On microphysical processes of noctilucent clouds (NLC): observations and modeling of mean and width of the particle size-distribution

G Baumgarten,J Fiedler,M Rapp

doi:10.5194/acp-10-6661-2010

Abstract

Abstract. Noctilucent clouds (NLC) in the polar summer mesopause region have been observed in Norway (69° N, 16° E) between 1998 and 2009 by 3-color lidar technique. Assuming a mono-modal Gaussian size distribution we deduce mean and width of the particle sizes throughout the clouds. We observe a quasi linear relationship between distribution width and mean of the particle size at the top of the clouds and a deviation from this behavior for particle sizes larger than 40 nm, most often in the lower part of the layer. The vertically integrated particle properties show that 65% of the data follows the linear relationship with a slope of 0.42±0.02 for mean particle sizes up to 40 nm. For the vertically resolved particle properties (Δz = 0.15 km) the slope is comparable and about 0.39±0.03. For particles larger than 40 nm the distribution width becomes nearly independent of particle size and even decreases in the lower part of the layer. We compare our observations to microphysical modeling of noctilucent clouds and find that the distribution width depends on turbulence, the time that turbulence can act (cloud age), and the sampling volume/time (atmospheric variability). The model results nicely reproduce the measurements and show that the observed slope can be explained by eddy diffusion profiles as observed from rocket measurements.

Highlights

Noctilucent clouds (NLC; called polar mesospheric clouds, or PMC, when seen from space) are an intriguing optical twilight phenomenon which can be observed throughout the summer months, most often at latitudes poleward of 50◦ (e.g. Thomas, 1984; Gadsden and Schroder, 1989)
We investigate in detail the mean and the width of the size distribution and compare the results to microphysical modeling of NLC to identify the processes affecting especially the distribution width
We have presented an extensive set of lidar observations of NLC particle properties and analyzed the dataset regarding the mean size (r) and distribution width (s)

Summary

Introduction

Noctilucent clouds (NLC; called polar mesospheric clouds, or PMC, when seen from space) are an intriguing optical twilight phenomenon which can be observed throughout the summer months, most often at latitudes poleward of 50◦ (e.g. Thomas, 1984; Gadsden and Schroder, 1989). 2-dimensional probability distribution for mean particle size (r) and distribution width (s) throughout the NLC layer sampled with 150 m range resolution (method 1). We observe a strong dependence of s and r for mean sizes below about 40 nm, while the mean distribution width is constant at about s = (14.7±1.5) nm for particles from 40 to 100 nm.

Results

Conclusion