Find in Library

<p>The responses of water vapour (H<span class="inline-formula">₂</span>O) and noctilucent clouds (NLCs) to the solar cycle are studied using the Leibniz Institute for Middle Atmosphere (LIMA) model and the Mesospheric Ice Microphysics And tranSport (MIMAS) model. NLCs are sensitive to the solar cycle because their formation depends on background temperature and the H<span class="inline-formula">₂</span>O concentration. The solar cycle affects the H<span class="inline-formula">₂</span>O concentration in the upper mesosphere mainly in two ways: directly through the photolysis and, at the time and place of NLC formation, indirectly through temperature changes. We found that H<span class="inline-formula">₂</span>O concentration correlates positively with the temperature changes due to the solar cycle at altitudes above about 82 km, where NLCs form. The photolysis effect leads to an anti-correlation of H<span class="inline-formula">₂</span>O concentration and solar Lyman-<span class="inline-formula"><i>α</i></span> radiation, which gets even more pronounced at altitudes below <span class="inline-formula">∼</span> 83 km when NLCs are present. We studied the H<span class="inline-formula">₂</span>O response to Lyman-<span class="inline-formula"><i>α</i></span> variability for the period 1992 to 2018, including the two most recent solar cycles. The amplitude of Lyman-<span class="inline-formula"><i>α</i></span> variation decreased by about 40 % in the period 2005 to 2018 compared to the preceding solar cycle, resulting in a lower H<span class="inline-formula">₂</span>O response in the late period. We investigated the effect of increasing greenhouse gases (GHGs) on the H<span class="inline-formula">₂</span>O response throughout the solar cycle by performing model runs with and without increases in carbon dioxide (CO<span class="inline-formula">₂</span>) and methane (CH<span class="inline-formula">₄</span>). The increase of methane and carbon dioxide amplifies the response of water vapour to the solar variability. Applying the geometry of satellite observations, we find a missing response when averaging over altitudes of 80 to 85 km, where H<span class="inline-formula">₂</span>O has a positive response and a negative response (depending on altitude), which largely cancel each other out. One main finding is that, during NLCs, the solar cycle response of H<span class="inline-formula">₂</span>O strongly depends on altitude.</p>