WATER VAPOUR TRENDS IN SWITZERLAND FROM RADIOMETRY, FTIR AND GNSS GROUND STATIONS

Water vapour in the atmosphere is not only a strong greenhouse gas, but also affects many atmospheric processes such as the formation of clouds and precipitation. With increasing temperature, Integrated Water Vapour (IWV) is expected to increase. Analysing how atmospheric water vapour changes in time is therefore important to monitor ongoing climate change.
To determine whether IWV increases in Switzerland as expected, we asses IWV trends from a Fourier Transform Infrared (FTIR) spectrometer at Jungfraujoch, from a tropospheric water radiometer (TROWARA) in Bern and from the Swiss network of ground-based Global Navigation Satellite System (GNSS) stations. In addition, trends are assessed from reanalyses data, using the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA5) and the Modern-Era Retrospecitve Analysis for Research and Applications (MERRA-2). We use a straightforward trend method to account for jumps in the GNSS data when instrumental changes were performed.
Comparing GNSS and FTIR data at Jungfraujoch, we found a clear-sky bias of the FTIR data of approximately 1mm. However, when coincident measurements are used, the data agree within their uncertainties. At Jungfraujoch, we found positive but insignificant IWV trends. For whole Switzerland, our data show mostly positive IWV trends between 2 and 5 % per decade. Generally, we found that IWV trends from GNSS data tend to be larger at higher altitudes. Further, we found that IWV scales to temperature as expected in Bern, except in winter. However, the correlation between IWV and temperature based on reanalyses data is not always clear.
Our results are generally consistent with the positive water vapour feedback in a warming climate and confirm the larger sensitivity to climate change at higher altitudes.