Cloud Detection and Distributions from Mipas Infra-red Limb Observations

Clouds are important for their radiative forcing, control of water vapour, chemical interactions, and effects on trace gas retrievals from remote sensing. Historically, clouds have been observed principally using nadir sounding but more recently limb-sounding instruments have demonstrated their sensitivity particularly to sub-visible cirrus. The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), launched on ENVISAT in March 2002, provides unprecedented infra-red emission spectra of clouds at high spectral resolution (0.025 cm unapodised), with vertical sampling of 3 km from 6 to 68 km. Here we show that the cloud index (CI) method of cloud detection, as implemented in the ESA operational processor for MIPAS, is reasonably robust to changes in altitude and water vapour concentrations at 12 km and above in the tropics and mid-latitudes, and in polar winter atmospheres between 15 and 27 km. Using CI, we then present the first cloud frequency distributions from MIPAS.