Mesospheric Inversion Layers at Mid-Latitudes and Coincident Changes of Ozone, Water Vapour and Horizontal Wind in the Middle Atmosphere

Abstract

We analyse middle atmospheric profiles of temperature, geopotential height, water vapour volume mixing ratio, and ozone volume mixing ratio above Bern (46.95 ∘ N, 7.44 ∘ E). These profiles were observed by the satellite experiment Aura/MLS and the ground-based microwave radiometers MIAWARA and GROMOS at Bern. The data series of Aura/MLS and GROMOS extend from the winter 2004/2005 to the winter 2017/2018 while the MIAWARA series starts in winter 2007/2008. Mesospheric inversion layers (MILs) above Bern, Switzerland are often present during the winter season, and the temperature peak of the MIL is located at an altitude of about 81 km in winter. The occurrence rate of the MIL during the winter season above Bern is about 42%. The MILs are possibly associated with planetary wave breaking processes in the mesospheric surf zone at mid-latitudes during winter. The study only evaluates daily averages in order to reduce tidal influences. Composite atmospheric profiles are computed for times when the MIL is present and for times when the MIL is absent. The difference of the composites indicates that middle and upper stratospheric ozone are reduced by up to 7% when the MIL is present while lower mesospheric water vapour is enhanced by up to 20% during the MIL occurrence. Using wind data of ECMWF operational analysis, we find that eastward and northward winds are decelerated by about 5–15 m/s in the lower mesosphere during the occurrence of an MIL. We also find that the occurrence of an MIL above Bern is not a regional process, but it depends on the movements and deformations of the polar mesospheric vortex. During an MIL, the location of Bern is outside of the lower mesospheric vortex. These new findings of atmospheric composition and circulation changes support the assumption that winter MILs at mid-latitudes are connected to planetary wave breaking in the middle atmosphere.