Quantifying dynamics and transport in the extratropical UTLS

Transcription

1 Quantifying dynamics and transport in the extratropical UTLS Peter Haynes DAMTP/CCfCS, University of Cambridge Université Fédérale Toulouse Midi-Pyrénées (contributions from Alison Ming ERC ACCI project)

2 Outline Historical development of theoretical/conceptual picture of dynamics and transport in the extratropical UTLS Organisational frameworks for interpreting chemical measurements pros and cons of different approaches Use of artificial tracers Forecast/reanalysis world is there convergence?

3 Early 1990s SPARC newsletter 1993 Building on Hoskins (1991) (Stratospheric) Overworld Lowermost Stratosphere Holton et al (1995) Emphasis on net fluxes STE flux of species with source in stratospheric overworld = flux across 380K

4 Early 2000s Law, Cox, H (WMO 2003) Stohl et al (2003) Extratropics: ExTL ( mixing layer, very lowermost stratosphere ), deep exchange vs shallow exchange, role of WCBs Tropics: Most convection does not reach the tropopause, TTL as layer with intermediate characteristics.

5 Evidence for tropospheric air entering the Lowermost Stratosphere Zahn (2001) Ray et al (1999)

6 Late 2000s Extratropics: Tropopause Inversion Layer (TIL), exchange above and below jets Gettelman et al (2011) Tropics: TIL, shallow branch of BDC

7 TIL in radiosonde data Birner et al (2002), Birner (2006) Miramar NAS, California (33 N, 117 W); Reno, Nevada (40 N, 120 W); Quillayute, Washington (48 N, 125 W); Yakutat, Alaska (60 N, 140 W) TIL now accepted as important feature of the extratropical UTLS

8 Dynamics-only simple GCM Ming 2015, following Son and Polvani 2007, also Kim and Son 2015 Thermal restoration state Statistical equilibrium state

9 How do we take account of meteorological variability in interpreting chemical and dynamical measurements? Troposphere: low static stability, low PV, high water vapour, low ozone, Stratosphere: high static stability, high PV, low water vapour, high ozone, Need to identify tropopause, but also define location relative to the tropopause.

10 Interpretation of single event e.g. Tropospheric Intrusion Homeyer et al (2011)

11 Seasonal to interannual climatology (e.g. water vapour) MLS ACE-FTS Randel and Jensen 2013 Randel et al 2012

12 Mapping the extratropical UTLS in the meridional (latitude-height) plane Thermal tropopause: z trop distinguished by dt/dz criterion then use geometric height z relative to : z trop Chemical species: use concentration to define a (single) meridional coordinate Jet tropopause : φ trop distinguished by jet axis, then use latitude φ relative to φ trop PV: e.g. use variation on θ surfaces to define equivalent latitude φ equiv(pv) Artificial tracer: use to define a single meridional coordinate, or on θ surfaces to define φ equiv(tracer) Each has advantages and disadvantages [?mapping longitudinal variation?]

13 Reversible changes in dt/dz (synoptic scale dynamics) cyclone anticyclone Wirth (2001)

14 Reversible changes in dt/dz (small-scale dynamics) N 2 profiles from CHAMP data (January 2002, 50-60N) (Ming 2015, following Randel et al 2007, Schmidt et al 2010, )

15 PV equivalent latitude φ equiv(pv) Potential Temperature θ approach Hoor et al 2004

16 Pan et al (2012) a cautionary note on using φ equiv(pv) [PV EqLat]-θ co-ordinates for aggregating chemical tracers in the UTLS In extratropical UTLS (in contrast with extratropical stratosphere) Convective processes can modify PV and redistribute chemical species on a time scale of a few hours Isentropes are strongly sloping (and intersect the boundary layer) φ equiv(pv) identifies stratospheric instrusions into the troposphere as tropospheric (and vice versa) Recent studies have identified double tropopauses as most frequent in late winter and spring, but φ equiv(pv) -- θ studies suggest the Lowermost Stratosphere is more ventilated in summer

17 Pan et al (2012)

18 Pan et al (2012)

19 Pan et al (2012) What next? PV structures do have a significant connection with STE events, If we average the chemical tracers based on their PV or EqLat, the STE events related to these PV structures will be largely concealed.

20 Scenario 1 Scenario 2 end state 1 end state 2

21 Double tropopause frequency Peevey et al 2012 Wu and Lu 2015

22 Extratropical UTLS visualised by effective diffusivity (H and Shuckburgh 2000)

23 ERA-I calculations of effective diffusivity Abalos et al 2015

24 Rossby wave breaking and exchange between tropics and extratropics Homeyer and Bowman 2013 Conclusion: Occurrence of DT does not imply effective irreversible transport to the extratropics?

25 Lagrangian characterisation of troposphere/stratosphere Berthet et al 2007 NH winter Proportion of back trajectories that visit boundary layer in 30 days NH summer PV=2? Suggested transition layer

26 An atmospheric chemist in search of the tropopause Prather et al 2011 e90 tracer surface emission, 100ppbv mean, 90 day lifetime

27 Summary Long-standing tradition of combining meteorological and chemical observations to study the extratropical UTLS. Operational analysis/reanalysis data is now vital resource where is the real world different from reanalysis world Different approaches needed for study of events vs study of climatology. No approach is perfect and any approach needs careful interpretation. Organisation of chemical measurements using PV/equivalent latitude or using artificial tracers has many advantages for climatology. How to define artificial tracers? Can we design for longitudinal localisation? Is there convergence with respect to re-analysis datasets? Quantitative diagnostics should be observable. Very high-resolution models potentially provide a self-consistent alternative world share datasets as much as possible.