Atmospheric Chemistry and Physics www.atmos-chem-phys.net 1680-7316 1680-7324 7 5 2007 10.5194/acp-7-1313-2007 http://www.atmos-chem-phys.net/7/1313/2007/ http://www.atmos-chem-phys.net/7/1313/2007/acp-7-1313-2007.html http://www.atmos-chem-phys.net/7/1313/2007/acp-7-1313-2007.pdf 1313 1332 2007-02-27 Stratospheric dryness: model simulations and satellite observations J. Lelieveld lelieveld@mpch-mainz.mpg.de C. Brühl P. Jöckel B. Steil P. J. Crutzen H. Fischer M. A. Giorgetta P. Hoor M. G. Lawrence R. Sausen H. Tost Max Planck Institute for Chemistry, J. J. Becherweg 27, 55128 Mainz, Germany Scripps Institution of Oceanography, UCSD, La Jolla, CA 92093-0221, USA Max Planck Institute for Meteorology, Bundesstrasse 53, 20146 Hamburg, Germany DLR-Institut für Physik der Atmosphäre, Oberpfaffenhofen, 82234 Wessling, Germany The mechanisms responsible for the extreme dryness of the stratosphere have been debated for decades. A key difficulty has been the lack of comprehensive models which are able to reproduce the observations. Here we examine results from the coupled lower-middle atmosphere chemistry general circulation model ECHAM5/MESSy1 together with satellite observations. Our model results match observed temperatures in the tropical lower stratosphere and realistically represent the seasonal and inter-annual variability of water vapor. The model reproduces the very low water vapor mixing ratios (below 2 ppmv) periodically observed at the tropical tropopause near 100 hPa, as well as the characteristic tape recorder signal up to about 10 hPa, providing evidence that the dehydration mechanism is well-captured. Our results confirm that the entry of tropospheric air into the tropical stratosphere is forced by large-scale wave dynamics, whereas radiative cooling regionally decelerates upwelling and can even cause downwelling. Thin cirrus forms in the cold air above cumulonimbus clouds, and the associated sedimentation of ice particles between 100 and 200 hPa reduces water mass fluxes by nearly two orders of magnitude compared to air mass fluxes. Transport into the stratosphere is supported by regional net radiative heating, to a large extent in the outer tropics. During summer very deep monsoon convection over Southeast Asia, centered over Tibet, moistens the stratosphere. Alcala C. M. and Dessler A. E.: Observations of deep convection in the tropics using the Tropical Rainfall Measuring Mission (TRMM) precipitation radar, J. Geophys. Res., 107, 4792, doi:10.1029/2002JD002457, 2002. Aumann, H. H., Broberg, S., Elliott, D., Gaiser, S., and Gregorich, D.: Three years of Atmospheric Infrared Sounder radiometric calibration validation using sea surface temperatures, J. Geophys. Res., D16S90, doi:10.1029/2005JD006822, 2006. Bannister, R. N., O'Neil, A., Gregory, A. R., and Nissen, K. M.: The role of the south-east Asian monsoon and other seasonal features in creating the "tape-recorder" signal in the Unified Model, Q. J. R. Meteorol. Soc. 130, 1531-1554, 2004. Bonazzola, M. and Haynes, P. H.: A trajectory-based study of the tropical tropopause region, J. Geophys. Res., 109, D20112, doi:10.1029/2003JD004356, 2004. Brewer, A. W: Evidence for a world circulation provided by the measurements of helium and water vapour distribution in the stratosphere, Q. J. R. Meteorol. Soc. 75, 351–363, 1949. Corti, T., Luo, B. P., Fu, Q., Vömel, H., and Peter, T.: The impact of cirrus clouds on troposphere-to-stratosphere transport, Atmos. Chem. Phys., 6, 2539–2547, 2006. Daley, R.: Atmospheric data analysis, Cambridge University Press, Cambridge, UK, 1991. Danielsen, E. F.: A dehydration mechanism for the stratosphere, Geophys. Res. Lett., 9, 605–608, 1982. Dessler, A. E., Palm, S. P., Hart, W. D., and Spinhirne J. D.: Tropopause-level thin cirrus coverage revealed by ICEsat/Geoscience Laser Altimeter System, J. Geophys. Res., 111, D08203, doi:10.1029/2005JD006586, 2006. Divakarla, M. G., Barnet, C. D., Goldberg, M. D., McMillin, L. M., Maddy, E., Wolf, W., Zhou, L., and Liu, X.: Validation of Atmospheric Infrared Sounder temperature and water vapor retrievals with matched radiosonde measurements and forecasts, J. Geophys. Res., 111, D09S15, doi:10.1029/2005JD006116, 2006. Eyring, V., Butchart, N., Waugh, D. W. et al.: Assessment of temperature, trace species and ozone in chemistry-climate model simulations in the recent past, J. Geophys. Res., 111, D22308, doi:10.1029/2006JD007327, 2006. Fischer, H., and Oelhaf, H.: Remote sensing of vertical profiles of atmospheric trace constituents with MIPAS limb-emission spectrometers, Appl. Opt., 35, 2787–2796, 1996. Folkins, I., Loewenstein, M., Podolske, J., Oltmans, S. J., and Proffitt, M.: A barrier to vertical mixing at 14 km in the tropics: Evidence from ozonesondes and aircraft measurements, J. Geophys. Res., 104, 22 095–22 102, 1999. Fu, R., Hu, Y., Wright, J. S., Jiang, J. H., Dickinson, R. E., Chen, M., Filipiak, M., Read, W. G., Waters, J. W., and Wu, D. L.: Short circuit of water vapor and polluted air to the global stratosphere by convective transport over the Tibetan Plateau, Proc. Nat. Acad. Sci., 103, 5664–5669, 2006. Fueglistaler, S., Wernli, H., and Peter, T.: Tropical troposphere-to-stratosphere transport inferred from trajectory calculations, J Geophys. Res., 109, D03108, doi:10.1029/2003JD004069, 2004. Fueglistaler, S. and Haynes, P. H.: Control of interannual and longer-term variability of stratospheric water vapor, J. Geophys. Res., 110, D24108, doi:10.1029/2005JD006019, 2005. Fueglistaler, S., Bonazzola, M., Haynes, P. H., and Peter, T.: Stratospheric water vapor predicted from the Lagrangian temperature history of air entering the stratosphere in the tropics, J. Geophys. Res., 110, D08107, doi:10.1029/2004JD005516, 2005. Fueglistaler, S., and Fu, Q.: Impact of clouds on radiative heating rates in the lower tropical stratosphere, J. Geophys. Res., 111, D23202, doi:10.1029/2006JD007273, 2006. Ganzeveld, L. N., van Aardenne, J., Butler, T., Lawrence, M. G., Metzger, S. M., Stier, P., Zimmerman, P., and Lelieveld, J.: Technical Note: Anthropogenic and natural offline emissions and the online EMissions and dry DEPosition (EMDEP) submodel of the Modular Earth Submodel System (MESSy), Atmos. Chem. Phys. Discuss., 6, 5457–5483, 2006. Geller, M. A., Zhou, X., and Zhang, M.: Simulations of the interannual variability of stratospheric water vapor, J. Atmos. Sci., 59, 1076–1085, 2002. Gettelman, A., Salby, M. L., and Sassi, F.: Distribution and influence of convection in the tropical tropopause region, J. Geophys. Res., 107, D10, 4080, doi:10.1029/2001JD001048 (2002). Gettelman, A., Forster, P.M. de F., Fujiwara, M., Fu, Q., Vömel, H., Gohar, L. K., Johanson, C., and Ammerman, M: Radiation balance of the tropical tropopause layer, J. Geophys. Res., 109, D07103, doi:10.1029/2003JD004190, 2004a. Gettelman, A., Weinstock, E. M., Fetzer, E. J., Irion, F. W., Eldering, A., Richard, E. C., Rosenlof, K. H., Thompson, T. L., Pittman, J. V., Webster, C. R., and Herman, R. L.: Validation of Aqua satellite data in the upper troposphere and lower stratosphere with in situ aircraft instruments, Geophys. Res. Lett., 31, L22107, doi:10.1029/2004GL020730, 2004b. Gettelman, A., Collins, W. D., Fetzer, E. J., Eldering, A., Irion, F. W., Duffy, P. B., and Bala, G.: Climatology of upper-tropospheric relative humidity from the Atmospheric Infrared Sounder and implications for climate, J. Climate, 19, 6104–6121, 2006a. Gettelman, A., Fetzer, E. J., Eldering, A., and Irion, F. W.: The global distribution of supersaturation in the upper troposphere from the Atmospheric Infrared Sounder, J. Climate, 19, 6089–6103, 2006b. Giorgetta, M. A. and Bengtsson, L.: Potential role of the quasi-biennial oscillation in the stratosphere-troposphere exchange as found in water vapor in general circulation model experiments, J. Geophys. Res., 104, 6003–6019, 1999. Giorgetta, M. A., Manzini, E., Roeckner, E., Esch, M., and Bengtsson, L.: Climatology and forcing of the quasi-biennial oscillation in the MAECHAM5 model, J. Climate, 19, 3882–3901, 2006. Hagemann, S., Arpe, K., and Roeckner, E.: Evaluation of the hydrological cycle in the ECHAM5 model, J. Climate, 19, 3810–3827, 2006. Harries, J. E., Russell III, J. M., Tuck, A. F., Gordley, L. L., Purcell, P., Stone, K. Bevilacqua, R. M., Gunson, M., Nedoluha, G., and Traub, W. A.: Validation of water vapor from the Halogen Occultation Experiment (HALOE), J. Geophys. Res., 101, 10 205–10 216, 1996. Hartmann, D. L., Holton, J. R., and Fu, Q.: The heat balance of the tropical tropopause, cirrus, and stratospheric dehydration, Geophys. Res. Lett., 28, 1969–1972, 2001. Haynes, P. H., Marks, C. J., McIntyre, M. E., Shepherd, T. G., and Shine, K. P.: On the "downward control" of extratropical diabatic circulations by eddy-induced zonal mean forces, J. Atmos. Sci., 48, 651–678, 1991. Heymsfield, A. J. and Donner, L. J.: A scheme for parameterizing ice-cloud water content in general circulation models, J. Atmos. Sci., 47, 1865–1877, 1990. Highwood, E. J. and Hoskins, B. J.: The tropical tropopause, Q. J. R. Meteorol. Soc., 124, 1579–1604, 1998. Holton, J. R., Haynes, P. H., McIntyre, M. E., Douglas, A. R., Rood, R. R., and Pfister, L.: Stratosphere-troposphere exchange, Rev. Geophys., 33, 403–439, 1995. Holton, J. R. and Gettelman, A.: Horizontal transport and the dehydration of the stratosphere, Geophys. Res. Lett., 28, 2799–2802, 2001. Jackson, D. R., Burrage, M. D., Harries, J. E., Gray, L. J., and Russell III, J. M.: The semi-annual oscillation in upper stratospheric and mesospheric water vapour as observed by HALOE, Q. J. R. Meteorol. Soc., 124, 2493–2515, 1998. Jensen, E. J., Pfister, L., Ackerman, A. S., Tabazadeh, A., and Toon, O. B.: A conceptual model of the dehydration of air due to freeze-drying by optically thin, laminar cirrus rising slowly across the tropical tropopause, J. Geophys. Res., 106, 17 237–17 252, doi:10.1029/2000JD900649, 2001. Jensen, E., and Pfister, L.: Transport and freeze-drying in the tropical tropopause layer, J. Geophys. Res., 109, D02207, doi:10.1029/2003JD004022, 2004. Jöckel, P., Sander, R., Kerkweg, A., Tost, H., and Lelieveld, J.: Technical Note: The Modular Earth Submodel System (MESSy) - a new approach towards Earth System Modeling, Atmos. Chem. Phys., 5, 433–444, 2005. Jöckel, P., Tost, H., Pozzer, A., Brühl, C., Buchholz, J., Ganzeveld, L., Hoor, P., Kerkweg, A., Lawrence, M. G., Sander, R., Steil, B., Stiller, G., Tanarhte, M., Taraborelli, D., van Aardenne, J., and Lelieveld, J.: The atmospheric chemistry general circulation model ECHAM5/MESSy: Consistent simulation of ozone from the surface to the mesosphere, Atmos. Chem. Phys., 6, 5067–5104, 2006. Kerkweg, A., Sander, R., Tost, H., and P. Jöckel: Technical Note: Implementation of prescribed (OFFLEM), calculated (ONLEM), and pseudo-emissions (TNUDGE) of chemical species in the Modular Earth Submodel System (MESSy), Atmos. Chem. Phys., 6, 3603–3609, 2006a. Kerkweg A., Buchholz, J., Ganzeveld, L., A. Pozzer, H. Tost, and P. Jöckel: Technical Note: An implementation of the dry removal processes DRY DEPosition and SEDImentation in, the Modular Earth Submodel System (MESSy), Atmos. Chem. Phys., 6, 4617–4632, 2006b. Kley, D., Stone, E. J., Henderson, W. R., Drummond, J. W., Harrop, W. J., Schmeltekopf, L., Thompson, T. L., and Winkler, R. H.: In situ measurements of the mixing ratio of water vapor in the stratosphere, J. Atmos. Sci., 36, 2513–2524, 1979. Kley, D., Russell III, J. M., and Phillips, C.: SPARC Assessment of upper tropospheric and stratospheric water vapour, WCRP-113, WMO/TD-No. 1043, SPARC Report No. 2, World Meteorological Organization, Geneva, 2000. Landgraf, J. and Crutzen, P. J.:An efficient method for online calculations of photolysis and heating rates, J. Atmos. Sci., 55, 863–878, 1998. Lin, S. J., and Rood, R. B.: Multi-dimensional flux-form semi-Lagrangian transport schenes, Mon. Wea. Rev., 124, 2046–2070, 1996. Lawrence, M. G. and Rasch, P. J.: Tracer transport in deep convective updrafts: plume ensemble versus bulk formulations, J. Atmos. Sci., 62, 2880–2894, 2005. Lohmann, U. and Roeckner, E. : Design and performance of a new cloud microphysics parameterization developed for the ECHAM4 general circulation model, Clim. Dyn., 12, 557–572, 1996. Lohmann, U., Roeckner, E., Collins, W. D. Heymsfield, A. J., McFarquhar, G. M., and Barnett, T. P.: The role of water vapor and convection during the Central Equatorial Pacific Experiment from observations and model simulations, J. Geophys. Res., 100(D12), 26 229–26 246, 1995. Luo, Z., and W. B. Rossow: Characterizing tropical cirrus life cycle, evolution and interaction with upper-tropospheric water vapor using Lagrangian trajectory analysis of satellite observations, J. Clim., 17, 4541–4563, 2004. Majewski, D.: Balanced initial and boundary values for a limited area model, Beitr. Phys. Atm., 58, 147–165, 1985. Manzini, E., Steil, B. Brühl, C., Giorgetta, M. A., and Krüger, K.: A new interactive chemistry-climate model: 2. Sensitivity of the middle atmosphere to ozone depletion and increase in greenhouse gases and implications for recent stratospheric cooling. J. Geophys. Res., 108(D14), 4429, doi:10.1029/2002JD002977, 2003. Manzini, E., Giorgetta, M. A., Esch, M., Kornblueh, L., and Roeckner, E.: The influence of sea surface temperatures on the northern winter stratosphere: ensemble simulations with the MAECHAM5 model, J. Climate, 19, 3863–3881, 2006. Massie S., Gettelman, A., Randel, W., and Baumgardner, D.: Distribution of tropical cirrus in relation to convection, J. Geophys. Res., 107 (D21), 4591, doi:10.1029/2001JD001293, 2002. McFarquhar, G. M., Heymsfield, A. J., Spinhirne, J., and Hart, B.: Thin and subvisual tropical tropopause cirrus, J. Atmos. Sci., 57, 1841–1853, 2000. Mlynczak, M. G., Mertens, C. J., Garcia, R. R., and Portmann, R. W.: A detailed evaluation of the stratospheric heat budget 2. Global radiation balance and diabatic circulations, J. Geophys. Res., 104, 6039–6066, 1999. Mote, P. W., Rosenlof, K. H., McIntyre, M. E., Carr, E. S., Gille, J. C., Holton, J. R., Kinnersley, J. S., Pumphrey, H. C., Russell III, J. M., and Waters, J. W.: An atmospheric tape recorder: The imprint of tropical tropopasue temperatures on stratospheric water vapor, J. Geophys. Res., 101, 3989–4006, 1996. Newell, R. E., and Gould-Stewart, S.: A stratospheric fountain?, J. Atmos. Sci. 38, 2789–2796, 1981. Nordeng, T. E.: Extended versions of the convective parameterization scheme at ECMWF and their impact on the mean and transient activity of the model in the tropics, ECMWF Technical Report, 206, Reading, UK, 1994. Norton, W. A.: Longwave heating of the tropical lower stratosphere, Geophys. Res. Lett., 28, 3653–3656, 2001. Peter, T. Luo, B. P., Wirth, M, Kiemle, C., Flentje, H., Yushkov, V. A., Khattatov, V., Rudakov, V., Thomas, A., Borrmann, S., Toci, G., Mazzinghi, P., Beuermann, J., Schiller, C., Cairo, F, Di Donfrancesco, G., Adriani, A., Volk, C. M., Ström, J., Noone, K., Mitev, V., MacKenzie, R. A., Carslaw, K. S., Trautmann, T., Santacesaria, V., and Stefanutti, L.: Ultrathin Tropical Tropopause Clouds (UTTCs): I. Cloud morphology and occurrence, Atmos. Chem. Phys., 3, 1083–1091, 2003. Plumb, R. A. and Eluszkiewiwicz, J.: The Brewer-Dobson circulation: dynamics of the tropical upwelling, J. Atmos. Sci., 56, 868–890, 1999. Potter, B. E. and Holton, J. R.: The role of monsoon convection in the dehydration of the lower tropical stratosphere, J. Atmos. Sci., 52, 1034–1050, 1995. Randel, W. J. and Wu, F.: Isolation of the ozone QBO in SAGE II data by singular-value decomposition, J. Atmos. Sci., 53, 2546–2559, 1996. Randel, W. J., Zawodny, J. M., and Oltmans, S. J. : Seasonal variation of water vapor in the lower stratosphere observed in Halogen Occultation Experiment data, J. Geophys. Res., 106(D13), 14 313–14 326, doi:10.1029/2001JD900048, 2001. Randel, W. J., Wu, F., Oltmans, S. J., Rosenlof, K., and Nedoluha G. E.: Interannual changes of stratospheric water vapor and correlations with tropical tropopause temperatures, J. Atmos. Sci., 61, 2133–2148, 2004. Randel, W. J., Wu, F., Vömel, H., Nedoluha, G. E., and Forster, P.: Decreases in stratsopheric water vapor after 2001: Links to changes in the tropical tropopause and the Brewer-Dobson circulation, J. Geophys. Res., 111, D12312, doi:10.1029/2005JD0006744, 2006. Reithmeier, C. and Sausen, R.: ATTILA – Atmospheric Tracer Transport in a Lagrangian model, Tellus, 54, 278–299, 2002. Remsberg, E. E., Bhatt, P. P., and Russell III, J. M.: Estimates of the water vapor budget of the stratosphere from UARS HALOE data, J. Geophys. Res., 101(D3), 6749–6766, 1996. Roeckner, E., Bäuml, G., Bonaventura, L., Brokopf, R., Esch, M., Giorgetta, M., Hagemann, S., Kirchner, I., Kornblueh, L., Manzini, E., Rhodin, A., Schlese, U., Schulzweida, U., and Tompkins, A.: The atmospheric general circulation model ECHAM5. PART I: Model description, Max Planck Institute for Meteorology, MPI-Report 349, http://www.mpimet.mpg. de/fileadmin/publikationen/Reports/max scirep 349.pdf, 2003. Roeckner, E., Brokopf, R., Esch, M., Giorgetta, M., Hagemann, S., Kornblüh, Manzini, L. E., Schlese, U., and Schulzweida, U.: Sensitivity of simulated climate to horizontal and vertical resolution in the ECHAM5 atmosphere model, J. Climate, 19, 3771–3791, 2006. Rosenlof, K. H., and Holton, J. R.: Estimates of the stratospheric residual circulation using the downward control principle, J. Geophys. Res., 98, 10 465–10 479, 1993. Rosenlof, K. H.: Seasonal cycle of the residual mean meridional circulation in the stratosphere, J. Geophys. Res., 100, 5173–5191, 1995. Russell, J. M. III, Gordley, L. L., Park, J. H., Drayson, S. R., Hesketh, W. D., Cicerone, R. J. Tuck, A. F., Frederick, J. E., Harries, J. E., and Crutzen, P. J.: The Halogen Occultation Experiment, J. Geophys. Res., 98, 10 777–10 798, 1993. Sander, R., Kerkweg, A., Jöckel, P., and Lelieveld, J.: Technical Note: The new comprehensive atmospheric chemistry module MECCA, Atmos. Chem. Phys., 5, 445–450, 2005. Sandu, A. and Sander, R.: Technical Note: Simulating chemical systems in Fortran90 and Matlab with the 5 kinetic preprocessor KPP-2.1, Atmos. Chem. Phys., 6, 187–195, 2006. Seidel, D. J., Ross, R. J., Angell, J. K., and Reid, G. C.: Climatological characteristics of the tropical tropopause as revealed by radiosondes, J. Geophys. Res., 106, 7857–7878, 2001. Sherwood, S. C.: A stratospheric "drain" over the maritime continent, Geophys. Res. Lett., 27, 677–680, 2000. Sherwood, S. C., and A. E. Dessler: A model for transport across the tropical tropopause, J. Atmos. Sci., 58, 765–779, 2001. Soden, B. J.: The impact of tropical convection and cirrus on upper tropospheric humidity: A Lagrangian analysis of satellite measurements, Geophys. Res. Lett., 31, L20104, doi:10.1029/2004GL020980, 2004. Solomon, S.: Stratospheric ozone depletion: A review of concepts and history, Rev. Geophys., 37, 275–316, 1999. Sparling, L. C.: Statistical perspectives on stratospheric transport, Rev. Geophys., 38, 417–436, 2000. Steil, B., Brühl, C., Manzini, E., Crutzen, P., Lelieveld, J., Rasch, P. J., Roeckner, E., and Krüger, K.: A new interactive chemistry climate model. I: Present day climatology and interannual variability of the middle atmosphere using the model and 9 years of HALOE/UARS data, J. Geophys. Res., 108, 4290, doi:10.1029/2002JD002971, 2003. Stohl, A. and Trickl, T. : A textbook example of long-range transport: Simultaneous observation of ozone maxima of stratospheric and North American origin in the free troposphere over Europe, J. Geophys. Res., 104(D23), 30 445–30 462, 1999. Thomas, A., Borrmann, S., Kiemle, C., Cairo, F., Volk, M., Beuermann, J., Lepuchov, B., Santacesaria, V., Matthey, R., Rudakov, V., Yushkov, V., MacKenzie, R. A., and Stefanutti, L.: In situ measurements of background aerosol and subvisible cirrus in the tropical tropopause region, J. Geophys. Res., 107 (D24), 4763, doi:10.1029/2001JD001385, 2002. Tiedtke, M.: A comprehensive mass flux scheme for cumulus parametrization in large-scale models, Mon. Weather Rev., 117, 1779–1800, 1989. Tost, H.: Global Modelling of Cloud, Convection and Precipitation Influences on Trace Gases and Aerosols, Ph.D. thesis, University of Bonn, Germany, 2006. Tost, H., Jöckel, P., Kerkweg, A., Sander, R., and Lelieveld, J.: Technical Note: A new comprehensive SCAVenging submodel for global atmospheric chemistry modelling, Atmos. Chem. Phys., 6, 565–574, 2006a. Tost, H., Jöckel, P., and Lelieveld, J.: Influence of different convection parameterisations in a GCM, Atmos. Chem. Phys., 6, 5475–5493, 2006b. Traub, M. A.: Lagrangian transport evaluation of atmospheric chemistry in the Mediterranean region Ph.D. Thesis, University of Mainz, Germany, 2004. Tuck A. F., Baumgardner, D., Chan, K. R., Dye, J. E., et al.: The Brewer-Dobson circulation in the light of high altitude in situ aircraft observations, Q. J. R. Meteorol. Soc., 123, 1–69, 1997. Van Aalst, M. K., van den Broek, M. M. P., Bregman, A., Brühl, C., Steil, B., Toon, G. C. Garcelon, S., Hansford, G. M., Jones, R. L., Gardiner, T. D., Roelofs, G. J., Lelieveld, J., and Crutzen, P. J.: Trace gas transport in the 1999/2000 Actic winter: comparison of nudged GCM runs with observations, Atmos. Chem. Phys., 4, 81–93, 2004. Volk, C. M., Elkins, J. W., Fahey, D. W., Salawitch, R. J., Dutton, G. S., Gilligan, J. M., Proffitt, M. H., Loewenstein, M., Podolske, J. R., Minschwaner, K., Margitan, J. J., and Chan, K. R.: Quantifying transport between the tropical and mid-latutude lower stratosphere, Science, 272, 1763–1768, 1996. Vömel, H., Oltmans, S. J., Johnson, B. J., Hasebe, F., Shiotani, M., Fujiwara, M., Nishi, N., Agama, M., Cornejo, J., Paredes, F., and Enriques, H.: Balloon-borne observation of water vapor and ozone in the tropical upper troposphere and lower stratosphere, J. Geophys. Res., 107, 4210, doi:10.1029/2001JD000707, 2002. Wang, D. Y., von Clarmann, T., Fischer, H., Funke, B., et al.: Validation of stratospheric temperatures measured by Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on Envisat, J. Geophys. Res., 110, D08301, doi:10.1029/2004JD005342, 2005. Webster, C. R. and Heymsfield, A. J.: Water isotope ratios D/H, $^18$O/$^16$O, $^17$O/$^16$O in and out of clouds map dehydration pathways, Science, 302, 1742–1692, 2003. Wernli H., and Bourqui, M.: A Lagrangian "1-year climatology" of (deep) cross-tropopause exchange in the extratropical Northern Hemisphere, J. Geophys. Res., 107 (D2), 4021, doi:10.1029/2001JD000812, 2002. Wild, M. and Roeckner, E.: Radiative fluxes in the ECHAM5 general circulation model, J. Climate, 19, 3792–3809, 2006. Wylie, D. P., and Wang P.-H.: Comparison of cloud frequency data from the high-resolution infrared radiometer sounder and the Stratospheric Aerosol and Gas Experiment II, J. Geophys. Res., 102, 29 893–29 900, 1997. Yulaeva, E., Holton, J. R., and Wallace, J. M.: On the cause of the annual cycle in tropical lower stratospheric temperatures, J. Atmos. Sci., 51, 169–174, 1994.