Noctilucent clouds and the mesospheric water vapour: the past decade

U. von Zahn (1) , G. Baumgarten (1) , U. Berger (1) , J. Fiedler (1) , P. Hartogh (2)

1 AIP - Leibniz-Institute of Atmospheric Physics
2 MPS - Max-Planck-Institut für Sonnensystemforschung = Max Planck Institute for Solar System Research

The topic of this paper is the sensitivity of the brightness of noctilucent clouds (NLC) on the ambient water vapour mixing ratio f(H₂O). 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(H₂O) 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(H₂O) 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(H₂O), performed in the Arctic summer at the ALOMAR observatory. The episodic change exhibited by the Arctic summer means of f(H₂O) turns out to be quite different from all those derived from annual means of f(H₂O). 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(H₂O) 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.

Domaines: Océan, Atmosphère

U. von Zahn, G. Baumgarten, U. Berger, J. Fiedler, P. Hartogh. Noctilucent clouds and the mesospheric water vapour: the past decade. Atmospheric Chemistry and Physics, 2004, 4 (11/12), pp.2449-2464. ⟨hal-00295564⟩