Comparison of M10 and M20 Meteomodem radiosondes relative humidity measurements with ECMWF ERA5 above France: focus on the upper troposphere

Abstract. Accurate knowledge of the relative humidity (RH) in the troposphere is important for predicting cloud formation, particularly in the upper troposphere where contrails can form and contribute to global warming. However, it is difficult to predict their formation due to the lack of precise RH measurements at these altitudes. This paper compares RH data from Meteomodem radiosondes (M10 and M20) acquired over a 5-years period (2020-2024) at the Trappes and Nîmes meteorological stations in France with ECMWF ERA5 analyses, with a focus on the upper troposphere. For Trappes, two datasets exist: one processed operationally by Météo France (MF) and a second processed using the GRUAN standard. Whatever the processing is, Meteomodem radiosondes RH values are on average higher than ERA5 ones, by about 2 % at 800 hPa up to 10 % at 200 hPa. The operational MF processing generally gives higher RH than the GRUAN processing. The median difference between both processing methods is lower than 2.2 % for pressures higher than 300 hPa and is maximum for lower pressures and nighttime measurements, the GRUAN processing showing more consistency between daytime and nighttime measurements. The evolution of MF processing over time does not seem to affect the comparison. The major differences observed between the relative humidities measured by the sondes and those provided by the ERA5 reanalysis are between 200 and 300 hPa. First, ERA5 indicates more occurrences of RH below 40 % than the sondes. Second, the sondes indicate supersaturation conditions (~20 %) more frequently than ERA5 (11 %), probably due to the cloud parameterization in the IFS model, which fixes the RH at 100 % as soon as a cloud forms, in agreement to the higher occurrence of saturation conditions observed by ERA5 in this study. A first comparison of the results obtained at Trappes and Nîmes between the year 2020 and the years 2022, 2023 and 2024 shows no major differences, suggesting that the switch from M10 to M20 sondes in March 2021 at Nîmes does not significantly affect the ability to combine RH data for long term trends. However, more detailed investigations are required to assess finer differences. Finally, this study underlines the need to continue efforts to assess the quality of RH measurements in the upper troposphere and to improve cloud parameterizations in the model to increase supersaturation frequency in the upper troposphere as observed by the sondes.