Observations of Gravity Waves in HIRDLS Data

1. Observations of Gravity Waves in HIRDLS Data Xiuping Yan, Neil Arnold, John Remedios EOS & RSPP, Department of Physics & Astronomy University of Leicester 26/6/2008

2. 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 Observations of Gravity Waves in HIRDLS Data

3. Motivation • Generate a global climatology dataset of atmospheric gravity waves • Provide constraints for the parameterization of gravity waves in global circulation models Observations of Gravity Waves in HIRDLS Data

4. 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 • Restoring force: buoyancy http://www.kowoma.de/en/gps/additional/atmosphere.jpg Observations of Gravity Waves in HIRDLS Data

5. 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 eastward forcing westward forcing westward winds eastward winds westward forcing westward forcing Fritts and Alexander, 2003 Observations of Gravity Waves in HIRDLS Data

6. Why are gravity waves important? Strong wind • 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 Observations of Gravity Waves in HIRDLS Data

7. 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 Observations of Gravity Waves in HIRDLS Data

8. 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) Observations of Gravity Waves in HIRDLS Data

9. 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 Observations of Gravity Waves in HIRDLS Data

10. 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 Observations of Gravity Waves in HIRDLS Data

11. Planetary waves • Aug 2006 • Top: low frequency planetary waves in background fields • Bottom: high frequency planetary waves in the temperature filter Observations of Gravity Waves in HIRDLS Data

12. Climatology of GW amplitude Jan 2006 Feb 2006 Mar 2006 Apr 2006 May 2006 Jun 2006 Sep 2006 Aug 2006 Jul 2006 Oct 2006 Nov 2006 Dec 2006 Observations of Gravity Waves in HIRDLS Data

13. Climatology of GW wavelength Jan 2006 Feb 2006 Mar 2006 Apr 2006 May 2006 Jun 2006 Jul 2006 Aug 2006 Sep 2006 Oct 2006 Nov 2006 Dec 2006 Observations of Gravity Waves in HIRDLS Data

14. 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 Observations of Gravity Waves in HIRDLS Data

15. 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) Observations of Gravity Waves in HIRDLS Data

16. Thank you very much for your attention!