Noctilucent clouds in a coupled atmosphere

University dissertation from Stockholm : Meteorologiska institutionen (MISU)

Abstract: Noctilucent clouds (NLC) at altitudes around 80 km are the highest clouds in our atmosphere. They have been a subject of research ever since their discovery in the late 19th century. This thesis takes an important step towards using NLC as a tool for studying dynamic coupling processes in the atmosphere on a global scale.Six years of NLC observations by the Odin satellite have provided information on global distribution, seasonal variation and hemispheric differences in cloud properties. Spectroscopic measurements by the OSIRIS instrument onboard Odin are used to retrieve cloud particle sizes. The ideas and challenges behind these retrievals are discussed in detail. A study of the latitudinal dependence of NLC shows a general increase of cloud occurrence, brightness, and particle sizes towards the pole. Microphysical modelling suggests that the particles grow larger near the pole because of a combined effect of colder temperature, availability of water vapour, and meridional transport times.To investigate the relationship between the cold summer mesopause region and the dynamics in the stratosphere, NLC properties derived from Odin have been used as a proxy for the state of the summer mesosphere. These data have been co-analysed with ECMWF temperatures as a proxy for the residual circulation in the stratosphere. The results of this study reveal a surprising coupling between the winter stratosphere and the summer mesopause. This interhemispheric link is suggested to be the principal cause for both year-to-year variability and hemispheric differences in summer mesosphere conditions. Further studies of the underlying dynamic coupling mechanisms were carried out using a comprehensive middle atmospheric model. As a conclusion from this study, the interannual variability in the summer polar mesopause region can be traced back to the planetary wave flux entering the winter stratosphere from the troposphere.

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