The distribution of carbon monoxide in the upper mesosphere and lower thermosphere during CRISTA-1 and -2

Abstract

Carbon monoxide concentrations are derived from the CRyogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) measurements in November 1994 and in August 1997. CRISTA is a helium-cooled infrared limb sounder. Its measurements include daytime spectra of the CO (Δ v=1) band at 4.7 μm. From the spectra CO densities are derived using a state of the art non-LTE model. During the first mission data were recorded between 58°S and about 68°N. In the second mission the latitude coverage was extended to ±71° in the mesosphere. The orbital characteristics, however, were such that daytime conditions prevailed in the northern (summer) hemisphere only. The derived CO concentrations agree well with other experimental data. As the CO mixing ratio increases steeply with altitude in the mesosphere and as the chemical lifetime is much longer than the time constants of dynamical motions this gas is an excellent tracer for dynamics. During the two CRISTA missions the CO distribution was found extremely variable both in time and in space, which is a result of the complex dynamics at these altitudes. The variations are compatible with the large-scale transport from the summer to the winter hemisphere. The influence of planetary waves is observed under winter as well as under summer conditions. Large tidal amplitudes were found at mid-latitudes during the second CRISTA mission. The overall wave activity observed in August by CRISTA suggests an abrupt change of wave damping in the vicinity of 97 km.