Noctilucent clouds and the mesospheric water vapour: the past decade

• Main Authors: von Zahn, U., Baumgarten, G., Berger, U., Fiedler, J., Hartogh, P.
• Other Authors: Leibniz-Institute of Atmospheric Physics (AIP), Max-Planck-Institut für Sonnensystemforschung = Max Planck Institute for Solar System Research (MPS), Max-Planck-Gesellschaft
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2004
• Subjects: [SDU.OCEAN]Sciences of the Universe [physics]/Ocean; Atmosphere; Alomar; Arctic; albedo
• Online Access: https://hal.science/hal-00295564; https://hal.science/hal-00295564/document; https://hal.science/hal-00295564/file/acp-4-2449-2004.pdf

International audience The topic of this paper is the sensitivity of the brightness of noctilucent clouds (NLC) on the ambient water vapour mixing ratio f(H 2 O). Firstly, we use state-of-the-art models of NLC layer formation to predict NLC brightness changes in response to changes in the 80km mixing ratio f(H 2 O) for the two cases of ground-based 532nm lidar observations at 69° N and for hemispheric satellite SBUV observations at 252nm wavelength. In this study, we include a re-evaluation of the sensitivity of NLC brightness to changes in solar Lyman ? flux. Secondly, we review observations of episodic changes in f(H 2 O) and those in NLC brightness, the former being available since 1992, the latter since 1979. To this review, we add a new series of observations of f(H 2 O), performed in the Arctic summer at the ALOMAR observatory. The episodic change exhibited by the Arctic summer means of f(H 2 O) turns out to be quite different from all those derived from annual means of f(H 2 O). The latter indicate that since 1996 a significant reduction of annually averaged upper mesospheric water vapour has occurred at low, mid, and high latitudes. These decreases of f(H 2 O) have been observed over the same time period in which a slow increase of SBUV NLC albedo has occurred. From this scenario and additional arguments we conclude that the cause for the observed long-term increase in NLC albedo remains to be identified. We close with comments on the very different character of decadal variations in NLC brightness and occurrence rate.