Trends of mesospheric water vapor due to the increase of methane - A model study particularly considering high latitudes

For the water vapor trend analyses by means of our global 3D-model COMMA-IAP we employed estimated methane, dinitrogen oxide and CO2 trends since the pre-industrial era. We especially consider in context with the increasing occurrence rate of NLC observations the change of the aeronomic conditions in the high latitude mesosphere. A particular problem for the model calculations is connected with the water vapor mixing ratio at the hygropause. We suppose that the hygropause was dryer at the pre-industrial time than currently. The middle atmosphere became more humid corresponding to the increasing methane concentration but depending on height and with a certain time delay due to the long transport time through the middle atmosphere. In order to get useful data of the solar Lyman-alpha flux back to the pre-industrial time we derived a quadratic fit using the sunspot number available since 1749 as the only solar proxy for the Lyman-alpha flux before 1947. The fit is based on Lyman-alpha flux values derived by Woods et al. [Woods, T.N., Tobiska, W.K., Rottman, G.J., Worden, JR. (2000). Improved solar Lyman-alpha irradiance modeling from 1947 through 1999 based on UARS observations, J. Geophys. Res., 105, 27, 195-227, 215], available since 1947. This Lyman-alpha flux values are used to determine the water vapor dissociation rate. The solar influence on the water vapor mixing ratio is insignificant at about 80 km within the NLC area but it becomes increasingly more important with growing altitudes. The rising water vapor concentration reduces the mesospheric ozone due to higher concentrations of the hydrogen radicals. This fact causes a positive feedback between both constituents above about 65 km. The dissociation of smaller amounts of ozone entails the production of less amounts of O(D-1) destroying a smaller quantity of water vapor. The effect is most pronounced in the vicinity of the daytime secondary ozone maximum around an altitude of 85-90 km where the relative increase of the water vapor mixing ratio is strongest. (C) 2006 COSPAR. Published by Elsevier Ltd. All rights reserved.