Articles | Volume 7, issue 13
Atmos. Chem. Phys., 7, 3399–3409, 2007

Special issue: HIBISCUS - investigating the impact of tropical convection...

Atmos. Chem. Phys., 7, 3399–3409, 2007

  02 Jul 2007

02 Jul 2007

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 T. Christensen et al.
  • 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 equatorial region. 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.

Final-revised paper