Observations of convective and dynamical instabilities in
tropopause folds and 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 folding. In both folds we found Kelvin-Helmholtz billows
with horizontal wavelength of ~900 m and thickness of ~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 aircraft ascended past the topside of
the fold that corresponded to regions of convective instability. We
interpreted this phenomenon as convectively breaking gravity waves.
Therefore we posit that convectively breaking gravity waves acting on
tropopause folds must be added to the list of important irreversible
mixing mechanisms leading to stratosphere-troposphere exchange.
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