Significant decline of mesospheric water vapor at the NDACC site Bern in the period 2007 to 2018

The middle atmospheric water vapor radiometer MIAWARA is located close to Bern in Zimmerwald (46.88°N, 7.46°E, 907m) and is part of the Network for the Detection of Atmospheric Composition Change (NDACC). Initially built in the year 2002, a major upgrade of the instruments spectrometer allowed to continuously measure middle atmospheric water vapor since April 2007. Thenceforward to Mai 2018, a time series of more than 11 years has been gathered, that makes a first trend estimate possible. For the trend estimation, a robust multi-linear parametric trend model has been used. The trend model encompasses a linear term, a solar activity tracker, the El Niño–Southern Oscillation (ENSO) index, the quasi-biennial oscillation (QBO) as well as the annual and semi-annual oscillation. In the time period April 2007 to Mai 2018 we find a significant decline in water vapor by −0.6±0.2ppmdecade−1 between 61 and 72km. Below the stratopause level (~48km) a smaller reduction of H2O of up to −0.3±0.1ppmdecade−1 is detected.