Articles | Volume 15, issue 22
Research article
19 Nov 2015
Research article |  | 19 Nov 2015

Impact of particle shape on the morphology of noctilucent clouds

J. Kiliani, G. Baumgarten, F.-J. Lübken, and U. Berger

Abstract. Noctilucent clouds (NLCs) occur during summer in the polar region at altitudes around 83 km. They consist of ice particles with a typical size around 50 nm. The shape of NLC particles is less well known but is important both for interpreting optical measurements and modeling ice cloud characteristics. In this paper, NLC modeling of microphysics and optics is adapted to use cylindrical instead of spherical particle shape. The optical properties of the resulting ice clouds are compared directly to NLC three-color measurements by the Arctic Lidar Observatory for Middle Atmosphere Research (ALOMAR) Rayleigh/Mie/Raman (RMR) lidar between 1998 and 2014. Shape distributions including both needle- and disc-shaped particles are consistent with lidar measurements. The best agreement occurs if disc shapes are 60 % more common than needles, with a mean axis ratio of 2.8. Cylindrical particles cause stronger ice clouds on average than spherical shapes with an increase of backscatter at 532 nm by ≈ 30 % and about 20 % in ice mass density. This difference is less pronounced for bright than for weak ice clouds. Cylindrical shapes also cause NLCs to have larger but a smaller number of ice particles than for spherical shapes.

Short summary
For the first time the shape of noctilucent cloud particles is analyzed with a 3-D Lagrangian model. Three-color lidar measurements are compared directly to optical modeling of NLC simulations with non-spherical shapes: a mix of elongated and flattened cylindrical ice particles consistent with measurements. Comparison is best if flattened particles form a majority, with mean axis ratio around 2.8. NLCs from cylindrical particles are slightly brighter and consist of fewer but larger ice particle.
Final-revised paper