Klemens Hocke

The ionospheric quasi-biennial oscillation (QBO) is the sum of the solar QBO and the terrestrial QBO from below. In the present study, we focus on the QBO modulations of diurnal variations in mesospheric temperature and in the daily range of total electron content (ΔTEC) values at low latitudes. The diurnal tide (DW1) in mesospheric temperature is observed by the Aura Microwave Limb Sounder (Aura/MLS) from 2004 to 2021 and shows a dominant modulation at the QBO period of 28 months. This QBO modulation is transfered into the equatorial ionosphere by diurnal electric currents in the ionospheric dynamo region and the equatorial plasma fountain effect. Thus, the QBO modulation of the ΔTEC variation is partly generated by the terrestrial QBO from below. We decomposed the observed ionospheric QBO in ΔTEC from Global Navigation Satellite System (GNSS) into the terrestrial QBO part (from Aura/MLS) and the solar QBO part where the latter is derived from the QBO modulation of the solar flux at 10.7 cm (F10.7). The magnitude of the terrestrial QBO is 34% of the magnitude of the solar QBO in its effect on diurnal TEC variation. The supercomposition of terrestrial and solar QBO explains 81% of the ionospheric QBO signal in ΔTEC. The correlation between the QBO modulation in ΔTEC and solar flux F10.7 is 0.97. The maximal cross-correlation between the mesospheric QBO modulation in DW1 and the stratospheric QBO modulation in zonal wind is 0.97 with a certain phase lag.

We derived distributions of inertia gravity waves in the polar middle atmosphere from 2005 to 2021 by using temperature profiles observed by the Microwave Limb Sounder (Aura/MLS) on the Aura satellite. In the southern polar atmosphere, there is enhanced gravity wave activity in the mesosphere above the Weddell sea. The enhanced gravity wave activity occurs with small interannual variations in January and in July from 2005 to 2021. The vertical evolution of gravity wave distributions is analysed by averaging all gravity wave maps in January or July from 2005 to 2021 and showing them separately at altitudes from 44 km to 90 km. In winter season (July), the well-known hotspot of gravity wave activity in the stratosphere over the southern Andes is isolated from the hotspot of gravity wave activity in the mesosphere over the Weddell sea. There is no evolution from the stratospheric gravity wave hotspot to the mesospheric gravity wave hotspot which is more south- and eastward. We also present the gravity wave distributions for the northern polar middle atmosphere which have no clear hotspots and generally smaller amplitudes compared to the southern polar middle atmosphere.

The Aura Microwave Limb Sounder (Aura/MLS) measures temperature profiles with a horizontal spacing of about 170 km along its near polar orbit. We highpass-filtered the horizontal temperature fluctuations along the suborbital track in the middle atmosphere. The characteristics of inertia gravity waves with horizontal wavelengths between 200 and 825 km are evaluated for the equatorial region (10° S to 10° N), northern polar region (70° N to 82° N), and southern polar region (70° S to 82° S) over the time interval from August 2004 to December 2021. A modulation of gravity wave activity by quasi-biennial oscillations is present in the equatorial stratosphere but not in the equatorial mesosphere. The gravity wave activity in the southern polar mesosphere is stronger by a factor of up to 2 than in the northern polar mesosphere. The seasonal variation in the vertical structure of gravity wave activity shows strong interhemispheric differences. There are double layers of enhanced gravity wave activity in the upper mesosphere over Antarctica in the summer and the winter, while the northern polar region does not show a double layer structure of gravity wave activity. In the northern polar region, upper mesospheric gravity wave activity is decreased after the onset of major sudden stratospheric warmings.

Because of the near-polar, sun-synchronous orbit of the Aura satellite, the Microwave Limb Sounder (Aura/MLS) observes the lunar tide as a lunar semimonthly variation of geopotential height and temperature in the middle atmosphere. The FFT spectrum of the mesospheric geopotential height time series from 2004 to 2021 shows a significant spectral peak at a period of 14.7653 d which is a half lunar month. The lunar tidal signal is clearer in geopotential height than in temperature. For the first time, the characteristics of the lunar tide in geopotential height (or pressure) are observed for the middle atmosphere. The latitudinal dependence of the observed mesospheric lunar tidal amplitude is in a good agreement with the numerical simulation of Geller. The climatology of the lunar tide shows larger amplitudes in January than in July at low latitudes, in agreement with the simulation. Generally, the observed lunar tide in geopotential height is smalller by a factor 2–3 than the simulated lunar tide. The vertical phase gradient of the observed lunar tide agrees well with the simulated vertical phase gradient.

The Aura Microwave Limb Sounder (Aura/MLS) measures temperature profiles with a horizontal spacing of about 170 km along its near polar orbit. We highpass-filtered the horizontal temperature fluctuations along the suborbital track in the middle atmosphere. The characteristics of inertia-gravity waves with horizontal wavelengths between 200 and 825 km are evaluated for the equatorial region (10°S to 10°N), northern polar region (70°N to 82°N), and southern polar region (70°S to 82°S) over the time interval from August 2004 to December 2021. The gravity wave activity over the southern polar region is stronger by a factor of up to 2 than over the northern polar region. The seasonal variation of the vertical structure of gravity wave activity shows strong interhemispheric differences. There are double layers of enhanced gravity wave activity in the upper mesosphere over Antarctica in summer and winter while the northern polar region does not show a double layer structure of gravity wave activity. In the northern polar region, the upper mesospheric gravity wave activity is decreased after the onset of major sudden stratospheric warmings.

The observations of the Aura Microwave Limb Sounder (Aura/MLS) indicate the existence of a polar mesospheric precursor of major sudden stratospheric warmings (SSWs). The geopotential height data of Aura/MLS from 2004 to 2021 show that a burst of a 4 day-oscillation (zonally symmetric wave with zonal wavenumber zero) occurs at the stratopause and in the mesosphere at 80° N about 17 days before the central date of the SSW. The detection of this precursor in forecast analyses of weather and climate models has the potential to extend the subseasonal forecast of SSWs from three to five weeks.

The observations of the Aura Microwave Limb Sounder (Aura/MLS) indicate the existence of a polar mesospheric precursor of major sudden stratospheric warmings (SSWs). The geopotential height data of Aura/MLS from 2004 to 2021 show that a burst of a 4 day-oscillation (zonally symmetric wave with zonal wavenumber zero) occurs at the stratopause and in the mesosphere at 80° N about 17 days before the central date of the SSW. The detection of this precursor in forecast analyses of weather and climate models has the potential to extend the subseasonal forecast of SSWs from three to five weeks.

Because of the near-polar, sun-synchronous orbit of the Aura satellite, the Microwave Limb Sounder (Aura/MLS) observes the lunar tide as a lunar semimonthly variation of geopotential height and temperature in the middle atmosphere. The FFT spectrum of the mesospheric geopotential height time series from 2004 to 2021 shows a significant spectral peak at a period of 14.7653 days which is a half lunar month. The lunar tidal signal is clearer in geopotential height than in temperature. For the first time, the characteristics of the lunar tide in geopotential height (or pressure) are observed for the middle atmosphere. The latitudinal dependence of the observed mesospheric lunar tidal amplitude is in a good agreement with the numerical simulation of Geller. The climatology of the lunar tide shows larger amplitudes in January than in July at low latitudes, in agreement with the simulation. Generally, the observed lunar tide in geopotential height is smaller by a factor 2–3 than the simulated lunar tide. The vertical phase gradient of the observed lunar tide agrees well with the simulated vertical phase gradient.

Because of the near-polar, sun-synchronous orbit of the Aura satellite, the Microwave Limb Sounder (Aura/MLS) observes the lunar tide as a lunar semimonthly variation of geopotential height and temperature in the middle atmosphere. The FFT spectrum of the mesospheric geopotential height time series from 2004 to 2021 shows a significant spectral peak at a period of 14.7653 days which is a half lunar month. The lunar tidal signal is clearer in geopotential height than in temperature. For the first time, the characteristics of the lunar tide in geopotential height (or pressure) are observed for the middle atmosphere. The latitudinal dependence of the observed mesospheric lunar tidal amplitude is in a good agreement with the numerical simulation of Geller. The climatology of the lunar tide shows larger amplitudes in January than in July at low latitudes, in agreement with the simulation. Generally, the observed lunar tide in geopotential height is smaller by a factor 2–3 than the simulated lunar tide. The vertical phase gradient of the observed lunar tide agrees well with the simulated vertical phase gradient.

For climate change research, it is important to have multiple independent measurement techniques. The ground-based microwave radiometer at Bern has been operated since 1994 and allows the independent derivation of the linear trend of the integrated water vapour column or integrated water vapour (IWV). According to the Clausius-Clapeyron equation, the water vapour saturation increases with increase of temperature. There is also a water vapour feedback since water vapour is a natural greenhouse gas and amplifies man-made global warming. In Switzerland, climate change is stronger than at many other places in the world. We analyse observations of the tropospheric water radiometer (TROWARA) which monitored IWV above Bern from 1995 to 2025. The relative IWV increase is 5.1 %/decade. Evaluation of coincident IWV data from ERA5 (reanalysis of European Centre for Medium-Range Weather Forecasts) gives a trend of 3.7 %/decade. The ERA5 surface air temperature in Bern increased by 0.47K/decade from 1995 to 2025. Thus, we get 10.9 % more IWV for a 1 K increase in case of TROWARA and 7.8 % more IWV for a 1K increase in case of ERA5. Though the IWV trends of TROWARA and ERA5 slightly differ, both datasets agree in the fact that water vapour above the Swiss Plateau significantly increased by 11 % or 16 % from 1995 to 2025. This strong increase of water vapour certainly has an impact on weather, climate, and hydrology.