deletions | additions       diff --git a/Characterisation_of_STTs.tex b/Characterisation_of_STTs.tex  index 131179d..6c1faa7 100644  --- a/Characterisation_of_STTs.tex  +++ b/Characterisation_of_STTs.tex 
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\subsection{Characterisation of STTs}  \label{Section:CharacterisationOfSTTs}  STT events are characterised in the vertical profiles of ozone as altitudes in the troposphere where the ozone mixing ratio exceeds a specified  threshold. Usually stratospheric ozone mixes irreversibly down into the troposphere in a synoptic-scale tongue of air: the vertical ozone profile observed by the ozonesonde depends upon the time in this cycle that it isobserved is observed  \citep{Sprenger2003}. As such, the altitude of the tropospheric ozone peak due to a an  STT event, and the amplitude of the event, event  above the background tropospheric ozone profile, vary in space and time. Firstly, two Two  definitions of the tropopause height are calculated: the standardWMO  lapse rate tropopause \citep{WMO1957}, and the ozone tropopause (following \citep{Bethan1996}. At Davis,  the definition of \citep{Bethan1996}, but ozone tropopause defintion is modificted  for Davis as modified by polar sites, following  \citep{Tomikawa2009,Alexander2013}. While the ozone tropopause can be less robust during stratosphere-troposphere exchange, it performs better than the lapse rate tropopause at polar latitudes in winter and near jet streams in the lower stratosphere \cite{Bethan1996}. The lower of these tropopause altitudes is taken referred to  as the tropopause for this study, because it is likely that of the  mixing of minor constituentsoccurs  in the upper troposphere and lower stratosphere, particularly when there is an ill-defined lapse rate tropopause.
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%While the ozone tropopause can be less robust during stratosphere-troposphere exchange, it performs better than the lapse rate tropopause at polar latitudes in winter and near jet streams in the lower stratosphere \cite{Bethan1996}.  %For many of the sonde profiles ozone disturbances occur between the lapse rate and ozone defined tropopauses, and since it is not clear that this area is actually the troposphere we only characterise events bound by the lower of the lapse rate and ozone tropopause heights.  First the The  vertical profiles of ozone volume mixing ratio  are linearly interpolated to a regular grid with 20m resolution up to 14km altitude  and are then bandpass filtered so as to retain perturbations on altitude scales between 0.5km - 5km. The choice of band limits is set empirically, but we note that to define an STT event, a clear increase above the background ozone level  is needed, and a vertical limit of ($\sim 5$~km) $\sim 5$~km  removes seasonal-scale effects \textbf{SA: to reword and make clearer...}. effects.  The ozone perturbation profile is then considered analysed  at altitudes from 2~km above the surface (to avoid surface pollution events) and 1~km below the tropopause (to avoid the sharp transition to stratospheric air producing spurious false positives). Perturbations above the 99~th percentile (locally) of all ozone levels are initially classified aspotential  STT events. %Then profiles are run through a Fourier bandpass filter which removes scales outside of 0.5km and 5km, leaving us with the ozone perturbation from background.  %In order to avoid spurious events caused by surface pollution or Fourier transform anomalies these perturbation profiles are trimmed to between 2km above the surface and 1km below the tropopause.  %Now using all the trimmed filtered profiles we extract those with points above the 99th percentile(locally) of all ozone levels.  In order to remove unclear 'near tropopause' anomalies we further filter out remove  events where the gradient between the maximum ozone peak and the altitude at 1~km below the is greater than -20 ppbv/km and require that the perturbation profile does  not drop below zero between the event peak and the tropopause. %as well as having the Fourrier filtered profile not drop below zero between the event peak and the tropopause. To provide a conservative estimate of ozone flux for each event, the ozone concentration is integrated vertically over the interval for which an STT event is identified. An example of an ozone profile is illustrated in Figure~\ref{fig:filterEG} and indicates how the algorithm detects an STT event. 
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%An initial conservative estimate of ozone flux in each event is made by interpolating to find the ozone concentration above the baseline at the peak of each event, bound above and below by where the Fourrier filtered profile becomes negative.  %An example illustrating the Fourier filter and how the flux is determined is provided in figure \ref{fig:filterEG}.