Transport of chemical species across the subtropical tropopause

Project Information

Grant agreement ID: ENV4970546

Project closed

Start date 1 December 1997

End date 31 May 2000

Funded under

• Specific programme of research and technological development in the field of environment and climate, 1994-1998

Total cost

No data

EU contribution

No data

Coordinated by

CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE
France

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Deliverables

Climatology of the ozone vertical layering near the tropopause

Layering in the lower stratosphere during winter, pole-ward of the subtropical jet stream was found using ozone sounding stations data, satellite measurements and radiosondes. In the 350-390 K altitude region, the lowermost stratosphere is strongly influenced by tropospheric air especially during winter. Layering of potential vorticity in ECMWF analyses is consistent with the radiosondes, and enables their results to be extended globally. A strong inter-hemispheric difference between the population of layers is found with Northern Hemisphere layers typically 100ppbv higher than those in the Southern Hemisphere. In the Southern Hemisphere layering was also found associated with the Asian Summer Monsoon. More data has been generated by the project by the means of two intensive campaigns in the Indian Ocean in July '1998 and in the Eastern Atlantic in February/April 1999. By comparing the ozone vertical profiles with the data already collected in these areas during the last five years, it was found that in the southern hemisphere biomass burning explains peaks in the ozone distribution in the upper troposphere from September to December, but inputs from the stratosphere strongly control the ozone variability at the tropopause during July and August. Large-scale stratospheric input in the tropical troposphere was observed in the southern hemisphere. No such feature is seen in the Northern Hemisphere, on the contrary, there is a strong evidence of subtropical upper tropospheric ozone, poor air intrusions into the mid-latitude lower stratosphere. In order to identify the dynamical mechanism responsible for the observed layering, the distribution of Rossby-wave breaking (RWB) in the subtropics was calculated by the RAL using isentropic potential vorticity (PV) from the ECMWF analyses, from a free-running GCM and from a transport chemical model to simulate the ozone distribution. At 350K, all the data sets show a maximum of RWB in summer in each hemisphere. RWB is found to maximise in winter above 450K. This is in good agreement with the observed distribution of the layers in the ozone data.

Use of new methods in data meteorological data analysis

Highly idealized Lagrangian advection technique, called Contour Advection with Surgery (CAS) allows one to provide a high resolution isentropic description of the evolution of both subtropical tropopause fold and the stratospheric/tropospheric filament. This technique was applied by Laboratoire d'Aerologie showing a large range of small-scale developments in the two case studies they selected in the MOZAIC data base. It is anticipated that future work providing quantitative estimates of mass transport arising from stratospheric intrusions of the type examined here will build on the above results. This technique was even further improved by CNRS-LMD considering also the non-conservative large change of the PV field. The mapping of the subtropical barrier efficiency limiting the air exchange across the tropopause was investigated by using two diagnostic approaches: (i) the effective diffusivity (ii) the Lyapunov exponent. The effective diffusivity is useful to quantify the efficiency of the diffusion equation to transport a tracer on a given surface. It was shown with this diagnostic that there is a low stirring at 350 K neat the subtropical jet, and an increased efficiency of the exchanges at 330 K (tropopause folding) and also to some extent at 370 K. The Lyapunov exponent characterizes the time needed to increase the fluid particle spatial separation. When looking at stable and unstable particle separation distribution along the subtropical barrier at 330 K and 350 K, one clearly sees: (i) the impact of the convergent flow of the Hadley cell circulation in the winter hemisphere (ii) crossing both ways of the tropopause at 330 K. New tools to calculate the gravity wave adjustment of jets have been developed. These tools predict that while unbalanced motion is usually emitted out of the jet, a trapping of the unbalanced component may occur leading to the long-time coexistence of the jet and the trapped wave. This open the possibility to calculate the effect of the unbalanced motion on transport across the jet using analysed winds, where the unbalanced component is filtered out.

Use of new technology: space observations and new airborne sensors

The observations of the GOME instrument on the ERS-2 platform are also capable of providing useful information for Stratospheric Tropospheric Exchanges (STE) analysis. University of Aberystwyth detected an intrusion of subtropical air into the lower stratosphere over Europe on 7th March 1997, using preliminary GOME ozone profiles provided by the Rutherford Appleton Laboratory, UK (RAL). This innovation in satellite ozone technology used retrievals at 12km altitude and an along-orbit resolution of 4Okm to validate the extent of the intrusion produced in ECMWF analyses. Observations of methane and water vapour from the Halogen Occultation Experiment (HALOE) satellite instrument were used to investigate the prime influence on inter-annual variations of tracer distributions in the subtropics. The two possible mechanisms are isentropic mixing and differential mean up welling. It was found that differential mean up welling was the major influence. We have also demonstrated that a new technology based on laser remote sensing can be easily used on a small size jet aircraft for ozone mapping in the upper troposphere and lower stratosphere.

Model assessment using high resolution observations

The ozone distribution near the subtropical jet stream with an excellent resolution was provided by an airborne radar flown by CNRS-SA onboard the French Mystere-20 aircraft. The mesoscale meteorological model Meso-NH was applied by CNRS-SA with high vertical resolution in order to study the strong layering observed in the upper troposphere and lower stratosphere. This has made the evaluation of the performance of this model in the subtropics possible. An estimation of the ozone transport across the subtropical barrier using this model (shows that the troposphere to stratosphere upward transport is dominant over the downward advection of stratospheric air into the tropical troposphere. The interpretation of MOZAIC data in the subtropical Atlantic has shown that two main types of STE events appear equator-wards of the subtropical barrier: -Subtropical tropopause folds during the boreal winter in the northern tropics (length of ozone events of about one hundred km). -Filamentation associated with baroclinic Rossby- wave breaking (RWB) during the austral summer in the southern tropics (length of ozone events of about a few hundreds km). Analysis of the performance of the model Meso-NH to simulate these events has shown that it improves over the interpretation of ECMWF analyses, the understanding of the STE processes involved in each case. However with a 4O-km horizontal resolution the model diffusion prevents the small-scale developments needed to further quantify STE exchanges and further work with Meso--NH would imply to employ the grid-nesting technique with horizontal resolutions of about a few km.