Trajectory matching of ozonesondes and MOZAIC measurements in the UTLS – Part 1: Method description and application at Payerne, Switzerland

With the aim of improving ozonesonde observations in the upper troposphere/lower stratosphere (UTLS), we use three-dimensional forward and backward trajectories, driven by ERA-Interim wind fields to match and compare ozonesonde measurements at Payerne (Switzerland) with observations from the MOZAIC aircraft program from 1994–2009. The uncertainties associated with the sonde–MOZAIC match technique were assessed using "self-matches", i.e. matches of instruments of the same type, such as MOZAIC–MOZAIC. Despite strong vertical ozone gradients of ozone at the tropopause, which render the match approach difficult, the method provides excellent results, showing mean differences between different MOZAIC aircraft of ±2% (typically with a few hours between the up- and downstream match points). Matches between MOZAIC aircraft and Payerne ozonesondes show an agreement of ±5% for sondes equipped with electrochemical concentration cells (ECC) and between <5% (not scaled to total ozone) and <10% (scaled) for the Brewer–Mast (BM) sondes after 1998. Prior to 1998, BM sondes show an offset of around 20% (scaled). No break can be identified through the change from the BM to ECC sonde types in September 2002. A comparison of BM sondes with ozone measurements from the NOXAR B747 project for the period 1995–1996 show a smaller offset of around 15% (scaled), which may indicate a small drift in the MOZAIC calibration.