Impact of particle shape on the morphology of noctilucent clouds

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.

The vapor pressure over nano-crystalline ice

“…However, noctilucent clouds (NLCs) form under conditions investigated in this work and light scattering models showed better agreement to NLC data retrieved by satellite and lidar remote sensing instruments when analyzed under the assumption of aspherical ice particle shapes (Eremenko et al, 2005;Hervig et al, 2012;Kiliani et al, 2015). At the particle temperatures investigated here (below T p = 130 K) water is most likely deposited as ASW onto the ice particles, which makes aspherical particle growth unlikely.…”

“…Below 160 K, estimated grain size diameters are in the range between 7 and 19 nm. According to our measurements, the crystal size does not depend on the formation temperature below 160 K and remains constant over a typical measurement period of 10 h. Small grain sizes like this have previously been observed after crystallization of vapor-deposited ASW: Jenniskens and Blake (1996) observed crystal diameters of 10 to 15 nm between 150 and 160 K and Kumai (1968) reported diameters of 5 to 30 nm at 113 to 143 K using electron diffraction. Dowell and Rinfret (1960) used X-ray diffraction and observed grain sizes of about 40 nm.…”

“…It is well known that the crystallization process below 166 K of ASW as well as the high-pressure ices II, IV, V and IX form nano-crystalline ice (Arnold et al, 1968;Backus and Bonn, 2004;Dowell and Rinfret, 1960;Jenniskens and Blake, 1996;Kondo et al, 2007;Kuhs et al, 1987;Kumai, 1968). The formation of nano-crystallites is believed to occur by nucleation of ice embryos followed by their diffusional isotropic three-dimensional growth within the remaining ASW matrix until all amorphous water is transformed to crystalline ice.…”

“…Using electron diffraction, Jenniskens and Blake (1996) observed crystal diameters of 10 to 15 nm between 150 and 160 K and Kumai (1968) reported diameters of 5 to 30 nm at 113 to 143 K. Dowell and Rinfret (1960) used X-ray diffraction and observed grain sizes of about 40 nm. Crystallization of the high-pressure ices II, IV, V and IX has been shown to produce nano-crystalline ice as well (Arnold et al, 1968;Kuhs et al, 1987).…”

“…This nano-granular structure may have significant effects on the properties of the ice polymorph. For example, Johari and Andersson (2015) attributed a reduction in the measured thermal conductivity of ice crystallized from ASW to enhanced phonon scattering at stacking faults and grain boundaries of the crystallites. Furthermore, the nano-crystallites might impact the vapor pressure over the ice phase, but to the best of our knowledge, this effect has not been quantified yet.…”