Strateole 2 balloons reveal persistent errors in reanalyzed winds and trajectory calculations in the tropical lower stratosphere

Abstract. Winds in the tropical lower stratosphere raise difficulties for numerical weather prediction models: without geostrophy, winds decouple from temperature and direct observations are scarce. The Strateole 2 project explores the tropical lower stratosphere using superpressure balloons that drift for up to three months between 18 and 21 km altitude. Wind is measured on all flights: eight in the first campaign (2019–2020) and seventeen in the second (2021–2022). These measurements are used to assess errors in the winds of the ERA5 reanalysis for latitudes between 18° S and 10° N. Two additional objectives of this study are to assess errors in modelled balloon trajectories, and to document the dispersion of air below the balloons, in order to facilitate the interpretation of observations made below the balloons. The comparison of measured and reanalyzed winds reveal significant errors, with standard deviations of 3.76 m s⁻¹ for zonal and 3.24 m s⁻¹ for meridional wind. Relative to a previous comparison in 2010, only a modest decrease of 20 and 10 % is found. Trajectory calculations have very variable skill, with median errors after 24 hours of 260 km, but a tenth of the errors larger than 600 km. Factors leading to large errors, such as initial wind error and latitude are identified. Similarly, trajectory dispersion of air below the balloon is very variable, depending on the initial shear. The persistent errors highlight the need for regular obsevations of winds in the tropical lower stratosphere, and emphasize the need for caution when using trajectory calculations for process studies.