Water vapour profiles by ground-based FTIR spectroscopy: study for an optimised retrieval and its validation

The sensitivity of ground-based instruments mea- suring in the infrared with respect to tropospheric water vapour content is generally limited to the lower and mid- dle troposphere. The large vertical gradients and variabili- ties avoid a better sensitivity for the upper troposphere/lower stratosphere (UT/LS) region. In this work an optimised re- trieval is presented and it is demonstrated that compared to a commonly applied method, it improves the performance of the FTIR technique. The reasons for this improvement and the possible deficiencies of the method are discussed. Only by applying the method proposed here and using measure- ments performed at mountain observatories can water vapour variabilities in the UT/LS be detected in a self-consistent manner. The precision, expressed as noise to signal ratio, is estimated at 45%. In the middle and lower troposphere, precisions of 22% are achieved. These estimations are con- firmed by a comparison of retrieval results based on real FTIR measurements with coinciding measurements of syn- optical meteorological radiosondes.