Colour indices for the detection and differentiation of cloud types in infra-red limb emission spectra

Simple radiance ratios have been used for the detection of clouds around the tropopause and in the winter polar stratosphere from the infra-red spectra of two remote sensing instruments, the cryogenic infrared spectrometers and telescopes for the atmosphere (CRISTA) flown on two space shuttle missions in 1994 and 1997 and the Michelson interferometer for passive atmospheric sounding (MIPAS) launched on ENVISAT in March 2002. This very successful approach was first applied to different wavelength regions of the CRISTA measurements and was then used as a pre-flight validation test for a cloud detection algorithm of the operational retrieval processor for MIPAS. Preliminary results are now available from the MIPAS instrument and are presented here. First, cloud top heights have been derived down to 12 km by the detection method and show quite reasonable results. In addition, modelled spectra and measurements show that the extension of the method to lower altitudes – potentially down to 6 km – should be possible. Second, the high spectral resolution of the MIPAS measurements allows in addition the detection of scattering effects in the spectra, which gives the future opportunity to retrieve information about the size of the scattering particles. In particular, an index has been developed which allows large particle clouds to be identified. Finally, investigations of CRISTA spectra have already shown that the differentiation of polar stratospheric cloud (PSC) types is possible based on their characteristic spectral features. Application of the differentiation method to the tropics shows no indication for clouds containing PSC-like nitric-acid-hydrate particles.