Water vapour transport in the tropical tropopause region in coupled Chemistry-Climate Models and ERA-40 reanalysis data Stefanie Kremser1,2, Ingo Wohltmann1, Markus Rex1, Ulrike Langematz2, Martin Dameris3, and Markus Kunze2 1Stiftung Alfred-Wegener Institute for Polar and Marine Research, Potsdam, Germany 2Institut für Meteorologie, Freie Universität Berlin, Berlin, Germany 3Deutsches Zentrum für Luft- und Raumfahrt, Institut für Physik der Atmosphäre, Oberpfaffenhofen, Germany
Abstract. In this study backward trajectories from the tropical lower stratosphere
were calculated for the Northern Hemisphere (NH) winters 1995–1996,
1997–1998 (El Niño) and 1998–1999 (La Niña) 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 Universität 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 Niña
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.
Citation: Kremser, Stefanie, Wohltmann, Ingo, Rex, Markus, Langematz, Ulrike, Dameris, Martin, and Kunze, Markus: Water vapour transport in the tropical tropopause region in coupled Chemistry-Climate Models and ERA-40 reanalysis data, Atmos. Chem. Phys., 9, 2679-2694, doi:10.5194/acp-9-2679-2009, 2009.