Royal Netherlands Meteorological Institute

 

Dr. Peter F.J. van Velthoven
Research into stratosphere-troposphere exchange (STE) and the Brewer-Dobson circulation
STE and the tropopause
Stratosphere-troposphere exchange is the transport of air and atmospheric constituents across the tropopause.

The tropopause is by definition the surface that forms the boundary between the troposphere and the stratosphere. In the troposphere the atmospheric stability is much smaller than in the stratosphere and therefore vertical (cross-isentropic) mixing is much more rapid in the troposphere than in the stratosphere. The tropopause thus acts as a barrier to upward transport of air and pollutants, in the sense that vertical transport above it is slower than below it.

Nowadays, it is recognized that the tropopause is a transition region of finite thickness (a few km thick) rather than an infinitesimally thin surface (e.g. Lelieveld et al, 1997).

Quite recently, it has been found that there is often a thin inversion layer at the tropopause (TIL) (Birner et al., 2002). This constitutes a real barrier to vertical transport, that can only be overcome by air parcels with excess buoyancy.

It has also been found that in the tropics most of the deep convection does not reach the tropopause. In the tropical transition layer (TTL), above the average level of neutral buoyancy of the tropical convection at about 14km, there is slow net upward ascent (Folkins, 1999).

Apart from the traditionnal definition by WMO of the thermal tropopause, one can also use potential vorticity to define a dynamical tropopause, or tracers such as ozone (O3) and carbon monoxide (CO) to define a chemical tropopause (Zahn and Brenninkmeijer, 2003).

In the tropics the height of the tropopause is supposed to be determined by deep convection (Reid and Gage, 1981). In the extratropics baroclinic waves, cyclones and anticyclones, seem to play a role in determining the tropopause height (Held, 1982; Lindzen, 1993).

The transport of air and trace constituents across the tropopause is still not well quantified. Some relatively accurate estimates of STE are based on output from Numerical Weather Prediction Models (NWPs) that assimilate meteorological observations and from (C)GCMs. It is also possible to make estimates of STE on the basis of measurements of trace gases that are (quasi-)conserved in the tropopause region, such as CFCs.

The Brewer-Dobson circulation
STE is part of the general circulation of the atmosphere that transports air upward in the tropics (Brewer, 1949), poleward in the stratosphere, and downward at midlatitudes and in polar regions (the Brewer-Dobson circulation). This circulation is driven by forces induced by extratropical disturbances (e.g. planetary and gravity waves) in the middle and upper stratosphere. This is called the extratropical wave pump (Holton et al., 1995). The strength and structure of the Brewer-Dobson circulation in models can be diagnosed by calculating the age-of-air distribution (Hall and Plumb, 1994). Traditionally this is done by simulating a tracer with a source function in the form of a pulse or step function in the tropical lower tropopshere (Hall and Plumb, 1994), but one can also use trajectories (Scheele et al., 2005). Such calculations showed that most weather forecast, (chemistry) climate and chemistry transport models tend to underestimate the age-of-air. This can be due to the use of an advection scheme that does not conserve air mass or to improper processing of the meteorological fields (Bregman et al., 2003). Ideally, trajectory-based calculations of the age-of-air should not suffer from this problem (Schoeberl et al, 2003). However, the assimilation of observations can adversely affect the age-of-air in meteorological (re-)analyses and in CTMs that use these meteorological fields to drive their transport (Meijer et al., 2004). As a consequence the influx of ozone into the tropopshere in chemistry-transport models can become much too large (Van Noije et al, 2004) and needs to be corrected by applying a suitable boundary condition (Van Noije et al, 2006). It has been shown that more recent weather forecast model versions and reanalyses (e.g. ERA-interim) perform better in this respect (Monge-Sanz et al., 2007).

Small scale processes
At the base of the downward branch of the Brewer-Dobson circulation in the extratropics, net irreversible transport from the stratosphere to the troposphere across the tropopause takes place through a multitude of processes :
  • Tropopause folding (Danielsen, 1968)
  • Isentropic cross-tropopause mixing (Chen, 1995). in general this will be reversible and includes tropopause folding.
  • The seasonal vertical displacement of the tropopause (Reiter, 1975; Appenzeller et al, 1996). This is affected by vertical advection (Brewer-Dobson), mixing by extratropical waves, radiative processes, and convective erosion.
  • Turbulent mixing near upper level fronts (tropopause folds) and jet streams (Shapiro, 1976, 1980)
  • Mixing by overshooting cloud turrets (and downdrafts) in cut-off lows (Price and Vaughan, 1993) and deep convective clouds (Danielsen, 1993).
  • Differential radiative heating, e.g. in anti-cyclones (e.g. Zierl and Wirth, 1997)
  • Irreversible mixing by breaking planetary and gravity waves (Danielsen et al., 1991)

In the initial decades of STE research, attention focussed on identifying the different mechanisms. Their relative importance has still not been fully quantified, although the seasonal variation of the tropopause height (incl. radiative changes), isentropic mixing incl. tropopause folds, and deep convection seem to be most important. Many of these processes manifest themselves during the life-cycles of extratropical cyclones and near upper level fronts and jet streams.
Relevance
The quantification of the transport of stratospheric ozone into the troposphere is essential to our understanding of the tropospheric ozone budget and to quantification of the impact of anthropogenic emissions of non-CO2 greenhouse gas percursors on climate. The understanding of the processes that transport water vapour from the tropical troposphere into the stratosphere is essential for the correct modelling of stratospheric water vapour and ozone depleting compounds i.e. the possible recovery of the ozone layer. Also, the residence time of aircraft emissions depends on whether they are emitted in the troposphere or in the stratosphere, and on the intensity of mixing across the tropopause.
Our work
In the past, we have studied STE by performing flux calculations, as suggested by (Wei, 1987), using meteorological analyses from ECMWF (Siegmund and Van Velthoven, 1996; Meloen et al., 2003) and by simulating air and ozone transport with the TM atmospheric chemistry-transport model (Van Velthoven and Kelder, 1996; Van Noije et al., 2004, 2006; Meijer et al., 2005). We have calculated the age of air also with our trajectory model (Scheele et al, 2005). Finally, we have studied the signatures of STE in observations obtained during dedicated measurement campaigns with instrumented aircraft (e.g. Lelieveld et al., 1997; Ovarlez et al., 1999), and from ozone soundings (e.g. Zachariasse et al., 2001).
(Historical) references
  • Evidence for a world circulation provided by the measurements of helium and water vapour distribution in the stratosphere. A.W. Brewer, Q.J. Roy. Meteor.Soc. 75, 351-363 (1949).
  • Evaluation of the potential vorticity changes near the tropopause and the related vertical motions, vertical advection of vorticity, and transfer of radioactive debris from stratosphere to troposphere. D.O. Staley, J.Meteor. 17, 591- 620 (1960).
  • Stratospheric-Tropospheric Exchange Based on Radioactivity, Ozone and Potential Vorticity. E.F. Danielsen, J.Atmos.Sci. 25, 502- 518 (1968).
  • Stratospheric-Tropospheric Exchange Processes. E.D.Reiter, Rev.Geophys.Space Phys. 13, 459-473 (1975).
  • The role of Turbulent Heat Flux in the Generation of Potential Vorticity in the Vicinity of Upper-Level Jet Stream Systems M.A. Shapiro, Mon.Weath.Rev. 102, 892-906 (1976).
  • Turbulent Mixing within Tropopause Folds as a Mechanism for the Exchange of Chemical Constituents between the Stratosphere and Troposphere. M.A. Shapiro, J. Atmos. Sci. 37, 994-1004 (1980).
  • On the annual variation in the height of the tropical tropopause. G.C. Reid, and K.S. Gage, J.Atmos.Sci. 38, 1928-1938 (1981).
  • On the height of the tropopause and the static stability of the troposphere. I.M. Held, J.Atmos.Sci. 39, 412-417 (1982).
  • A New Formulation of the Exchange of Mass and Trace Constituents between the Stratosphere and Troposphere. Ming-Ying Wei, J.Atmos.Sci. 44, 3079-3086 (1987).
  • Irreversible transport in the stratosphere by internal waves of short vertical wavelength. E. F. Danielsen, R.S. Hipskind, W.L.Starr, J.F. Vedder, S.E. Gaines, D. Kley and K.K. Kelly, J. Geophys.Res. 96, 17,433-17,452 (1991).
  • In Situ Evidence of Rapid, Vertical, Irreversible Transport of Lower Tropospheric Air Into the Lower Tropical Stratosphere By Convective Cloud Turrets and by Larger-Scale Upwelling in Tropical Cyclones. E. Danielsen, J. Geophys. Res., 98, 8665-8681 (1993).
  • Baroclinic neutrality and the tropopause. R.S. Lindzen, J.Atmos.Sci. 50, 1148-1151 (1993).
  • The potential for stratosphere- troposphere exchange in cut-off low systems. J.D. Price and G. Vaughan, Q.J.Roy.Meteor.Soc. 119, 343-365 (1993).
  • Age as diagnostic of stratospheric transport. T. M. Hall and R. A. Plumb, J. Geophys. Res., 99, 1059-1070 (1994).
  • Stratosphere-troposphere exchange. Holton et al., Rev. Geophys. 33, 403-439 (1995).
  • Isentropic cross-tropopause mass exchange in the extratropics. P. Chen, J. Geophys. Res. 16661-16673 (1995).
  • Seasonal variation of mass transport across the tropopause. C. Appenzeller, J. R. Holton and K. Rosenlof, J. Geophys. Res., 101, 15 (1996).
  • The influence of radiation on tropopause behavior and stratosphere-troposphere exchange in an upper tropospheric anticyclone. B. Zierl and V. Wirth, J.Geophys.Res. 102, 23,883-23,894 (1997).
  • A barrier to vertical mixing at 14 km in the tropics: Evidence from ozonesondes and aircraft measurements. I. Folkins, M. Loewenstein, J. Podolske, S.Oltmans and M. Proffitt, J. Geophys. Res., 104, 22,095 (1999).
  • How sharp is the tropopause at midlatitudes? T. Birner, A. Dornbrack, and U. Schumann , Geophys. Res. Lett., 29, 1700 (2002).
  • New directions: a chemical tropopause defined. A. Zahn, and C.A.M. Brenninkmeijer, Atmos. Environ., 37, 439-440 (2003).
  • A comparison of the lower stratospheric age spectra derived from a general circulation model and two data assimilation systems. M. R. Schoeberl, A. R. Douglass, Z. Zhu, and S. Pawson, J. Geophys. Res., 108, 4113 (2003).
  • Mean age of air and transport in a CTM: Comparison of different ECMWF analyses. B. M. Monge-Sanz, M. P. Chipperfield, A. J. Simmons, S. M. Uppala, Geophys. Res. Lett., 34 (2007).
Some of our own publications:
  • Estimates of Stratosphere-Troposphere Exchange: Sensitivity to Model Formulation and Horizontal Resolution. P.F.J. van Velthoven and H. Kelder, J.Geophys.Res. 101, 1429-1434 (1996). (abstract)
  • Cross-tropopause transport in the extratropical northern winter hemisphere, diagnosed from high-resolution ECMWF data. P.C. Siegmund, P.F.J. van Velthoven and H. Kelder, Quart.J.Roy.Meteo.Soc. 122, 1921-1942 (1996). (abstract)
  • Chemical perturbation of the lowermost stratosphere through exchange with the troposphere. J. Lelieveld, B. Bregman, F. Arnold, V. Burger, P. J. Crutzen, H. Fischer, A. Waibel, P. Siegmund and P. F. J. van Velthoven, Geophys. Res. Lett. 24, 603-606 (1998).
  • Water vapor measurements from the troposphere to the lowermost stratosphere: Some signatures of troposphere to stratosphere exchanges. J. Ovarlez, P.F.J. van Velthoven, and H. Schlager, J.Geophys.Res., 104, 16,973-16,978 (1999). (abstract)
  • Cross-tropopause and interhemispheric transports into the free troposphere over the Indian Ocean. M. Zachariasse, H.G.J. Smit, P.F.J. van Velthoven and H. Kelder, J.Geophys.Res. 106, 28,441-28,452 (2001).
  • Stratosphere-troposphere exchange: a model and method intercomparison. J. Meloen, P. Siegmund, P. van Velthoven, H. Kelder, M. Sprenger, H. Wernli, A. Kentarchos, G. Roelofs, J. Feichter, C. Land, C. Forster, P. James, A. Stohl, W. Collins and P. Cristofanelli, J.Geophys.Res., 108, D12 (2003).
  • On the use of mass-conserving wind fields in chemistry-transport models. B. Bregman, A. Segers, M. Krol, E. Meijer and P. van Velthoven, Atmos. Chem. Phys., 3, 447-457 (2003).
  • The influence of data assimilation on the age of air calculated with a global chemistry-transport model using ECMWF wind fields. E. W. Meijer, E. W., B. Bregman, A. Segers and P. F. J. van Velthoven, Geophys. Res. Lett., 31, L23114 (2004).
  • Implications of the enhanced Brewer-Dobson circulation in European Centre for Medium-Range Weather Forecasts reanalysis ERA-40 for the stratosphere-troposphere exchange of ozone in global chemistry transport models. Noije, T.P.C. van, H.J. Eskes, M. van Weele and P.F.J. van Velthoven, J. Geophys. Res., 2004, 109, D19308 (2004).
  • Stratospheric age of air computed with trajectories based on various 3D-Var and 4D-Var data sets. R. Scheele, R., P. Siegmund, and P. van Velthoven, Atmos.Chem.Phys. 5, 1-7 (2005).
  • Time series of the stratosphere-troposphere exchange of ozone simulated with reanalyzed and operational forecast data. T. P. C. van Noije, A. J. Segers, and P. F. J. van Velthoven, J. Geophys. Res., 111, D03301 (2006).
 


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