Heavy Water – HDO
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 content of heavy water (HDO). Therefore, the isotopic composition contains information about the history of water. SCIAMACHY’s measurements permit obtaining a global view on the water vapour isotopic composition in the atmosphere (Frankenberg et al. 2009). These are the first global isotope measurements with high sensitivity towards the lowest layers of the atmosphere down to the surface, where most of the water vapour resides. By exploiting the capability of SCIAMACHY to retrieve H2O and its heavier isotopologue HDO, new insights into the hydrological cycle are provided.
Fig. 3-4 presents the global distribution of the water isotope HDO shown as relative abundance of water vapour. High fractions of HDO are found in the tropics and sub-tropics where water evaporates from the oceans and is then transported towards the poles. The relative amount of heavy water in the remaining water vapour will be reduced as the heavy isotope rains out preferentially resulting in lower abundances at higher latitudes. The same occurs when moist air from the oceans travels over the continents as e.g. clearly seen in North America. The satellite data bear the potential to rigorously test and subsequently improve the description of such cycles in climate models. This will eventually even result in better predictions of the changes in the hydrological processes, e.g. drought and precipitation in a future climate.