| 03 Mar 2026
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.
Competing interests: Author Antoine Farah is employed by the Meteomodem company.
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This paper compares radiosonde relative humidity measurements in the extratropical upper troposphere using Meteomodem M10 or M20 with collocated relative humidities from ERA5 over an extended period. Results show systematically higher relative humidities in the radiosondes compared to ERA5, particularly around 200 hPa. Two methods of raw data processing are compared, where GRUAN processing leads to systematically lower relative humidities than the Meteomodem processing at some altitudes. Little difference is found between M10 and M20 radiosondes.
Major issues: The paper does pay too little attention to the international radiosonde intercomparison in Lindenberg (Dirksen et al. 2024). This paper is cited and it is mentioned that M20 has a wet bias during nighttime compared to the standard defined in Dirksen's report. It seems highly likely that much of the presented discrepancy between ERA5 and M10/20 radiosondes is related to that wet bias. The wet bias can be partly reduced with GRUAN processing, which is the most important result of this study. The authors however attribute the discrepancy to potential errors in ERA5 (too little supersaturation over ice events due to deficient parameterization). To me this is highly doubtful, first because ERA5 relative humidity is reported relative to water, and it is unclear to me how the authors converted this relative humidity to relative humidity with respect to ice. There are uncertainties involved due to interpolation and averaging in ERA5 boxes at least.
Parts of the conclusions are written rather vaguely, giving the impression of inconclusive results. This does not help the reader at all. It may help to give uncertainty estimates instead of formulations such as "shouldn't prevent climatological studies". At least some of these uncertainty values can be found in the main body of the paper. It is important to state these also in the conclusions since often only the conclusions are read.
MInor revisons:
ERA5 (Hersbach et al. 2020) should be cited a lot earlier at the first occurence in the introduction.
lines 262 and 270/271: This appears redundant, consider shortening this paragraph
387: Data processing evolution is a bit awkward in this context. It is more stepwise changes. Thus I would say "changes" or "upgrades"
line 213f: It is not entirely clear to me if this (taking RH from ERA5, and T from the sondes) could be a reason for the different supersaturation frequencies in ERA5 and the radiosondes. This should be elaborated with some sensitivity experiments/uncertainty estimates
line 464: daytime! radiosoundings
line 506: larger (not greater)
lines 524, 548: IFS
line 540: Is there a public document describing this campaign
lines 529ff: as noted above: the conclusions are formulated very vaguely here ("shouldn't prevent", "appears reproducible"). It would be better to have uncertainty ranges