ARA - Atmospheric Radiation Analysis

Research themes

Our planet is exposed to important natural variations resulting from numerous complex, internal, or external processes. These variations are at the basis of the natural evolution of the Earth climate. Since the late 1880s, human activities add to natural influences that have been present over Earth's history. Scientific evidence strongly indicates that natural influences cannot explain the rapid increase in global near-surface temperatures observed since the second half of the 20^th century.

Ongoing climate research largely relies on an intricate coupling between modelling, which tends to account for an increasing number of processes and their interaction, and observation, which allows complex mechanisms and their parameterization to be studied in details. In particular, space borne observing systems bring global scale data essential to the evaluation of model results and of the assumptions they carry.

Interpretation of space measured radiative fluxes in terms of atmospheric thermodynamics and chemical variables is, in general, extremely complex and mixes numerous scientific domains as: quantum mechanics, from which emission and absorption spectra may be described; the forward modelling of radiative transfer, which allows radiance space measurements to be expressed in terms of the sate of the atmosphere; inverse problem theory and its application to the inversion of the radiative transfer equation; last, statistical methods and tools which open the way to interpreting huge data bases and analysing results of long-term time series.

The Atmospheric Radiation Analysis (ARA) team of the Laboratoire de Météorologie Dynamique (LMD) is one of the few groups in the world to gather all these theoretical skills. This has allowed developing a complete chain designed for processing satellite data at global scale on which rely different research activities aiming at improving our knowledge of the climate variability and evolution.

Research activities of the team focus on numerous space borne sensors (vertical and limb sounders or imagers) carrying out observations in the infrared or in the microwaves. The list of retrieved variables is long: temperature and water vapour vertical profiles, cloud characteristics (cloud cover, physical and microphysical properties), distribution of major greenhouse gases (in particular CO2 and CH4) in the troposphere and in the stratosphere , surface characteristics (temperature, spectral emissivity, particularly over land), and aerosol properties (infrared optical depth, altitude, particle size).

The ARA team was selected, in the early 1990s, by NOAA and NASA to reanalyse space observations made by the 1^st generation sounders TIROS-N Operational Vertical Sounder (TOVS) on board the NOAA polar satellites since 1979 (Pathfinder project). The ARA team also directly participates in the analysis of second generation high spectral resolution instruments as the Atmospheric Infrared sounder (AIRS), launched on board the NASA/Aqua platform in May 2002, the Infrared Atmospheric Sounder Interferometer (IASI), launched on board the European MetOp platform in October 2006, or the Atmospheric Chemistry Experiment - Fourier Transform Interferometer- (ACE-FTS), launched on board the Canadian SCISAT platform in August 2003, whose very high spectral resolution and continuous spectral coverage offer an unique opportunity of a major step forward in our knowledge of climate processes. The team is also involved in the design of the next generation instruments, such as the IASI-NG mission which will be part of the EUMETSAT post-EPS system. However, the large increase in the number of spectral channels measured enhances the complexity of associated analysis methods and softwares. The ARA team, owing to its expertise in the development of processing and analysis tools and through its main research themes has the necessary skills to overcome the present challenges.

These researches are conducted in tight cooperation with national and international research teams, in particular through European (EU, Eumetsat, ESA) or national (CNES, INSU, ONERA) contracts. They also participate in the European initiative GMES (Global Monitoring for Environment and Security).