Evaluation of ECMWF ERA-40 temperature and wind in the lower tropical stratosphere since 1988 from past long-duration balloon measurements T. Christensen1, B. M. Knudsen1, J.-P. Pommereau2, G. Letrenne3, A. Hertzog4, F. Vial4, J. Ovarlez4, and M. Piot2,5 1Danish Meteorological Institute, Copenhagen, Denmark 2Service d'Aéronomie, IPSL/CNRS, Verrières-Le-Buisson, France 3Centre National d'\'Etudes Spatiales, Toulouse, France 4Laboratoire de Météorologie Dynamique, IPSL/CNRS, Palaiseau, France 5Institut für Umweltphysik, University of Heidelberg, Heidelberg, Germany
Abstract. The temperature and wind of the ECMWF ERA-40 reanalysis in the tropical
lower stratosphere during the period 1988–2001 has been evaluated by
comparison with independent in situ measurements of 21 IR Montgolfier
and superpressure long-duration balloon flights performed by CNES from
Pretoria (26° S) in South Africa in 1988–1989, Latacunga (1° S)
in Ecuador in 1991–1998 and Bauru (22° S) in Brazil in 2000–2001.
The ERA-40 temperature displays a bias varying progressively from +1.16 K
in 1988–1989, to +0.26 K in 1994–1996 and −0.46 K after 1998,
the latter being fully consistent with recent evaluations of ECMWF
operational analysis from radio occultation and in situ long-duration
balloon observations. The amplitude of the bias and its evolution are
very similar to the results of a previous evaluation from radiosondes
in 1991–2003, suggesting that the origin of the drift of ERA-40 might be
mainly due to errors in the series of satellite measurements of MSU,
replaced by AMSU in 1998, assimilated in the model.
The ERA-40 zonal wind speed in the lower stratosphere appears slightly
overestimated by 0.7–1.0 m/s on average in both the tropics and
equatorial region, that is by 5–10% compared to the average 10–20 m/s
wind speed. This bias, fully consistent with a recent evaluation of
ECMWF operational analysis in 2004, is found constant during the whole
1988–2001 period, suggesting
a shortfall in the variabililty of ERA-40
horizontal winds in the lower stratosphere in the tropics and the
Finally calculated trajectories
using ERA-40, frequently used for analysing field observations, are found
in error compared to that of the balloons by ±500 km after 5 days
and ±1000 km after 10 days.
Citation: Christensen, T., Knudsen, B. M., Pommereau, J.-P., Letrenne, G., Hertzog, A., Vial, F., Ovarlez, J., and Piot, M.: Evaluation of ECMWF ERA-40 temperature and wind in the lower tropical stratosphere since 1988 from past long-duration balloon measurements, Atmos. Chem. Phys., 7, 3399-3409, doi:10.5194/acp-7-3399-2007, 2007.