Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 5 5 2005 10.5194/acpd-5-9493-2005 http://www.atmos-chem-phys-discuss.net/5/9493/2005/ http://www.atmos-chem-phys-discuss.net/5/9493/2005/acpd-5-9493-2005.html http://www.atmos-chem-phys-discuss.net/5/9493/2005/acpd-5-9493-2005.pdf 9493 9545 2005-09-29 Water vapour profiles by ground-based FTIR spectroscopy: study for an optimised retrieval and its validation M. Schneider F. Hase T. Blumenstock IMK-ASF, Forschungszentrum Karlsruhe and Universität Karlsruhe, Germany The sensitivity of ground-based instruments measuring in the infrared with respect to tropospheric water vapour content is generally limited to the lower and middle troposphere. The large vertical gradients and variabilities avoid a better sensitivity for the upper troposphere/lower stratosphere region. In this work an optimised retrieval is presented and it is demonstrated that compared to a commonly applied method, it widely improves the performance of the FTIR technique with respect to upper tropospheric water vapour. Within a realistic error scenario it is estimated that the optimised method reduces the upper tropospheric uncertainties by about 25–30%, leading to a noise to signal ratio of 50%. The reasons for this improvement and the possible deficiencies of the method are discussed. The estimations are confirmed by a comparison of retrieval results based on real FTIR measurements with coinciding measurements of synoptical meteorological radiosondes.