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| Titel |
Refinements in the use of equivalent latitude for assimilating sporadic inhomogeneous stratospheric tracer observations, 1: Detecting transport of Pinatubo aerosol across a strong vortex edge |
| VerfasserIn |
P. Good, J. Pyle |
| 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 ; 4, no. 7 ; Nr. 4, no. 7 (2004-09-13), S.1823-1836 |
| Datensatznummer |
250002031
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| Publikation (Nr.) |
 copernicus.org/acp-4-1823-2004.pdf |
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| Zusammenfassung |
| The use of PV equivalent latitude for assimilating stratospheric tracer
observations is discussed - with particular regard to the errors in the equivalent
latitude coordinate, and to the assimilation of sparse data. Some example
measurements are assimilated: they sample the stratosphere sporadically and
inhomogeneously. The aim was to obtain precise information about the isentropic
tracer distribution and evolution as a function of equivalent latitude. Precision
is important, if transport across barriers like the vortex edge are to be detected
directly. The main challenges addressed are the errors in modelled equivalent
latitude, and the non-ideal observational sampling. The methods presented allow
first some assessment of equivalent latitude errors and a picture of how good or
poor the observational coverage is. This information determines choices in the
approach for estimating as precisely as possible the true equivalent latitude
distribution of the tracer, in periods of good and poor observational coverage.
This is in practice an optimisation process, since better understanding of the
equivalent latitude distribution of the tracer feeds back into a clearer picture
of the errors in the modelled equivalent latitude coordinate. Error estimates
constrain the reliability of using equivalent latitude to make statements like
"this observation samples air poleward of the vortex edge" or that of more
general model-measurement comparisons. The approach is demonstrated for
ground-based lidar soundings of the Mount Pinatubo aerosol cloud, focusing on the
1991-92 arctic vortex edge between 475-520K. Equivalent latitude is estimated at
the observation times and locations from Eulerian model tracers initialised with
PV and forced by UK Meteorological Office analyses. With the model formulation
chosen, it is shown that tracer transport of a few days resulted in an error
distribution that was much closer to Gaussian form, although the mean error was
not significantly affected. The analysis of the observations revealed a small
amount of irreversible transport of aerosol across the vortex edge during late
January 1992, coincident with a strongly disturbed vortex. |
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