1. Trajectory-based studies of dehydration in the tropical tropopause region  
Peter Haynes, University of Cambridge.
Collaboration with:
Stefan Fueglistaler, ETH/Univ. of Washington,
Marine Bonazzola, University of Cambridge/ECMWF,
Rob Mackenzie, University of Lancaster,
Chuansen Ren, University of Lancaster
Work reported in Fueglistaler et al (JGR 2004), Bonazzola and H (JGR 2004), Fueglistaler et al (JGR 2005), Fueglistaler and H (JGR 2005).

2. Rationale
Relevance to modelling of deep convection and chemistry and their roles in the Tropical Tropopause Layer?
What does the trajectory-based view tell us about dehydration and water vapour?
What can it tell us about chemistry?

3. Outline
Trajectory calculations and implications for dehydration
Seasonal climatology of stratospheric H2O
Interannual variability of stratospheric H2O
Summary/Implications

4. Questions
How does dehydration occur? (convection scale versus large scale)
Apparent lack of large-scale cirrus focussed attention on convective scale processes (Danielsen 1982).
Widespread ‘subvisible’ cirrus now observed by SAGEII, LITE, etc. Large-scale dehydration viable.
Where does dehydration occur?
Newell and Gould-Stewart (1981) ‘stratospheric fountain’ vs Dessler (1998).

5. LITE observations over W. Pacific (Winker and Trepte 1998)

6. Trajectory-based approach to studying dehydration
Builds on Holton and Gettelman (2001), Jackson et al (2001).
Consider trajectories from troposphere to stratosphere through the TTL. Stratospheric water vapour determined by Lagrangian temperature history and effect on microphysics: particle formation, growth and fall-out.
Dehydration occuring remotely from convection implies calculations based on large-scale temperatures and velocity fields are useful.
Simple picture: ‘Lagrangian cold point’ (Lcp) sets entry value of water vapour

7. Trajectory calculations
Methven trajectory code based on ECMWF analyses (T106, 31 levels, 6 hours).
Ensembles of initialised on 430K (30S-30N, 1 deg ×1 deg), 3 months duration.
NH winter 98-99: to (W99)
NH summer 99: to (S99)
Ensembles of 5580 initialised on 400K (30S-30N, 2 deg ×2 deg), 3 months duration.
NH winter 98-99: to (W99B)
NH winter 97-98: to (W98)
TS ensemble: those back trajectories that originate below 355K.

8. Cold Point Layer (CPL)
Potential temperatures corresponding to temperature minima along the trajectories for the TS ensemble.
CPL is ∼ 360K-380K for NH winter 98-99, NH summer 99, ∼ 365K-385K for NH winter

9. NH winter 98-99
Implied [H2O]e 1.5ppmv

10. NH summer 99
Implied [H2O]e 3.8ppmv

11. Fueglistaler et al (2004) NH winter NH summer H2O -- TS trajectories
Lagrangian cold point level
340K level
H2O -- all trajectories
NH winter
NH summer

12. NH winter 97-98 vs
NH winter 98-99
Implied [H2O]e 2.0 ppmv vs 1.5ppmv

13. Distinguishing between different transport contributions
Backward trajectory from P passes through CPL during time interval [t1, t2] and intersects specified isentropic surface at Q.
(P) Minimum smvr in column through P during time [t1, t2].
(Q) Minimum smvr in column through Q during time [t1, t2].
(R) Minimum smvr along trajectory.
P-Q defines a ‘stratospheric fountain effect’. Q-R defines ‘horizontal transport effect’.

14. Estimates for contribution of different transport processes to dehydration

15. Back-trajectory calculations at 1-month time intervals for using ERA-40 (Fueglistaler et al, 2005)
Summary results

16. (Fueglistaler et al, 2005) ATMOS (Michelsen et al 2001)
Arctic balloon (Engel et al 1996)
mid-latitude balloon (Zogel et al 1999)
HALOE
(Fueglistaler et al, 2005)

17. Interannual variation: absolute values of H2O
Fueglistaler and H (2005)

18. Interannual variation: anomalies from average seasonal cycle
Fueglistaler and H (2005)

19. T100 anomalies are good predictor of TLcp anomalies
Fueglistaler and H (2005)

20. Source region for (upper) TTL?
JF 2001
JA 2001
Fueglistaler et al 2004
Gettelman et al 2004

21. AWI Diabatic Trajectories
Immler et al 2006 (under review)

22. Transport timescales for (upper) TTL?
Fueglistaler et al 2004
Trajectory studies suggest 20days
Folkins 2005
1-D models suggest >50days?

23. Implications/Questions
Summary
Inhomogeneity in both vertical and horizontal transport significantly reduces TLcp below T100 (or whatever).
TLcp calculated from ERA40 is good predictor of absolute values of stratospheric H2O, including seasonal and interannual variation.
Results support view that at leading order stratospheric water vapour is controlled by large-scale transport and large-scale temperatures
Implications/Questions
Three-dimensionality of upper TTL important for chemical processing? (no single level of zero radiative heating, air parcels move large horizontal distances)
What is (surface) source region for stratosphere `(‘overworld’ vs ‘lowermost stratosphere’?
Physical mechanisms for variation in vertical transport in upper TTL?

24. Questions
Physical reality of velocity and temperature fields in TTL (e.g. magnitude and geographical variation of vertical velocities).
Realistic microphysics (e.g. necessity of invoking small-scale temperature fluctuations).
Critical evaluation of roles of large-scale vs convective-scale processes --implications for isotopes, transport/processing of short-lived chemical species.

25. Winds
Temperatures
Distribution of Lcp
Distribution on 340K
(Fueglistaler et al, 2005)