Improved cloud detection for upper tropospheric clouds

Over the past decade, limb sounding instruments have increasingly been able to sound the upper troposphere where clouds become an important issue both for retrieval sensitivity and for investigations of atmospheric composition. The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) is providing unprecedented, high spectral resolution measurements of clouds in the upper troposphere. These measurements will be presented and discussed in terms of the sensitivity of infra-red spectra to cloud extinction, and the frequency of occurrence of clouds which are expected to be mainly sub-visible cirrus. It will also be shown that the concentration of water vapour is critical to the detection of clouds in the MIPAS data. Combination of a water index with the currently operational cloud index method is shown to deliver improved results below 10 km in the tropical atmosphere, where water vapour mixing ratios exceed 2000 ppmv. This strengthens the possibility to retrieve water vapour at low altitudes and has implications for the design of new infra-red instruments which aim to measure profiles of clouds with high vertical resolution.