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| Titel |
A new global real-time Lagrangian diagnostic system for stratosphere-troposphere exchange: evaluation during a balloon sonde campaign in eastern Canada |
| VerfasserIn |
M. S. Bourqui, A. Yamamoto, D. Tarasick, M. D. Moran, L.-P. Beaudoin, I. Beres, J. Davies, A. Elford, W. Hocking, M. Osman, R. Wilkinson |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 12, no. 5 ; Nr. 12, no. 5 (2012-03-13), S.2661-2679 |
| Datensatznummer |
250010869
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| Publikation (Nr.) |
 copernicus.org/acp-12-2661-2012.pdf |
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| Zusammenfassung |
| A new global real-time Lagrangian diagnostic system for
stratosphere-troposphere exchange (STE) developed for Environment Canada (EC)
has been delivering daily archived data since July 2010. The STE calculations
are performed following the Lagrangian approach proposed in
Bourqui (2006) using medium-range, high-resolution operational global
weather forecasts. Following every weather forecast, trajectories are started
from a dense three-dimensional grid covering the globe, and are calculated
forward in time for six days of the forecast. All trajectories crossing
either the dynamical tropopause (±2 PVU) or the 380 K isentrope and
having a residence time greater than 12 h are archived, and also used to
calculate several diagnostics. This system provides daily global STE
forecasts that can be used to guide field campaigns, among other
applications. The archived data set offers unique high-resolution information
on transport across the tropopause for both extra-tropical hemispheres and
the tropics. This will be useful for improving our understanding of STE
globally, and as a reference for the evaluation of lower-resolution models.
This new data set is evaluated here against measurements taken during a
balloon sonde campaign with daily launches from three stations in eastern
Canada (Montreal, Egbert, and Walsingham) for the period 12 July to 4 August
2010. The campaign found an unexpectedly high number of observed
stratospheric intrusions: 79% (38%) of the profiles appear to show the
presence of stratospheric air below than 500 hPa (700 hPa). An objective
identification algorithm developed for this study is used to identify layers
in the balloon-sonde profiles affected by stratospheric air and to evaluate
the Lagrangian STE forecasts. We find that the predictive skill for the
overall intrusion depth is very good for intrusions penetrating down to 300
and 500 hPa, while it becomes negligible for intrusions penetrating below
700 hPa. Nevertheless, the statistical representation of these deep
intrusions is reasonable, with an average bias of 24%. Evaluation of the
skill at representing the detailed structures of the stratospheric intrusions
shows good predictive skill down to 500 hPa, reduced predictive skill
between 500 and 700 hPa, and none below. A significant low statistical bias
of about 30% is found in the layer between 500 to 700 hPa. However,
analysis of missed events at one site, Montreal, shows that 70% of them
coincide with candidate clusters of trajectories that pass through Montreal,
but that are too dispersed to be detected in the close neighbourhood of the
station. Within the limits of this study, this allows us to expect a
negligible bias throughout the troposphere in the spatially averaged STE
frequency derived from this data set, for example in climatological maps of
STE mass fluxes. This first evaluation is limited to eastern Canada in one
summer month with a high frequency of stratospheric intrusions, and further
work is needed to evaluate this STE data set in other months and locations. |
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