Gravity waves above deep convection: Modeling results showing wave breaking, secondary generation,

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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

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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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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.

Lane and Knievel, JAS 2005.
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Change in momentum flux at 15 km due to
resolution.
Momentum flux _at_ 15 km ?x2 km (solid) ?x125 m
(dashed)
(N/m2)/ (m/s)
?x2 km lt w2(10 km) gt 6 m2/s2,
lt ?uw(25 km) gt 0.04 N/m2 ?x125 m
lt w2(10 km) gt 10 m2/s2, lt ?uw(25 km) gt
0.02 N/m2
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2D Breaking
U(z)
From Lane, Sharman, Clark, Hsu, JAS 2003.
?U 10 m/s will give (U-C) 0 for downshear
waves. ?5-10 km.
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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?

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3D CRM
?x?y?z 150 m 674 (L) x 338 (W) x 234 (H) 100
km x 50 km x 35 km Anelastic,
nonhydrostatic. Simplified microphysics -
Kessler warm rain Smagorinsky turbulence. Midlat
itude, 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
Similar pattern of breaking - breaking of
downshear waves. - Less mixing in 3D
But what does the 3D breaking look like?
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(No Transcript)
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New waves emanating from breaking region. -
secondary waves.
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z15 km, t40 mins

• -above convective overshoot
• no condensation
• adiabatic
• qv passive tracer
• dqv/dz gt 0 at this height
• ? and qv in phase

Pert. Pot. Temp (K) Pert. qv (ppm)
X (km)
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z15 km, t75 mins
Pert. Pot. Temp (K) Pert. qv (ppm)
At this time - 0.05 average reduction in Qv on
400 K surface.
X (km)
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• Summary
• Still a lot about the spectrum of these waves we
  dont 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.