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New Insights into Upward Transport across the Extratropical Tropopause derived from Extensive in situ Measurements during the SPURT Project

Peter Hoor, MPI for Chemistry, Mainz, Germany (hoor@iac.umnw.ethz.ch)

H. Bönisch3, D. Brunner2, A. Engel3, H. Fischer1, C. Gurk1, G. Günther4, M. Hegglin2, M. Krebsbach4, R. Maser5, Th. Peter2, C. Schiller4, U. Schmidt3, N. Spelten4, and H. Wernli2,6, V. Wirth6

1Max Planck - Institute for Chemistry, Dept. of Air Chemistry, Mainz, Germany
2Institute for Atmospheric and Climate Science, Swiss Federal Institute of Technology, Zürich, Switzerland
3Institute for Meteorology and Geophysics, Johann Wolfgang Goethe - University, Frankfurt am Main, Germany
4Institute of Chemistry and Dynamics of the Geosphere, ICG-1, Research Center Jülich, Germany
5Enviscope GmbH, Frankfurt am Main, Germany
6Institute for Atmospheric Physics, University of Mainz, Germany

Introduction

Long-term observations from 1970 to 2000 indicate a substantial ozone decrease in the northern mid latitude stratosphere accompanied by a negative temperature trend [WMO, 2003]. Changes in ozone in the lowermost stratosphere where it acts as an efficient greenhouse gas and participates in local chemistry are of particular importance, but difficult to detect due to the high degree of dynamical and chemical variability in this region. In particular, troposphere-to-stratosphere transport (TST) in the extratropics involving diabatic processes (e.g. radiative processes associated with the decay of anticyclones, turbulent mixing in the vicinity of the jets, convection) perturb local chemistry with feedbacks on temperature or ozone budget.
Model studies indicate that TST occurs throughout the year below the isentropic surface defined by a potential temperature of Q = 345 K, predominantly to the north of 50°N with a relatively weak zonal variability [Sprenger and Wernli, 2003]. During summer a secondary maximum of TST occurs at low latitudes at Q = 360 K, which is partly associated with weaker PV gradients at the subtropical tropopause in summer [Haynes and Shuckburgh, 2000].

The effect of TST and subsequent mixing was identified by in situ trace gas measurements [e.g. Danielsen, 1968; Dessler et al.1995; Fischer et al., 2000; Ray et al. 1999], which however were too limited to allow conclusions about the overall effect of these individual processes on the lowermost stratosphere. In particular, relatively little is known about the spatial extent, which is affected by extratropical TST and subsequent mixing within the lowermost stratosphere.

The SPURT Measurement Strategy

To address these issues an improved measurement strategy was developed for the project SPURT (SPURenstofftransport in der Tropopausenregion, Trace Gas Transport in the Tropopause Region), which has been conducted as part of the German atmospheric research program AFO 2000 funded by BMBF (German Federal Ministry of Education and Research). During SPURT airborne in situ measurements of dynamical trace gases were performed on a regular basis to obtain an overview on their spatial distribution in the UT/LS-region in all four seasons and over a broad latitude range. A Lear Jet 35 based in Hohn (Germany, 52°N/6°E) was used as the measurement platform. The area investigated covered the tropopause region up to 13.7 km from the southwestern tip of Europe to polar latitudes (Figure 1). In total, 160 flight hours were spent on eight measurement campaigns over a time period of three years.

Figure 1. Flight itineraries and dates for the eight missions performed during the SPURT-project.

A highly modular set of in situ instruments was developed to keep preparation time for each individual campaign short and to maximize a high reliability for all the instruments. The whole set of observed species is given in Table 1. The trace gas measurements were supplemented by measurements of several meteorological parameters, such as temperature, pressure and horizontal wind components.

Table 1: In situ techniques combined in the payload employed during the SPURT missions

Species
Technique
Time resolution
Total uncertainties, 1 s-level
Institute
CO, N2O, CH4 TDLAS
5 s
1.5%, 1.5%, 2.5%
MPI-Mainz
CO2 NDIR
1 s
0.2 ppm
MPI-Mainz
O3 UV-absorption
9 s
5%
FZ Jülich
H2O Lyman-a fluorescence
1 s
6%
FZ Jülich
N2O, F12, SF6, H2 in situ GC
75 s
1%, 1%, 1%, 2%
University Frankfurt/Main
O3, NO, NOy CLD, gold catalyst (NOy)
1 s
5%, 8%, 13%
ETH-Zürich

Typically, a campaign was performed over five days including three days for technical integration of the combined payload and for ground tests and two successive days for the measurements, of which one day was dedicated to the lowermost stratosphere over southern Europe followed by a day with flights heading to polar latitudes (Fig. 1). Each individual flight ideally consisted of two long flight legs, one at tropopause altitude level and one leg high above the tropopause. At the end of each flight a climb to maximum altitude was performed to sample undisturbed stratospheric air, followed by a slow descent to obtain a high resolution vertical profile. The return flight to Germany on the same day mirrored this flight pattern. Flight planning was based on meteorological forecast products provided by the ETH Zürich using operational ECMWF forecasts (60 levels). From the predicted potential vorticity (PV)-fields the location of the tropopause (PV = 2 PVU) was deduced to select the flight levels. Itineraries and flight levels were chosen to cover different tropopause altitudes associated with various meteorological conditions and different types of air masses.

Meteorological post-flight analyses included the calculation of ten-day backward trajectories, which were initialised every ten seconds along the flight track at the exact location and time of the aircraft.

Measurements and Results

During SPURT the influence of extratropical cross tropopause mixing was investigated using in situ measurements of different species and subsequent model studies. In this article we focus on the spatial distribution of carbon monoxide, CO. Tropospheric sources of CO are mainly combustion processes and the oxidation of hydrocarbons leading to average tropospheric mixing ratios ranging from 70 ppbv in the tropics to 130 ppbv in the Northern Hemisphere extratropics. In the stratosphere the major photochemical source is oxidation of methane (CH4), which is rather slow compared to CO degradation. The photochemical lifetime of CO in the lowermost stratosphere is on the order of three months. Thus, in the stratosphere CO mixing ratios of 10 - 15 ppbv would be expected for photochemical steady state, if no additional transport and subsequent mixing of tropospheric air occurs. Therefore, CO is an ideal tracer to investigate TST and subsequent mixing on time scales of days to weeks in the lowermost stratosphere, as any excess above the equilibrium value must stem from the troposphere.

Meridional advection of (sub-)tropical tropospheric air or southward excursions of stratospheric air lead to strong displacements of the local tropopause from its climatological mean. Equivalent latitude, jeq accounts for these deviations as long as PV is conserved, i.e. under adiabatic conditions [Strahan et al., 1999]. It transforms the undulating PV-contours on a given isentropic surface to the arc of the equal area circle resulting in a tropopause-following coordinate system. Thus, lower stratospheric trace gas distributions in jeq-Q coordinates are displayed according to the distance from the local tropopause. Note that no averaging has to be applied for calculating jeq and that the transition from PV to equivalent latitude on a given isentrope is unique. Extratropical TST requires an air parcel to increase its PV or jeq, conserved quantities unless diabatic processes occur, such as mixing, radiative heating or the release of latent heat.

As evident in Figure 2, measured CO distributions in jeq-Q coordinates appear to be rather similar for the whole set of campaigns during SPURT. Highest mixing ratios are found in the troposphere ranging from 75 ppbv to more than 130 ppbv. During winter the tropospheric latitudinal gradient becomes evident, being maximum at high latitudes. Patchy tropospheric structures illustrate the variability of CO and its sources in the troposphere. The lowest CO values between 20 and 30 ppbv were encountered above Q = 370 K and PV-levels exceeding 8 PVU.

Figures 2. CO as a function of equivalent latitude and potential temperature for all SPURT missions. Also given is the location of the tropopause (PV = 2PVU, thick black line) and the PV = 4PVU surface (thin line). Note that CO isopleths just above the tropopause rather follow the tropopause than isentropes, indicating that the influence of extratropical TST is mainly related to the local tropopause than to isentropic surfaces..

The gradient of CO at the tropopause, as well as intermediate CO mixing ratios between upper tropospheric and lowest stratospheric values, indicate that the tropopause is a barrier against TST and subsequent mixing but it is not totally impermeable. In case of rapid mixing of tropospheric air along isentropes within the lowermost stratosphere, one should expect a homogenous isentropic CO distribution (e.g. SPURT 6, Q = 335 K) for the whole lowermost stratosphere. Instead, CO-gradients on isentropic surfaces extend further into the lowermost stratosphere resulting in isopleths of CO that are not parallel to isentropes. The region of the strongest CO decline from upper tropospheric CO values down to 50 ppbv forms a band roughly following the local tropopause indicating that mixing of tropospheric air across the extratropical tropopause is too weak to balance photochemical CO degradation and the diabatic descent of CO-depleted stratospheric background air. Note that this observation is independent of the choice of the PV threshold for the tropopause as a higher PV-value would not affect isentropic CO-gradients (PV = 4 PVU in Fig. 2).

Obviously, a transition region, which is strongly affected by TST and subsequent mixing, becomes established close to the tropopause, exhibiting chemical characteristics both of the troposphere and stratosphere. The isentropic gradient of CO in the lowermost stratosphere indicates that the influence of TST and subsequent mixing decreases with distance from the local tropopause.

However, even at the largest distances from the tropopause (Q > 45 K above the local tropopause) the air cannot be regarded as purely stratospheric. The lowest CO values range from 20 ppbv to 30 ppbv, well above the CO equilibrium value and still indicate a significant tropospheric contribution. A detailed analysis of long-lived trace gases (e.g. CO2 and N2O) reveals that air originating from the tropics may contribute significantly to the trace gas budget of the lowermost stratosphere [Hoor et al., 2003].

Summary

The new operational SPURT measurement strategy facilitated a broad overview of the seasonal and latitudinal trace gas distribution in the tropopause region and lowermost stratosphere over Europe. The results of the airborne in situ observations during the SPURT project illustrate that TST and subsequent mixing significantly alter the chemical composition in a narrow band above the local tropopause. Isentropic CO gradients mark a transition region, which follows the tropopause and indicate that the influence of extratropical tropospheric air is a function of distance from the local tropopause rather than potential temperature Q.

The new seasonally resolved SPURT dataset furthermore provides the possibility to perform detailed process-oriented case studies on TST [e.g. Hegglin et al., 2003]. Ongoing work investigates TST on global scales, including the determination of lag times (using SF6 and CO2 measurements) and condensation processes occurring at the tropopause (based on the observations of H2O) in combination with related models and theoretical studies.

Acknowledgements

Without the excellent support by the company GFD (Gesellschaft für Flugzieldarstellung) in cooperation with the company enviscope in operating the Lear Jet 35, SPURT would not have been possible. We are grateful to the German Ministry for Education and Research and to the Swiss National Fond for financial support.

References

Danielsen; E.F., Stratospheric-tropospheric exchange based upon radioactivity, ozone and potential vorticity, J. Atmos. Sci, 25, 502-518, 1968.

Dessler, A.E., et al., Mechanisms controlling water vapour in the lower stratosphere: "A tale of two stratospheres”, J. Geophys. Res., 100, 23167-23172, 1995.

Fischer, H., et al., Tracer correlations in the northern high latitude lowermost stratosphere: Influence of cross-tropopause mass exchange, Geophys. Res. Lett., 27, 97 – 100, 2000.

Haynes, P. and E. Shuckburgh, Effective diffusivity as a diagnostic of atmospheric transport, 2., Troposphere and lower stratosphere, J. Geophys. Res., 105, 22795-22810, 2000.

Hegglin, M.I., et al., Tracing troposphere to stratosphere transport within a mid-latitude deep convective system, ACPD, submitted, 2003.

Hoor, P., et al., The extent of cross tropopause mixing in the extratropical lowermost stratosphere, ACPD, 2003.

Ray, E.A., et al., Transport into the Northern Hemisphere lowermost stratosphere revealed by in situ tracer measurements, J. Geophys. Res., 104, 26562-26580, 1999.

Sprenger M. and H. Wernli, A northern hemispheric climatology of cross-tropopause exchange for the ERA15 time period (1979-1993), J. Geophys. Res., 108, doi:101029/2002JD002636, 2003.

Strahan, S. et al., Climatology and small scale structure of lower stratospheric N2O based on in situ observations, J. Geophys. Res., 104, 30463 -30480, 1999.

WMO (World Meteorological Organization), Scientific Assessment of Ozone Depletion: 2002, Global Ozone Research and Monitoring Project – Report N°. 47, World Meteorological Organization, Geneva, 2003.

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