Spectral emission measurements of polar mesospheric clouds by CRISTA-2

The second flight of the CRyogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA-2) experiment took place in the first part of August 1997 on the Space Shuttle mission STS-85. Infrared limb emission spectra in the mesosphere were collected between about 72°S and 72°N during northern high latitude late summer with its very low mesopause temperatures. The mesospheric data set collected by CRISTA includes CO2 4.3 μm and 15 μm spectra from which CO2 densities and atmospheric temperatures are derived using a non-local thermodynamic equilibrium model. At wavelengths near 12 μm thermal emissions of polar mesospheric clouds were recorded on several orbits and at latitudes north of 65°N. The positions at which clouds were detected agree well with the simultaneously determined thermal structure, which is governed by planetary waves of wave numbers 2 and 1. The CRISTA data yield the first thermal emission spectrum of these clouds. The spectral shape is in agreement with the assumption that the cloud particles are composed of water ice. The total amount of water stored in the clouds corresponds to a gas phase water concentration of 2.9 ppm.