Observations of convective and dynamical instabilities in tropopause foldsand their contribution to stratosphere-troposphere exchange

With aircraft-mounted in situ and remote sensing instruments for dynamical, thermal, and chemical measurements we studied two cases of tropopause fold ing. In both folds we found Kelvin-Helmholtz billows with horizontal wavele ngth of similar to 900 m and thickness of similar to 120 m. In one case the instability was effectively mixing the bottomside of the fold, leading to the transfer of stratospheric air into the troposphere. Also, we discovered in both cases small-scale secondary ozone maxima shortly after the aircraf t ascended past the topside of the fold that corresponded to regions of con vective instability. We interpreted this phenomenon as convectively breakin g gravity waves. Therefore we posit that convectively breaking gravity wave s acting on tropopause folds must be added to the list of important irrever sible mixing mechanisms leading to stratosphere-troposphere exchange.