Kazuyuki Miyazaki

1. Transport and mixing in the extratropical tropopause region in a high vertical resolution GCM Kazuyuki Miyazaki Japan Agency for Marine-Earth Science and Technology Co-authors: Kaoru Sato1, Shingo Watanabe2, Yoshio Kawatani2, Yoshihiro Tomikawa3, MasaakiTakahashi1 1. Univ. of Tokyo, 2. JAMSTEC, 3. NIPR

2. Just above the extratropical tropopause: strong temperature gradient forms a TIL, and the chemical transition layer also lies at this level. • The similar locations of the TIL and ExTL imply that the chemical and thermal structures interact with each other in the extratropical tropopause region (e.g., Randel et al., 2007). • However, the relationship between the mechanisms of formation of the TIL and the ExTL is still unclear. • In addition, relative importance of transport processes at different scales has not been fully understood. • To clarify these processes, we used a vertically high resolved GCM.

3. KANTO project: A high vertical resolution GCM T213L256 GCM： vertical resolution = 300 m, top=85km horizontal resolution = 0.5625° No gravity wave drag parameterization ・Gravity wave propagations and the induced circulationare explicitly simulated. ・allows analyzing fine thermal/dynamical structure around the tropopause. ・The importance of variously scaled atmospheric processes can be examined. Watanabe et al., JGR2008, JGR2009, Tomikawa et al., JGR2008, Kawatani et al., JAS2009a(revised)&JAS2009b(revised), Sato et al., GRL2009, Miyazaki et al., JAS2009a(revised)&JAS2009b(submitted) - study various aspects of small-scale phenomena including gravity waves and their role on the large-scale fields in the atmosphere.

4. QBO variations Gravity wave propagations (Watanabe et al., 2008)

5. Comparisons with a coarse-resolution GCM (Δz≒1km) N2 MPV tropopause based-coord. Lat.

6. Mean-meridional circulation: January Mean vertical wind W* Mean meridional wind V* 4PVU Compared to a coarse resolution model, the high resolution model has ・stronger convergence of the mean downward velocity in the lowermost stratosphere ・stronger and sharper meridional divergent flows in both the tropical Hadley circulation and the extratropical direct circulation

7. The PV is a material invariant (i.e., passive tracer) under inviscid (frictionless) and adiabatic approximation (Hoskins, 1985). Continuity equation of isentropic PV; Zonal mean equation based on the mass-weighted isentropic zonal means (MIM) Non-conservative term reflects changes in static stability by diabatic processes (e.g., heating at higher altitudes stabilizes the atmosphere and produces the PV). If the source/sink effect is negligible, this analysis provides insights into transport processes related to formation of long-lived tracer concentration gradients Analysis of maintain mechanisms of vertical PV gradients

8. PV budget analysis results

9. Maintain mechanisms of PV gradients +EDY+EDZ+source July January imply that the formation of large tracer concentration gradients around the ExTL mainly arises from mean downward transport in the lower levels and isentropic (winter) and vertical (summer) mixing in the upper levels. MEY1 MEY2 MEY1 MEY2 MEZ1 MEZ1 MEZ2 MEZ2 EDZ EDZ EDY EDY

10. N2: 50N-60N TIL formation mechanisms • Stratification by radiation and downward advection of the static stability profile are important in determining seasonal variation of the TIL. • The summertime maximum in static stability is a result of radiation. dN2/dt: 50N-60N at TIL In the isentropic formulation, eddy heat transport terms are eliminated. Thermodynamic analysis

11. dN2/dt: vertical profiles • N2 increases during summer • In the upper part TIL: • Radiation processes • In the lower part TIL: • Downward advection of heat Thermodynamic analysis

12. July dN2/dt by radiation Log(H2O) • Coarse-resolution model • Excessive diffusion (adiabatic and diabatic eddies) too small H2O gradient and N2 increase in the TIL Gradient genesis analysis & Thermodynamic analysis ↓ • - ExTL and TIL may have similar locations, as a result of common dynamic processes and interactions between constituent distributions and thermal structure • downward advection of constituent (gradient) and heat at the lower part • eddy mixing (suppression) of constituents (e.g., H2O) and radiative stratification (related to large constituent concentration gradients) at the upper part

13. Relative contributions of atmospheric motions with different scales to mean-meridional circulation and mixing

14. Downward control calculation PW: n=1-3 MW: n=4-20 GW: n=21- Div. EP-F Stream func. January θ V* Lat.

15. Seasonal variation of horizontal and vertical mixing Horizontal diffusion coefficient Vertical diffusion • - Strong isentropic mixing between 20 K below and 10 K above the tropopause • Vertical eddy mixing are substantial in the tropopause region during summer, but are strongly suppressed just above the well-mixed region

16. 50N-60N January Overworld stratosphere 400K Middleworld stratosphere Isentropic mixing TIL θ Small horizontal-scale motions (s>21) provide an important contribution to the total isentropic mixing at TIL levels. Zonal wave number(s) Overworld stratosphere 400K Middleworld stratosphere TIL θ Integrated flux

17. - very strong small-scale vertical mixing just above the tropopause  active 3-D mixing in the LMS 50N-60N January Overworld stratosphere 400K Middleworld stratosphere Vertical mixing TIL θ Zonal wave number(s) Overworld stratosphere 400K Middleworld stratosphere TIL θ Integrated flux

18. PV’ for n>21 at 360K ← Snap-shot picture The small-scale disturbances occur over possible sources of gravity waves(high mountains, cyclones, fronts, and convection), suggesting that the propagation and breaking of gravity waves lead to active 3-D mixing in the TIL. January ←Monthly mean amplitude

19. Discussions: GWs and small-scale mixing around the TIL • Following the linear wave theory, a large static stability allows the occurrence of large-amplitude GWs due to increase of the saturation amplitude. • Even when GWs do not break, large-amplitude GWs can be dissipated due to radiative relaxation and can cause eddy vertical dispersions (under non-uniform diabatic heating rate fields). - In the upper part of and just above the TIL, GWs may easily reach their saturation limit and breaking level because of a decrease in static stability (and atmospheric density) with height. - Dynamic instability occurs more frequently in the tropopause region than at other levels, (which also may lead to the saturation of gravity waves). - These situations associated with rapid changes in static stability and the dissipation and saturation of GWs seem to cause small-scale 3-D mixing around the TIL. N2 & Probability of the occurrence of Ri<0.25

20. Summary • The formation of large PV (or tracer) gradients around the ExTL mainly arises from mean downward transport in the lower levels and isentropic (during winter) and vertical (during summer) mixing in the upper levels. • The locations of the TIL and the ExTL can be similar due to common dynamic processes (for constituent and heat) and interactions between constituent distributions and thermal structure. • The small-scale dynamics associated with GWs play important roles in driving tracer transports by both the mean-meridional circulation and 3-D mixing. It may be important to include these small-scale dynamic effects in GCMs or CTMs to obtain better simulations of the TIL and ExTL. Miyazaki et al., Transport and mixing in the extratropical tropopause region in a high resolution GCM - Part 1: Potential vorticity and heat budget analysis, JAS (revised) - Part 2: Relative importance of large-scale and small-scale dynamics, JAS (submitted)

24. Eddy PV transport fluxes & U Eddy transport plays an important role in exchanging air between the stratosphere and troposphere through processes such as stratospheric intrusions, cutoff lows, and gravity wave breakings, especially around the subtropical jet stream.