Multiwavelength lidar observation of a strange noctilucent cloud at Kühlungsborn, Germany (54°N)

On July 6/7, 1997, we observed a noctilucent cloud (NLC) by lidar at Kuhlungsborn, Germany (54°N, 12°E) using four laser wavelengths (393, 423, 532, and 770 nm). While at the near-ultraviolet and visible wavelengths clear backscatter signals were detected, the NLC did not produce significant backscattering at the near-infrared wavelength 770 nm. The latter signature can not be explained by backscattering on any size distribution of homogeneous water ice spheres. In this work we discuss the spectral backscatter signatures of particles with various shapes and compositions. We found three different particle types matching the experimental results, but there are physical arguments excluding these candidates as well. Our lidar observations of this strange NLC event appear to contradict the common understanding of NLC particle formation and sublimation. We emphasize the importance of multiwavelength observations for an in-depth interpretation of noctilucent cloud lidar backscatter data, including also the infrared part of the spectrum.