Abstract
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.
Highlights
Noctilucent clouds (NLCs), called polar mesospheric clouds (PMCs), occur in the polar region at altitudes around 83 km
1 ε for ε we find the best agreement to the Arctic Lidar Observatory for Middle Atmosphere Research (ALOMAR) lidar, the ε value of 2.8 for r > 20 nm is more suitable for comparing mean axis ratios to SOFIE satellite measurements than the lower value of 2.4 for the initial distribution since the backscatter signal is caused by large ice particles
Size and shape of noctilucent cloud particles have long been an important topic in characterizing ice formation in the upper mesosphere region
Summary
Noctilucent clouds (NLCs), called polar mesospheric clouds (PMCs), occur in the polar region at altitudes around 83 km. NLCs only form during summer, when the upper mesosphere is coldest (below 130 K) and the amount of water vapor is enhanced due to transport by atmospheric circulation (Holton, 1983). NLCs consist of ice particles with r ≈ 50 nm which form by heterogeneous nucleation, for example around meteoric dust particles (Turco et al, 1982). The size of mesospheric ice particles can be inferred with optical instruments such as lidar, which measure backscattered light at multiple wavelengths. Using microphysical modeling in this context requires simulating particle shape, since measurements indicate that NLC particles in general are not spherical. For example Baumgarten et al (2002) indicate needlelike particles with a diameter-over-length ratio (ε) of less than 0.4. The most extensive data set is from the SOFIE (Solar Occultation For Ice Experiment) instrument, with mean ε ≈ 0.5 or ε ≈ 2 (Hervig et al, 2009a; Hervig and Gordley, 2010)
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