wikipedia.org/wiki/Image:WestCoastAirFloatplane.jpg

http://en.wikipedia.org/wiki/Image:Yellow.balloon.takesoff.in.bath.arp.jpg




HIRDLS instrument

• HIRDLS (HIgh Resolution Dynamics Limb Sounder): ainfrared limb-scanning radiometer

• An international joint US-UK development project between the University of Colorado at Boulder and theUniversity of Oxford

• Platform: NASA Earth Observing System (EOS) AURA satellite, launched on July 15, 2004

• An orbit period of ~100 minutes (14.4 orbits per day)• Horizontal along track sampling: ~100 km

• Longitudinal resolution: an orbital spacing of 24.72º

• Vertical resolution: 1km

• Scan rate: each scan takes ~ 15.5 Secs• Measurements: day and night

• Global coverage: 65ºS to 82ºN

• 4 temperature channels: 14.71 – 16.67 μm




GW temperature perturbations

• HIRDLS temperaturemeasurements (T): basic state

of rest + large-scale waves +small-scale waves

• Background field (Tbk): basicstate of rest + low frequencyplanetary waves

• The temperature filter(Tf): high frequency

planetary waves

• Gravity wave temperatureperturbations (T’): T – Tbk - Tf




Planetary waves

• Aug 2006

• Top: low frequency

planetary waves inbackground fields

• Bottom: high frequencyplanetary waves in thetemperature filter




Climatology of GW amplitude

Jan 2006

Apr 2006

Oct 2006 Nov 2006 Dec 2006

Jul 2006 Aug 2006 Sep 2006

May 2006 Jun 2006

Mar 2006Feb 2006




Climatology of GW wavelength

Jan 2006

Apr 2006

Jul 2006 Aug 2006 Sep 2006

May 2006 Jun 2006

Mar 2006Feb 2006

Oct 2006 Nov 2006 Dec 2006

C ti f lit d d l th




Cross-section of amplitude and wavelength(May 2006)

• Top: Alexander

et al., 2008• Bottom: our

results

• Same data butdifferent

techniques• Qualitativelyagree witheach other

• Details aredifferent as a

result of usingdifferenttechniques




Summary

• A global climatology of gravity wave amplitude and vertical

wavelength has been developed• The studies of gravity wave amplitude and wavelength show

that the observed wave activity is highly variable spatially with apronounced seasonal dependence

– This suggests that orography isn’t the only important source of thegravity waves observed in the lower stratosphere

– Convection is probably another important source for the wavesobserved, especially at tropics in the stratosphere

• Monthly mean of vertical wavelengths is in between 5 to 12 km.Waves with wavelength in this domain are typically internalgravity waves (D. G. Andrews et al., Middle Atmosphere

Dynamics , 1987)

Gravity Waves Yan

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