Approach to the Cross-Validation of MIPAS and CHAMP Temperature and Water Vapour Profiles Gabriele Petra Stiller, Tilman Steck, Mathias Milz, Thomas von Clarmann, Udo Grabowski, and Herbert Fischer Institut f¨ ur Meteorologie und Klimaforschung (IMK), Forschungszentrum Karlsruhe GmbH / Universit¨at Karlsruhe, Postfach 3640, 76021 Karlsruhe, Germany gabriele.stiller@imk.fzk.de Summary. The Michelson Interferometer for Passive Atmospheric Sounding (MI- PAS) is a space-borne limb viewing mid-infrared high-resolution spectrometer launched on ENVISAT into polar orbit on 1 March 2002. Water vapour and tem- perature profiles are retrievable from spectral data in an altitude range from ap- proximately 5 km up to the mesosphere. Complementing the operational analysis of ESA, a non-operational MIPAS level 2 processor has been developed at IMK to de- rive geophysical data from the MIPAS observations. The IMK MIPAS level 2 data processing scheme supports a multitude of regularization approaches. Full retrieval diagnostics in terms of retrieval covariance matrices, averaging kernel matrices etc. are available. Vertical profiles of temperature and humidity have been identified to be candidate quantities for comparison with corresponding CHAMP data as part of commissioning phase as well as long-term cross-validation activities. The accuracy of MIPAS water vapour and temperature measurements is investigated by test re- trievals based on simulated data. The approach to cross-validate vertical profiles of temperature and humidity from CHAMP and MIPAS, respectively, is based on the χ 2 of the difference of MIPAS and CHAMP retrieved profiles under consideration of error correlations, averaging kernels, and spatial and temporal mis-matching of the observations. Key words: Remote sensing, atmospheric observation, MIPAS, water vapour 1 Introduction On 1 March 2002, the high-resolution mid-IR FTS limb emission sounder MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) has successfully been brought into space on board of ENVISAT. ENVISAT is a sun-synchronous polar orbiter with 98.55 ◦ inclination in approximately 800 km altitude; subsequent orbits will provide global coverage about every 3 days with 14.4 orbits per day. MIPAS is a limb-viewing Fourier transform infrared (FTIR) emission spectrometer with 0.035 cm −1 spectral resolution (unapodised), covering the mid infrared from 685 cm −1 to 2410 cm −1 (14.6 – 4.15 μm). Tangent heights from about 5 to 150 km are supported. The