Visible spectra and chromaticity of noctilucent clouds

Abstract

A computational scheme was developed to predict the visible spectra and chromaticity of noctilucent clouds (NLC) for arbitrary viewing geometries. The spectrum of a cloud in the visible is computed at 5‐nm intervals using the spherical shape of the stratified atmosphere, the measured solar irradiance spectrum, wavelength‐dependent extinction by air molecules, ozone, and aerosols, and scattering by a specified size distribution of cloud particles. The chromaticity calculated from each spectrum is used to evaluate the cloud's color. A comparison is made of the abilities of two proposed cloud particle size distributions to account for published spectra measured from the ground and from Earth orbit. A lognormal distribution of “small” particles with an effective radius of 42.6 nm is found to be better at reproducing measured spectra of NLC than a Junge (power law) distribution of “large” particles with an effective radius of 700 nm. Chromaticities of the small‐particle spectra show much better agreement with those of spectra measured from orbit and with the visual appearance of NLC viewed from the ground. Chromaticities are found to be weakly dependent on the atmospheric aerosol loading. However, NLC brightness is strongly dependent upon both tropospheric and stratospheric aerosols.