The size of noctilucent cloud particles above ALOMAR (69N,16E): Optical modeling and method description

Noctilucent clouds (NLC) have been observed by a Rayleigh/Mie/Raman (RMR) lidar in Northern Norway (69N,16E) capable to measure the aerosol backscatter coefficients at three wavelengths (355 nm, 532 nm, 1064 nm) and since 1998 also under sunlight conditions. For strong noctilucent cloud events the three backscatter coefficients are used to calculate the parameters of a monomodal particle distribution describing the observed optical properties. We deduce the radius of the particle size distribution, the width, and the average number density of the particle ensemble. To minimize the smearing by a changing structure of the noctilucent clouds we use the lowest possible time and altitude resolutions, which are 14 and 150 min, respectively. Using this, the vertical structure of the particle properties below and above the peak is investigated. From a statistical comparison of the observed ratios of backscatter coefficients we find that under the assumption of spherical particles the lognormal distribution describes the optical properties of NLC above ALOMAR better than the Gaussian distribution. If the optical analysis is extended to aspherical particles then both Gaussian and lognormal distribution can sufficiently describe the optical properties when the axis ratio of the particles is allowed to vary from 1/10 to 10. Using the traditional optical model but the new data inversion method on the summer 1998 NLC data leads to the same results found by von Cossart et al. [von Cossart, G., Fiedler, J., von Zahn, U. Size distributions of NLC particles as determined from 3-colour observations of NLC by ground-based lidar, Geophys. Res. Lett., 26 (11), 1513–1516. doi: 10.1029/1999GL900226, 1999]. Using the new optical model we find that independent of the assumed particle shape the average size of strong NLCs above ALOMAR in 2005 is 47 ± 3 nm for spheroids or cylinders with Gaussian distribution while it is 54 ± 3 nm for the same particles but assuming a lognormal size distribution. The distribution widths are s = 16 ± 1 nm or σ = 1.25 ± 0.03 for Gaussian or lognormal distribution while the particle number-density is N = 120 cm−3 for both cases. This comparison shows that the combination of three widely separated wavelengths used in backscatter geometry allows a robust measurement of the particle size. We recommend performing the sensitivity test of the deduced particle properties on the particle shape also for other optical instruments sounding the NLC particle sizes.