Study of the hydrological cycle using satellite-borne isotopologue measurements and model results

The circulation of water on Earth that describes the continuous movement of water from the land to the atmosphere and back again is called the hydrological cycle. The transport of water vapor through the hydrological cycle is crucial for life on Earth and for our climate. In the atmosphere, water vapor is transported over large distances and undergoes often several phase changes, from evaporation to condensation, before returning to the ocean. These processes leave a characteristic imprint on the isotopic composition of atmospheric water vapor and the corresponding precipitation. Over the last-decade, global scale measurements of atmospheric water vapor isotopologues (HDO/H2O) have become available from different remote-sensing instruments that operate on satellites and overcome sampling difficulties of water vapor. The main objective of this thesis, therefore, is to investigate how these measurements and related modeling of water vapor isotopologues can provide new insight into the hydrological cycle. This thesis uses mainly two satellite datasets, from the Tropospheric Emission Spectrometer (TES) and the SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) and results from the isotope-enabled General Circulation Model (GCM) ECHAM4. Related datasets such as precipitation, isotopic composition of precipitation from the Global Network of Isotopes in Precipitation (GNIP) network, the Niño-3 index, and results from other iso-GCMs are also used. Analyses have been carried out using monthly averaged data both for the satellite HDO/H2O measurements and the model results. Using these data, a number of interesting conclusions of the hydrological cycle have been obtained. 1) Transpiration is the largest contributor to the evaporation fluxes from continental area and the results from both isotope and non-isotope based methods are consistent. 2) A number of established isotope effects on the global-scale as well as in the tropics, linked to the movement of InterTropical Convergence Zone (ITCZ), are observed in the TES version 5, SCIAMACHY and the ECHAM model, demonstrating the improvement of the new version of TES dataset (version 5). 3) The isotopic composition of water vapor is a good climatic indicator for drought and flooding events and performs even better than the Niño-3 index. 4) For the lower atmosphere, our results show that rainout processes, less rain re-evaporation of falling droplets, and increase of convective updrafts and diffusive exchange within the convective systems, play an important role in producing the “isotope amount effect” during ENSO events. Simultaneously, convective updrafts control the water vapor at higher altitudes. 5) The regions with intense mixing and strong convection are marked by a flatter isotopic gradient. However, a model intercomparison does not show a similar relation: models simulating steeper or flatter isotopic gradients are not necessarily marked by weaker or stronger mixing (or smaller/larger ECS). The results of this study show that the measurement of water isotopologues from satellite-borne instruments in conjunction with GCM models provide interesting new information when studying the land-surface-, and atmospheric hydrological cycle