Variability of mesospheric water vapor above Bern in relation to the 27-day solar rotation cycle

Abstract

a b s t r a c tMany studies investigated solar-terrestrial responses (thermal state, O 3 , OH, H 2 O) with emphasis on the tropical upper atmosphere. In this paper the focus is switched to water vapor in the mesosphere at a mid-latitudinal location. Eight years of water vapor profile measurements above Bern (°°46.88 N/7.46 E) are investigated to study oscillations with the focus on periods between 10 and 50 days. Different spectral analyses revealed prominent features in the 27-day oscillation band, which are enhanced in the upper mesosphere (above 0.1 hPa, ∼64 km) during the rising sunspot activity of solar cycle 24. Local as well as zonal mean Aura MLS observations support these results by showing a similar behavior. The relationship between mesospheric water and the solar Lyman-α flux is studied by comparing the similarity of their temporal oscillations. The H 2 O oscillation is negatively correlated to solar Lyman-α oscillation with a correlation coefficient of up to −0.3 to −0.4, and the phase lag is 6-10 days at 0.04 hPa. The confidence level of the correlation is ≥99%. This finding supports the assumption that the 27-day oscillation in Lyman-α causes a periodical photodissociation loss in mesospheric water. Wavelet power spectra, cross-wavelet transform and wavelet coherence analysis (WTC) complete our study. More periods of high common wavelet power of H 2 O and solar Lyman-α are present when amplitudes of the Lyman-α flux increase. Since this is not a measure of physical correlation a more detailed view on WTC is necessary, where significant (two sigma level) correlations occur intermittently in the 27 and 13-day band with variable phase lock behavior. Large Lyman-α oscillations appeared after the solar superstorm in July 2012 and the H 2 O oscillations show a well pronounced anti-correlation. The competition between advective transport and photodissociation loss of mesospheric water vapor may explain the sometimes variable phase relationship of mesospheric H 2 O and solar Lyman-α oscillations. Generally, the WTC analysis indicates that solar variability causes observable photochemical and dynamical processes in the mid-latitude mesosphere.

Quasi 18 h wave activity in ground-based observed mesospheric H<sub>2</sub>O over Bern, Switzerland

“…In Studer et al (2012) the numerical structure of the band-pass filter was shown. Lately the filter has been used to investigate the impact of the 27-day solar rotation cycle on mesospheric water vapor (Lainer et al, 2016) and to analyze the quasi 16-day planetary wave during boreal winter (Scheiben et al, 2014). We follow the advice from Oppenheim et al (1989) and run the filter with a zero phase lag forward and backward along the measurement time series.…”

“…In the stratosphere the vertical resolution of the water vapor profiles is 11 km and degrades to about 14 km in the mesosphere (Deuber et al, 2005). A recent validation against the Aura MLS v4.2 water vapor product (Livesey et al, 2015) revealed that for most months and altitudes the relative differences between MI-AWARA and Aura MLS are below 5 % (Lainer et al, 2016).…”

Long-term observation of midlatitude quasi 2-day waves by a water vapor radiometer

“…A recent validation against the Aura MLS v4.2 water vapor product revealed that for most months and altitudes the relative differences between MIAWARA and Aura MLS are below 5 % (Lainer et al, 2016 MIAWARA water vapor data set used during this study has a temporal resolution of 6 h. This is useful to study not only the Q2DW but also possible interactions with waves of shorter periods like tides. Compared to an even higher temporally resolved H 2 O data set like the one used in Lainer et al (2017) with a 3 h time interval, the 6 h interval ensures usability also during summer when the measurement sensitivity is lower.…”

“…9a the highest wavelet power (label A) is found at coordinates (48, > 64) and could be related to an interference of the Q2DW with the quasi 18 h wave, which itself is likely to originate from a nonlinear wave-wave coupling between the diurnal tide and the westward-traveling quasi 2-day wave (W2) (Lieberman et al, 2017). A recent study by Lainer et al (2016) revealed dominant oscillations in mesospheric water vapor profiles with a period close to 18 h in Northern Hemispheric winter months. However, such oscillations within a sub-diurnal period spectrum in the MLT can also be related to low-frequency inertia-gravity waves, as shown by Li et al (2007) with measurements from a sodium lidar system over Fort Collins, Colorado (41 • N, 105 • W).…”

“…In the mesosphere H 2 O is photochemically stable for weeks (Brasseur and Solomon, 2006) and this circumstance is used to investigate middle atmospheric wave dynamics from ground-based observations (Scheiben et al, 2014;Tschanz and Kämpfer, 2015;Lainer et al, 2016Lainer et al, , 2017. In this study we present quite continuous observations of the Q2DW signature in middle atmospheric water vapor for 7 years (84 months) by the middle atmospheric water vapor radiometer MIAWARA at 7.46 • E).…”