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Todd Lane The University of Melbourne, Australia. NCAR Gravity Wave Retreat 26 June 2006

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. Important issues regarding convectively generated gravity waves.

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Todd Lane The University of Melbourne, Australia. NCAR Gravity Wave Retreat 26 June 2006

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  1. 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

  2. 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

  3. 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.

  4. 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.

  5. Change in momentum flux at 15 km due to resolution. Momentum flux @ 15 km: x=2 km (solid) x=125 m (dashed) (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

  6. 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.

  7. 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?

  8. 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. Midlatitude, real sounding case. Moderate negative shear above cloud top. (Same scenario as previous 2D cases).

  9. Potential temperature (2 K intervals), t = 60 mins

  10. 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?

  11. “New” waves emanating from breaking region. - secondary waves.

  12. 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 Pert. Pot. Temp (K) Pert. qv (ppm) X (km)

  13. z=15 km, t=75 mins Pert. Pot. Temp (K) Pert. qv (ppm) At this time - 0.05% average reduction in Qv on 400 K surface. X (km)

  14. 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.

  15. 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).

  16. 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.

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