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1. Ground-based observations of noctilucent clouds With the shortest possible wavelength (308 nm)

Noctilucent clouds (NLC) remain a fascinating phenomenon of the upper atmosphere to study. The questions about the typical particle density and particle size distribution within a NLC are very prominent ones, to which a number of answers have been given, though some of the answers contradict each other. The parameters of particle size distributions can be derived from groundbased lidar measurements of the spectral dependence of the volume backscatter coefficient of an NLC. Such studies have been performed during a number of NLC events by e.g. the ALOMAR Rayleigh/Mie/Raman (RMR) lidar (von Cossart et al., GRL, 26, 1513, 1999). A drawback of these experiments is the wavelength limitation of the RMR lidar, the shortest wavelength of which is 355 nm. At this wavelength, the sensitivity of the lidar to particles with sizes smaller than, say, 25 nm is minimal. Because a considerable part of the entire particle population may have sizes below that threshold, a lingering question remains whether or not this drawback matters for typical NLC distributions. Using the ALOMAR ozone lidar, a measurement of the NLC volume backscatter coefficient at 308 nm becomes possible. Due to the l-4 -dependence of the backscatter coefficients, the latter are almost a factor of 2 larger at this wavelength than at 355 nm. For this reason and in order to gain a fourth wavelength to the spectral distribution, we aim at using the ozone lidar for the outlined project.

Atmospheric processes Climate NLC Arctic Atmosphere lidar