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).

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?

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

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).

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

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

Trajectory calculations Methven trajectory code based on ECMWF analyses (T106, 31 levels, 6 hours). Ensembles of 21960 initialised on 430K (30S-30N, 1 deg ×1 deg), 3 months duration. NH winter 98-99: 28-02-99 to 01-12-98. (W99) NH summer 99: 31-08-99 to 01-06-99. (S99) Ensembles of 5580 initialised on 400K (30S-30N, 2 deg ×2 deg), 3 months duration. NH winter 98-99: 28-02-99 to 01-12-98. (W99B) NH winter 97-98: 28-02-98 to 01-12-97. (W98) TS ensemble: those back trajectories that originate below 355K.

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 97-98.

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

NH summer 99 Implied [H2O]e 3.8ppmv

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

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

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’.

Estimates for contribution of different transport processes to dehydration

Back-trajectory calculations at 1-month time intervals for 1979-2001 using ERA-40 (Fueglistaler et al, 2005) Summary results

(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)

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

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

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

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

AWI Diabatic Trajectories Immler et al 2006 (under review) fimmler@awi-potsdam.de, fimmler@awi-bremerhaven.de

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

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?

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.

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