Observations of stratosphere-to-troposphere transport events over the eastern Mediterranean using a ground-based lidar system



[1] In order to cover a substantial amount of stratospheric intrusions during the project Influence of Stratosphere-Troposphere Exchange in a Changing Climate on Atmospheric Transport and Oxidation Capacity (STACCATO), coordinated measurements were carried out at several locations in central and southern Europe, based on quasi-operational stratosphere-troposphere exchange forecasts. In this context, lidar measurements of tropospheric ozone were performed at Thessaloniki (23°E, 40.5°N), Greece, for the investigation of stratosphere-to-troposphere transport (STT) events over the southeastern Mediterranean region, during 2000–2002. The study of STT in this area is of particular interest not only because of its geographic location, which is more southern than the typical position of the polar front jet, but also because of the sparseness of detailed studies in this area. A summary of the main characteristics of the STT events that were detected by lidar during the investigation period reveals a tropospheric ozone increase of the order of 10% between 4.5 and 6.5 km above sea level, a coincident decrease in relative humidity, and elevated values of potential vorticity, thus providing a direct indication of the occurrence of stratospheric air in the middle troposphere. No direct effect on surface ozone was recorded. Furthermore, the majority of the STT cases detected reveal a common pathway of the stratospheric air masses that reached Thessaloniki originating from the North Sea. In addition, the event of 9 January 2001, during which the clearest and strongest descent of stratospheric air occurred, is further analyzed. An ozone-rich layer of the order of 60–90 ppbv between 5 and 6.5 km above sea level is clearly depicted, which was the result of the intermixture of originally stratospheric and boundary layer air originating from North America.