Observations of Gravity Waves in HIRDLS Data

Xiuping Yan, Neil Arnold, John RemediosEOS & RSPP, Department of Physics & Astronomy

University of Leicester26/6/2008

2Observations 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

3Observations of Gravity Waves in HIRDLS Data

Motivation

• Generate a global climatology dataset of atmospheric gravity waves

• Provide constraints for the parameterization of gravity waves in global circulation models

4Observations of Gravity Waves in HIRDLS Data

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 fluid except for the planetary boundary layer (0 to ~100-3000 m), which means a quasi-steady slowly changing background

– capable of supporting gravity waves

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

http://www.kowoma.de/en/gps/additional/atmosphere.jpg• Restoring force: buoyancy

5Observations of Gravity Waves in HIRDLS Data

Why are gravity waves important?

• Gravity waves modify the atmospheric circulation and influence the atmospheric thermal structures

– The gravity wave zonal mean forces cause reversals of zonal mean jets and drive a mean meridional transport circulation that leads to a warm winter mesopause and a cold 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

6Observations of Gravity Waves in HIRDLS Data

• Gravity waves influence the atmospheric compositional structures

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

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

Why are gravity waves important?

Strong wind

7Observations of Gravity Waves in HIRDLS Data

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 buoyancy frequency

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

8Observations of Gravity Waves in HIRDLS Data

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 small

scale wave features – spatially discontinuous in the horizontal direction– MST radar observation for troposphere and lower

stratosphere and upper mesosphere in the summer– Lidar has similar altitude range as HIRDLS, but requires

clear skies• Satellite remote sensors:

– consistent global coverage for observing global activities of gravity waves

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

9Observations of Gravity Waves in HIRDLS Data

HIRDLS instrument

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

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

10Observations of Gravity Waves in HIRDLS Data

GW temperature perturbations

• HIRDLS temperature measurements (T): basic state of rest + large-scale waves + small-scale waves

• Background field (Tbk): basic state of rest + low frequency planetary waves

• The temperature filter (Tf): high frequency planetary waves

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

11Observations of Gravity Waves in HIRDLS Data

Planetary waves

• Aug 2006

• Top: low frequency planetary waves in background fields

• Bottom: high frequency planetary waves in the temperature filter

12Observations of Gravity Waves in HIRDLS Data

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

13Observations of Gravity Waves in HIRDLS Data

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

14Observations of Gravity Waves in HIRDLS Data

Cross-section of amplitude and wavelength (May 2006)

• Top: Alexander et al., 2008

• Bottom: our results

• Same data but different techniques

• Qualitatively agree with each other

• Details are different as a result of using different techniques

15Observations of Gravity Waves in HIRDLS Data

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 a pronounced seasonal dependence

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

– Convection is probably another important source for the waves observed, 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 internal gravity waves (D. G. Andrews et al., Middle Atmosphere Dynamics, 1987)