Noctilucent clouds and the mesospheric water vapour: the past decade U. von Zahn1, G. Baumgarten1, U. Berger1, J. Fiedler1, and P. Hartogh2 1Leibniz-Institute of Atmospheric Physics, Kühlungsborn, Germany 2Max-Planck-Institute for Solar System Research, Katlenburg-Lindau, Germany
Abstract. 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 α 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.
Citation: von Zahn, U., Baumgarten, G., Berger, U., Fiedler, J., and Hartogh, P.: Noctilucent clouds and the mesospheric water vapour: the past decade, Atmos. Chem. Phys., 4, 2449-2464, doi:10.5194/acp-4-2449-2004, 2004.