Turbopause and Gravity Waves

1. Turbopause and Gravity Waves Han-Li Liu HAO National Center for Atmospheric Research

2. Turbopause • Turbopause: separates the homosphere and heterosphere (also called homopause). • Location: base of the thermosphere and above mesopause. Regulator for mesosphere/thermosphere exechange. • Determination: Turbulent~Molecular diffusion. Richmond, 1983

3. Variability of Turbopause Hall et al, 1998 • Turbopause shows variability, but in general very stable and persistent. Why is eddy diffusion (supposedly due to gravity wave breaking) so consistently ~100m2/s at 100-120km?

4. A somewhat different question: • Eddy diffusion or molecular diffusion: Turbulent or laminar flow (in the vertical direction)? • It then becomes a problem of calculating the Reynolds number of the atmosphere:

5. Some assumptions • In a strongly stratified fluid, large scale vortical motion could be treated as a two dimensional turbulence, thus may not contribute significantly to vertical mixing. • The hydrostatic assumption is generally valid, so that the vertical spatial scale of strong vertical advection/mixing is on the order of scale height (or less). • For 3D turbulence, the horizontal scale is comparable to the vertical scale. • The wind is capped by acoustic wave speed.

6. Reynolds Number Re < 800: most likely to be laminar; Re > 5000: most likely to be turbulent.

7. W=100m/s W=10m/s W=1m/s

9. Discussion (1) • The Reynolds number is primarily tied to the atmospheric temperature profile. The transitional region between turbulent-laminar regime is quite stable over seasons and locations. • For the transitional region to be in the recognized turbopause region (100-120km), the vertical wind has to be on the order of 10-100 m/s. This most likely comes from gravity wave or acoustic wave perturbations (zonal mean vertical wind: 0.01-0.1 m/s, tidal perturbation: 0.1-1 m/s, in this region).

10. Discussion (2) • Some scale requirement are put on the perturbations, but much less stringent than the requirement for nearly constant eddy diffusion at a nearly constant height (which in turn requires specific gravity wave characteristics). • Eddy diffusion coefficient may actually be estimated in the transition region, assuming eddy and molecular diffusion are comparable therein. • Numerical models will not properly resolve turbopause if these scales are not properly resolved.

11. Temperature Structures near Turbopause • The turbopause is in a region when the temperature increases rapidly from the minimum at the mesopause, due to the heating by exothermic reactions and SRB and SRC. Both temperature gradient and curvature are large in the mesopause-turbopause region. It is found and BV frequency reaches maximum where the temperature curvature maximizes.

13. Implication for Wind Structure • Stronger static stability can support larger wind shear, before the atmosphere becomes dynamically unstable.

15. Larsen, 2002

16. Discussion (3) • The atmospheric static stability is strongest near the turbopause, because of large temperature curvature there. This region show modulation over local time/longitude, season, and latitude. • Very large wind shears (and large wind) could be obtained as a result, suggested also by observations. • Even though the agreement of the maximum static stability/large wind shear height and the turbopause height appears coincidental, the large wind nevertheless supports the scale argument in Reynolds number calculation. • The large wind shears can support sporadic-E in the same altitude range.