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Abstract:
Water vapour is one of the most important trace gases in the Earth’s atmosphere and plays a key role in atmospheric processes from a few micrometres up to climate-relevant scales. In this work, the long-term changes (2005-2020) of the global water vapour distribution were investigated using satellite measurements. To this end, first a retrieval algorithm was developed to determine the total vertical column of water vapour (TCWV) in the visible blue spectral range. This spectral range is particularly well suited for TCWV retrievals, as it provides a similar sensitivity over land and ocean surfaces. The retrieval is based on measurements of the novel satellite instrument TROPOMI, which combines an unprecedented signal-to-noise ratio with a high spatial resolution and a daily global coverage. In the new retrieval, for the first time an iterative scheme was implemented, which finds an optimal a priori profile shape of water vapour. Furthermore, an optimised surface albedo was developed. The novel, iterative approach provides reliable results even under high cloud cover and thus also enables the investigation of atmospheric water vapour phenomena (e.g. atmospheric rivers) on a fine spatial scale. Moreover, comparisons with microwave satellites, reanalyses models and GPS measurements show an excellent agreement. This new retrieval was then modified such that it could be applied to the long-term measurement series of the OMI instrument (2005-2020). From these retrieval results the MPIC OMI TCWV climate data record (CDR) was generated. This CDR is unique as it is solely based on one instrument so that cross-calibrations between different sensors are not necessary, but it still provides a global coverage with an almost homogeneous and high surface sensitivity. An extensive validation study with various reference data sets demonstrates a very good agreement over ocean, but also a systematic overestimation over land surfaces. Nevertheless, the CDR proves to be highly temporally stable and is therefore predestined for climate studies. The trend analyses based on the MPIC OMI TCWV CDR revealed on average a global TCWV increase, with almost all local, statistically significant trends being positive. It was shown that the assumption of temporally constant relative humidity is not always fulfilled even over ocean and that indirectly determined precipitation trends do not correspond to the "dry-gets-drier, wet-gets-wetter" paradigm. Furthermore, an increase in the water vapour residence time was found, implying a slow down of the atmospheric branch of the hydrological cycle. In addition, the CDR has been used to investigate changes in the (global) meridional circulation which revealed a poleward expansion of the southern tropical width across the entire Pacific Ocean.
