Scientific context

The radiative budget evolution of the stratosphere (layer above 10 km) is not properly taken into account in the recent projections of climate change from IPCC (2007). This issue has been addressed in the paper of Baldwin et al. (Nature, 2007) entitled "How will the stratosphere affect climate change?" The radiative budget of the stratosphere is controlled by ozone, greenhouse gases and aerosol abundance and depends on the latitude and season. For example its changes have already modified the wind distribution and then the surface temperature in the Antarctic region but "the mechanisms by which stratospheric circulation changes are communicated to the surface are not well understood" (Forster et al., JGR, 2005). An important scientific question concerning the arctic stratosphere is "how chemical and aerosol constituents change and how this relates to dynamics" (SPARC, 2008). One of the key parameters to quantify stratosphere-climate interactions is the ozone budget.

The ozone budget at high-latitudes critically depends on the balance between chemical and dynamical processes. The chemistry is governed through complex pathways (photochemistry, gaseous and heterogeneous chemistry) depending on the abundances of nitrogen and halogenated (chlorine and bromine) species. Dynamics are mostly driven by the Brewer-Dobson circulation; slow ascent of air masses in tropical regions, subsequent poleward transport and subsidence at mid-latitudes and in the polar region. However, some observations have revealed potential acceleration of the large scale circulation whereas recent study of Engel et al. (Nature Geoscience, 2009) shows rather a deceleration. This question is an opened question in the international scientific community.

Given the dependency of ozone on the chemistry of the atmosphere, it is of crucial importance to understand how ozone interacts with other chemical compounds, specifically in winter vortex conditions, and how the chemical composition evolves during and after the vortex breakdown in springtime. In the polar winter air masses are isolated in the polar vortex, leading to a specific chemistry. However, when the vortex breaks down, the polar chemical composition is modified when air from lower latitudes mixes in. It is therefore mandatory to study dynamical processes in parallel.

The aim of the project is to enrich the existing dataset in the Arctic region in order to follow the evolution and potential modifications of the coupled chemical-dynamical system in climate change context. Particularly, balloon-borne remote sounding instrumentation offers an important ability to sample with high accuracy and high vertical resolution over a large part of the stratosphere (from ~10 km to ~35 km) covering various solar zenith angles. ENRICHED is designed to substantially enhance the data gathered during the EU-funded aircraft campaign RECONCILE (up to about 19 km altitude) that took place in winter 2010 from Kiruna. Long-lived species measured by balloon-borne measurements are also relevant to study and identify dynamical key processes that constrained the summer stratosphere conditions investigated during the STRAPOLETE campaign funded by ANR CNES and IPEV. In addition, the dataset obtained will serve the long-term validation of ESA's ENVISAT (MIPAS, SCIAMACHY and GOMOS instrumentation).

It is proposed to follow the Arctic region from vortex to springtime conditions by balloon-borne observations. The data analysis will first be made by direct comparison of the different geophysical conditions to highlight the main characteristics of the system and its evolution as a function of time. The data set obtained will be enriched by satellite data which offers the global distribution of some species. Model simulations will be performed to establish the spatial and temporal link between the different measurements. A systematic comparison of measurements and modelling results will allow to evaluate the ability of models to represent the evolution of the system. With the aid of sensitivity tests on chemical and dynamical processes within the model, it will be possible to perform detailed process studies and also to improve the model.

The project is organised into four work packages (WPs): Chemistry, Dynamics, Cross comparisons and Satellite validation. The following schematic representation gives a general view of the project, showing the link between each WP and where the measurements occur in the overall plan.

The balloon measurements to be obtained are the core of the project. To investigate chemistry it is necessary to know the dynamical geophysical conditions and the evolution of the dynamical conditions constraining the chemical species abundance and ozone. Balloon instrumentations will thus allow to perform cross comparisons on each payload and to contribute to satellite validation. Satellite measurements will give the spatial coverage evolution needed. All measurements will be integrated to constraint models in order to perform detailed process studies.