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Journal article

Water vapour transport in the tropical tropopause region in coupled Chemistry-Climate Models and ERA-40 reanalysis data

Kremser S, Wohltmann I, Rex M, Langematz U, Dameris M, Kunze M...(+6 more)

Atmos. Chem. Phys., vol. 9, issue 8 (2009) pp. 2679-2694

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Abstract

In this study backward trajectories from the tropical lower stratosphere
were calculated for the Northern Hemisphere (NH) winters 1995-1996,
1997-1998 (El Nino) and 1998-1999 (La Nina) and summers 1996, 1997 and
1999 using both ERA-40 reanalysis data of the European Centre for
Medium-Range Weather Forecast (ECMWF) and coupled Chemistry-Climate
Model (CCM) data. The calculated trajectories were analysed to determine
the distribution of points where individual air masses encounter the
minimum temperature and thus minimum water vapour mixing ratio during
their ascent through the tropical tropopause layer (TTL) into the
stratosphere. The geographical distribution of these dehydration points
and the local conditions there determine the overall water vapour entry
into the stratosphere. Results of two CCMs are presented: the
ECHAM4.L39(DLR)/CHEM (hereafter: E39/C) from the German Aerospace Center
(DLR) and the Freie Universitat Berlin Climate Middle Atmosphere Model
with interactive chemistry (hereafter: FUB-CMAM-CHEM). In the
FUB-CMAM-CHEM model the minimum temperatures are overestimated by about
9 K in NH winter and about 3 K in NH summer, resulting in too high water
vapour entry values compared to ERA-40. However, the geographical
distribution of dehydration points is fairly similar to ERA-40 for NH
winter 1995-1996 and 1998-1999. The distribution of dehydration points
in the boreal summer 1996 suggests an influence of the Indian monsoon
upon the water vapour transport. The E39/C model displays a temperature
bias of about +5 K. Hence, the minimum water vapour mixing ratios are
higher relative to ERA-40. The geographical distribution of dehydration
points is fairly well in NH winter 1995-1996 and 1997-1998 with respect
to ERA-40. The distribution is not reproduced for the NH winter
1998-1999 (La Nina event) compared to ERA-40. There is an excessive
water vapour flux through warm regions e. g. Africa in the NH winter and
summer. The possible influence of the Indian monsoon on the transport is
not seen in the boreal summer 1996. Further, the residence times of air
parcels in the TTL were derived from the trajectory calculations. The
analysis of the residence times reveals that in both CCMs residence
times in the TTL are lower compared to ERA-40 and the seasonal variation
is hardly present.

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