Todd Lane The University of Melbourne, Australia. NCAR Gravity Wave Retreat 26 June 2006
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Transcript of Todd Lane The University of Melbourne, Australia. NCAR Gravity Wave Retreat 26 June 2006
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Gravity waves above deep convection:
Modeling results showing wave breaking, secondary generation, & mixing.
Gravity waves above deep convection:
Modeling results showing wave breaking, secondary generation, & mixing.
Todd Lane
The University of Melbourne, Australia.
NCAR Gravity Wave Retreat
26 June 2006
Todd Lane
The University of Melbourne, Australia.
NCAR Gravity Wave Retreat
26 June 2006
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Important issues regarding convectively generated gravity waves.
Do we know the “true” spectrum of waves?
This talk - focus on “small-scale” waves.
How do these waves behave near the cloud top?
When do these waves break?
What are the details of the breakdown and subsequent mixing?
Clusters / organized systems~100-1000 km
Individual clouds~1-10 km
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Vertical Velocity, 0.5 m s-1 intervals.
Fovell et al. (1992), Alexander & Holton (1997), Piani et al. (2000), Lane et al. (2001).
X ~ O( 1km)
20 - 30 km, N(troposphere),C 25-35 m/s
Vertically propagating waves that could reach upper stratosphere and mesosphere in situations with moderate shear.
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Lane and Knievel, JAS 2005.
Vertical Velocity, 0.5 m s-1 intervals.
X ~ O( 100 m)
5 - 10 km N(troposphere) C 5-15 m/s
Encountering a critical level is likely.
Evanescence also likely.
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Momentum flux @ 15 km: x=2 km (solid) x=125 m (dashed)
Change in momentum flux at 15 km due to resolution.
(N/m2)/(m/s)
x=2 km: < w2(10 km) > = 6 m2/s2, < u’w’(25 km) > = 0.04 N/m2
x=125 m < w2(10 km) > = 10 m2/s2, < u’w’(25 km) > = 0.02 N/m2
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From Lane, Sharman, Clark, & Hsu, JAS 2003.
U(z)
U ~ 10 m/s will give (U-C) = 0 for downshear waves. ~5-10 km.
2D Breaking
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Questions raised…Questions raised…To date - this breaking in lower-stratosphere (in a high-resolution complete CRM simulation) has only been demonstrated in 2D.
What happens in 3D?
- Do waves break at same location?
- 2D case should maximize breaking.
3D required to quantify turbulence and mixing - determine details of breakdown. Are these breaking waves efficient mixers?
To date - this breaking in lower-stratosphere (in a high-resolution complete CRM simulation) has only been demonstrated in 2D.
What happens in 3D?
- Do waves break at same location?
- 2D case should maximize breaking.
3D required to quantify turbulence and mixing - determine details of breakdown. Are these breaking waves efficient mixers?
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3D CRM3D CRMx=y=z= 150 m674 (L) x 338 (W) x 234 (H)100 km x 50 km x 35 km
Anelastic, nonhydrostatic.
Simplified microphysics - Kessler warm rain
Smagorinsky turbulence.
Midlatitude, real sounding case. Moderate negative shear above cloud top. (Same scenario as previous 2D cases).
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Potential temperature (2 K intervals), t = 60 mins
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Potential temperature (2 K intervals), t = 75 mins
But what does the 3D breaking look like?
Similar pattern of breaking - breaking of downshear waves.- Less mixing in 3D
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“New” waves emanating from breaking region. - secondary waves.
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X (km)
Pert. Pot. Temp (K)Pert. qv (ppm)
z=15 km, t=40 mins
-above convective overshoot-no condensation-adiabatic-qv passive tracer-dqv/dz > 0 at this height and qv in phase
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X (km)
Pert. Pot. Temp (K)Pert. qv (ppm)
z=15 km, t=75 mins
At this time - 0.05% average reduction in Qv on 400 K surface.
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Summary
Still a lot about the spectrum of these waves we don’t fully understand.
- Spectrum in real conditions - (not idealized squall-lines).
- Combined spectrum of individual clouds & clusters.
3D - Wave breaking:-Many similarities to 2D case - breaking in similar locations.
-Breaking close to cloud top due to interaction of short wavelength (~ 5-10 km) waves with critical level.
-Breaking causes (what appears to be) secondary wave generation.
-Coherent bands of 2 km wavelength waves emanating from wave breaking region.
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Summary (continued):
Mixing:-Breaking waves cause cross-isentropic transport of water vapour. This generalizes to other constituents that have vertical gradients in wave breaking region (e.g., ozone).
-These waves are inefficient mixers - mixing is highly localized & vertical displacements are small ~ 200 m.
- Caveat: Diabatic process comes directly from sub-grid turbulence parameterization.
- Parameterizations have much uncertainty - and need to be better constrained (by observations and DNS).
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Future Directions:
-More real case studies. - Cloud system focus rather than individual clouds - which
waves are more important? - Could be achieved with better utilization of cloud-resolving
NWP(-like) models.
- Mixing & turbulence studies. -Observations are crucial to provide reliable estimates of mixing.