Particle properties and water content of noctilucent clouds and their interannual variation

Abstract

Noctilucent clouds (NLC) have been observed by a multicolor lidar in northern Norway (69°N, 16°E). From three backscatter coefficients we calculate the parameters of a monomodal particle size distribution. We deduce the mean of the size distribution, the width, and the average number density of the ensemble. Using the backscatter coefficients at the peak of the layer the particle size above ALOMAR is investigated by comparing the observations with model results for spherical and aspherical particles assuming either lognormal or Gaussian size distribution. From the analysis of 645 particle size soundings (142 h of measurements) we find that the average size of all NLC particles above ALOMAR from 1998 to 2005 is 47.7 ± 1 nm for cylinders with Gaussian distribution while it is 39.7 ± 1 nm for the traditional model having spherical particles with lognormal distribution. The distribution width is 16.6 ± 0.5 nm for Gaussian distributed cylinders while the particle number density is 85 ± 6 cm−3. We compare our results in detail to previously published measurements and find a satisfying agreement between the observations taking into account the limitations of previous studies and the different locations of the measurements. From the particle properties we calculate a mean surface density of (4.4 ± 0.2) × 10−8 cm2/cm3 and a mean volume density of (6.0 ± 0.2) × 10−14 cm3/cm3. The mean volume density of faint and strong clouds is 1.6 × 10−14 cm3/cm3 and 7.9 × 10−14 cm3/cm3, respectively. From the volume density we calculate the year‐to‐year variation of the seasonal mean cloud water content to be about 40% and the average observable NLC ice mass flux through 70°N to be about 11 kilotons.