Noctilucent clouds and the mesospheric water vapour: the past decade

• Published in: Atmospheric Chemistry and Physics
• Main Authors: Zahn, U., Baumgarten, G., Berger, U., Fiedler, J., Hartogh, P.
• Format: Text
• Language: English
• Published: 2018
• Subjects: Alomar; Arctic; albedo
• Online Access: https://doi.org/10.5194/acp-4-2449-2004; https://www.atmos-chem-phys.net/4/2449/2004/

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 α 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.