Closing the gap in the tropics: the added value of radio-occultation data for wind field monitoring across the Equator

Abstract. Globally available and highly vertically resolved wind fields are crucial for the analysis of atmospheric dynamics for the benefit of climate studies. Most observation techniques have problems to fulfill these requirements. Especially in the tropics and in the Southern Hemisphere more wind data are required. In this study, we investigate the potential of radio-occultation (RO) data for climate-oriented wind field monitoring in the tropics, with a specific focus on the equatorial band within ± 5° latitude. In this region, the geostrophic balance breaks down, due to the Coriolis force term approaching zero, and the equatorial-balance equation becomes relevant. One aim is to understand how the individual wind components of the geostrophic-balance and equatorial-balance approximations bridge across the Equator and where each component breaks down. Our central aim focuses on the equatorial-balance approximation, testing its quality by comparison with ERA5 reanalysis data. The analysis of the zonal and meridional wind components showed that while the zonal wind was well reconstructed, it was difficult to estimate the meridional wind from the approximation. However, we still found a somewhat better agreement from including both components in the zonal-mean total wind speed in the troposphere. In the stratosphere, the meridional wind component is close to zero for physical reasons and has no relevant impact on the total wind speed. In general, the equatorial-balance approximation works best in the stratosphere. As a second aim, we investigated the systematic data bias between using the RO and ERA5 data and find it smaller than the bias resulting from the approximations. We also inspected the monthly-mean RO wind data over the full example year of 2009. The bias in the core region of highest quality of RO data, which is the upper troposphere and lower stratosphere, was generally smaller than ± 2 m s−1. This is in line with the wind field requirements of the World Meteorological Organization. Overall, the study encourages the use of RO wind fields for regional-scale climate monitoring over the entire globe, including the equatorial region, and also showed a small improvement in the troposphere when including the meridional wind component in the zonal-mean total wind speed.