Regional and global tropopause fold occurrence and related ozone flux across the tropopause

This paper gives a synthesis of three algorithms to detect the presence of tropopause folds from vertical ozone/radio-sounding profiles and from meteorological analysis. Also an algorithm to identify injection of stratospheric air into the lower troposphere from ozone/(7)beryllium time series is presented. Differences in the results obtained from the algorithms are observed and discussed with respect to the criteria for fold detection and input data used. Spatial gradients in the obtained folding frequencies are made evident on a global scale from the algorithm based on meteorological analysis (Q-vector/potential vorticity) and probably also on a regional European scale from algorithms both based meteorological analyses and on ozone/PTU soundings. The observed seasonal variation of folding occurrence is rather hat except during summer when also some differences appear between the algorithms. By combining the folding frequencies with literature estimates of the cross-tropopause ozone transfer in single folding events, an average stratospheric ozone influx into the troposphere of 5.7 +/- 1.3 x 10(10) mol. cm(-2) s(-1) is obtained for the Northern hemisphere and 12 +/- 2.7 x 10(10) mol. cm(-2) s(-1) for Western Europe. Potential additional contributions due to other stratosphere-troposphere exchange processes than folds are not yet included in these estimates. Finally, the link between statistics from ozone/(7)beryllium data and folding statistics is briefly discussed.