High-ozone layers in the middle and upper troposphere above Central Europe: potential import from the stratosphere along the subtropical jet stream

Specific very dry high-ozone layers, starting roughly two days after the onset of high-pressure periods during the warm season, have been reproducibly observed in the middle and upper troposphere with the ozone lidar in Garmisch-Partenkirchen (Germany). These episodes, previously not understood, were recently analysed based on extending backward simulations with the FLEXPART particle dispersion model to as many as twenty days and on jet-stream analyses including calculations with the LAGRANTO transport model. In all six cases analysed the model results indicate ozone import from the stratosphere on an extremely long path along the subtropical jet stream over the Pacific Ocean, Asia and, in part, all the way back to the Atlantic Ocean. The analysis suggests that stratospheric influence is the most important factor for the increase in ozone and is related to rather shallow transfer of air from the stratosphere into the upper- and mid-tropospheric air streams observed with the lidar. Contributions from the boundary layers of East Asia and North America are just occasionally present, in one case documented by a very dense aerosol plume from the Asian deserts. The considerable vertical and temporal extent of many of these layers and peak ozone mixing ratios between 80 and 150 ppb suggest that the observations are related to an important mechanism for stratosphere-to-troposphere transport (STT) and also confirm the model predictions of pronounced and persistent STT along the subtropical jet stream.