WIFT: Water vapour Isotopologues From TROPOMI

Living Planet Fellowship research project carried out by Andreas Schneider.

The role of atmospheric water vapour in the hydrological cycle, the atmospheric circulation, and the radiation and energy budgets is largely uncertain. Improving knowledge on these is one of the key challenges in atmospheric sciences and of great importance for projections of climate change. Measurements of water isotopologues provide information about the history of a sampled air parcel due to isotopic fractionation during evaporation and condensation and so give significant constraints for the processes involved. Global observations are especially useful for constraining general circulation models, but to date no satellite data set with high sensitivity in the lowermost troposphere (where most water vapour resides) and decent spatial and temporal resolution and data quality is available. The new Tropospheric Monitoring Instrument (TROPOMI) aboard the Sentinel 5 Precursor satellite is expected to be a ‘game-changer’, as it measures sunlight reflected by Earth’s atmosphere in the shortwave infrared spectral range with unprecedented spatial resolution up to 7 km × 7 km, daily global coverage, and high radiometric performance. The subject of this project is to exploit these measurements to retrieve the water vapour isotopologues H216O, HDO and, if possible, H218O. To this end, the SICOR retrieval algorithm suite, which has high software maturity, is employed. After setting up a processing pipeline at a high performance computing infrastructure, results will be validated against ground-based observations from the Total Carbon Column Observing Network (TCCON). Should recent data from the Multi-platform remote Sensing of Isotopologues for investigating the Cycle of Atmospheric water (MUSICA) project become available, these will also be used for validation. The influence of state-of-the-art molecular water vapour spectroscopy on the data product will be investigated. Moreover, the feasibility to infer the H218O isotopologue from TROPOMI measurements will be assessed. In this context, the spectral window will be optimised to minimise cross-dependencies between different isotopologues. Finally, the maturity and use of the new data product will be demonstrated. First, it will be compared to simulations with an isotope-enable global circulation model. The topic of the following investigation depends on the outcome of the feasibility study. In case the deuterium excess parameter can be obtained with sufficient accuracy, water vapour originating from combustion shall be studied on city scale. Alternatively, the role of evapotranspiration on the isotopic composition (HDO/H216O) in dependence of land usage will be examined. The outcome of this project will be an additional mature data product in ESA’s portfolio from the Sentinel missions.