Noctilucent clouds and the mesospheric water vapour: the past decade

[摘要] The topic of this paper is the sensitivity of thebrightness of noctilucent clouds (NLC) on the ambient water vapour mixingratio f(H₂O). Firstly, we use state-of-the-art models of NLC layerformation to predict NLC brightness changes in response to changes in the 80kmmixing ratio f(H₂O) for the two cases of ground-based 532nm lidarobservations at 69° N and for hemispheric satellite SBUV observations at252nm wavelength. In this study, we include a re-evaluation of thesensitivity of NLC brightness to changes in solar Lyman α flux.Secondly, we review observations of episodic changes in f(H₂O) andthose in NLC brightness, the former being available since 1992, the lattersince 1979. To this review, we add a new series of observations off(H₂O), performed in the Arctic summer at the ALOMAR observatory. Theepisodic change exhibited by the Arctic summer means of f(H₂O) turnsout to be quite different from all those derived from annual means off(H₂O). The latter indicate that since 1996 a significant reduction ofannually averaged upper mesospheric water vapour has occurred at low, mid,and high latitudes. These decreases of f(H₂O) have been observed overthe same time period in which a slow increase of SBUV NLC albedo hasoccurred. From this scenario and additional arguments we conclude that thecause for the observed long-term increase in NLC albedo remains to beidentified. We close with comments on the very different character ofdecadal variations in NLC brightness and occurrence rate.

[学科分类] 大气科学