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

Abstract

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 instrument's spectrometer allowed middle atmospheric water vapor to be continuously measured since April 2007. Thenceforward to May 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 multilinear parametric trend model has been used. The trend model encompasses a linear term, a solar activity tracker, the El Niño-Southern Oscillation (ENSO) index and the Quasi-Biennial Oscillation (QBO) as well as the annual and semi-annual oscillation. In the time period April 2007 to May 2018 we find a significant decline in water vapor by-0:6±0:2 ppm per decade between 61 and 72km. Below the stratopause level (∼ 48km) a smaller reduction of H2O of up to-0:3±0:1 ppm per decade is detected.