A new look at the atmospheric water cycle: Isotope measurements from space using SCIAMACHY

The atmospheric physics and chemistry group of IMAU is well-known for the innovative use of isotope measurements in atmospheric research. The head of the group, Prof. Thomas Röckmann, in collaboration with the Netherlands Institute for Space Research (SRON) and various international partners, has now obtained and analyzed the first global observations of water isotopes in the lowest part of our atmosphere. The new measurements from the satellite instrument SCIAMACHY provide new insights into the hydrological cycle that are expected to further improve climate models. The researchers published their results this week in Science.

Water is vital to the Earth's climate system. As a vapour, it is the strongest greenhouse gas and as precipitation it makes our planet habitable. When water evaporates from the Earth's oceans and surface, moves through the atmosphere and falls back as rain, evaporation and condensation processes change the relative amount of heavy water, HDO. Therefore, the isotopic composition contains information about the history of water. SRON scientists used the SCIAMACHY instrument onboard the European research satellite ENVISAT to provide a global view on the isotopic composition of water in the atmospheric vapour. These are the first global isotope measurements with high sensitivity towards the lowest layers of the atmosphere, where most of the water vapour resides.

The researchers focussed on a comparison with atmospheric General Circulation Models in two regions, the Sahel zone and the high arctic around Spitsbergen. In both cases current models cannot fully reproduce the measurements. This points to misrepresentations of both the tropical-subtropical hydrological cycle and the water transport to Arctic latitudes.

Satellite data bear the potential to rigorously test and subsequently improve the hydrological cycle in climate models. Eventually this will improve predictions of potential changes to the hydrological cycle (e.g. drought, precipitation events) in a future warming climate and reconstructions of climates in the past.

The SCIAMACHY measurements are a proof of concept and open up new research opportunities. The future instrument TROPOMI, scheduled to be launched in 2014, will further improve the accuracy of the measurements and, most importantly, their coverage in space and time promising more detailed insights into the global hydrological cycle.