Evaluation of ECMWF ERA-40 temperature and wind in the lower tropical stratosphere since 1988 from past long-duration balloon measurements

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 degrees S) in South Africa in 1988-1989, Latacunga (1 degrees S) in Ecuador in 1991-1998 and Bauru (22 degrees 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 & minus;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.