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| Titel |
A Comparative Study of ERA-40 and ERA-Interim Reanalyses: Uncertainties in Estimating Wave Forcing in Northern Winter |
| VerfasserIn |
Hua Lu, Thomas Bracegirdle, Tony Tony Phillips, John Turner |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250093430
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| Publikation (Nr.) |
 EGU/EGU2014-8135.pdf |
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| Zusammenfassung |
| The eddy heat flux is a fundamental quantity for understanding stratospheric variability. At
100 hPa it is a direct measure of the wave activity that propagates from the troposphere into
the stratosphere. In the Northern winter it is dominated by the contribution from stationary
waves and forms a significant component of the vertical Eliassen-Palm (E-P) flux. Based on
the transformed Eulerian mean (TEM) equation, the divergence of the E-P flux acts as a body
force on the mean flow and causes temperature variations in the stratosphere. Thus the
accuracy and homogeneity of the heat flux and the associated E-P flux divergence are vital for
studying stratospheric dynamics.
The objective of this study is to detect possible errors in these wave forcing parameters
estimated from the widely used ERA-40 and ERA-Interim reanalysis data sets produced by
the European Centre for Medium-range Weather Forecasts (ECMWF). We focus on the
December to February mean, during which the magnitude and variation of the wave forcing
are largest. We examine two types of discrepancies. The first is based on the simple
composite difference between the two data sets across their common period of the
1979/1980-2001/2002 winters. The second is to detect a discontinuity, or a sudden change of
mean in each data set.
We find four regions where significant discrepancies of zonal mean E-P flux divergence
exist. They are: 1) polarward of 20°N in the upper stratosphere where the discrepancies are
found to be associated with both the eddy heat flux v′T′and the eddy momentum flux u′v′; 2)
poleward of 45°N in the middle to lower stratosphere where the discrepancies are
primarily associated with the eddy heat flux v′T′; 3) the tropical to subtropical
upper troposphere where the discrepancies are mainly caused by the difference
in vertical eddy flux w′u′; and 4) the middle latitude upper troposphere where a
large cancellation of error is detected between the vertical eddy flux w′u′ and eddy
heat flux v′T′. It appears that these discrepancies are mainly related to analysis
increments in the 3D variation assimilation used by the ERA-40 reanalysis. In addition,
we find two significant sudden changes of mean in the eddy heat flux. The first
change is marked by a significant sudden drop of ERA-40 stationary component of
the eddy heat flux in 1991 over the latitude band of 10-30°N and at 100hPa. The
drop could be directly related to the contaminating effects of the major volcanic
eruption of Mount Pinatubo and a lack of radiosonde measurements over the Pacific
and Atlantic oceans. The second change is characterized by a sudden increase of
eddy heat flux magnitude in both ERA-40 and ERA-Interim at 10hPa in 1998.
This shift of the mean was associated with a significant drop of v′T′ difference
between ERA-40 and ERA-Interim poleward of 35°N over the land surfaces. The
implications of these discrepancies on the long-term trend of the Brewer-Dobson
circulation and the annual cycle in tropical tropopause temperature are discussed. |
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