An airborne ozone lidar has been operated on March 26 and 27, 1999 in order to document the ozone distribution near the subtropical jet (STJ) in the 5 ¿W--15 ¿W longitude band which corresponds to an area where the STJ accelerates and interacts strongly with mid-latitude cyclones. The two main features observed are a tropopause fold in the region where the STJ accelerates and an intrusion of subtropical tropospheric air into the mid-latitude lower stratosphere. A mesoscale meterological model is applied with high vertical resolution in order to study the strong layering in the upper troposphere and lower stratosphere. ¿ 2000 American Geophysical Union