| 22 Apr 2026
Impact of High-Volume GNSS Radio Occultation Data on the Navy's Global Numerical Weather Prediction
Abstract. This study assesses the impact of assimilating high-volume Radio Occultation (RO) data from the RO modeling experiment (ROMEX) on the Navy's global operational Naval Global Environment Model (NAVGEM). A series of observation system experiments were conducted, including a control run, a standard assimilation of all ROMEX data, and two sensitivity tests: one with an empirical bias correction and another with a modified refractivity coefficient. Results indicate that while the standard assimilation of ROMEX data improved free-tropospheric moisture forecasts, it amplified existing model biases in temperature and geopotential height, leading to forecast degradation. In contrast, both sensitivity experiments led to substantial improvements in forecast skill. The empirical bias correction method proved most effective, yielding consistent forecast improvements across temperature, moisture, and geopotential height. A Forecast Sensitivity to Observation Impact (FSOI) analysis confirmed the positive contribution of all ROMEX missions, with Spire missions providing the largest total impact and COSMIC-2 showing the highest per-observation effectiveness. The findings underscore that an adjustment to the current treatment of the observation was critical to fully realize the benefits of the large volume of RO observations. While the empirical bias correction delivers the greatest forecast improvements, it may obscure and reinforce persistent model biases. The refractivity coefficient adjustment offers an alternative that preserves the unbiased nature of RO observations.
Competing interests: At least one of the (co-)authors serves as editor for the special issue to which this paper belongs.
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General comments
This paper describes experiments at NRL with the high-volume GNSS-RO data from ROMEX. Similar to other centres, forecast degradations are seen when the observations are used without any alteration. Experiments using an empirical bias correction and an adjustment of the refractivity formulation are more sucessful and overall forecast improvements are seen.
The paper is well written, and the conclusions drawn are supported by the evidence that is presented. The paper is a useful addition to the literature, and provides an interesting perspective on the use of large numbers of GNSS-RO observations. My specific comments are largely minor details of presentational clarity.
Specific comments
L30, L34 and L40: I would suggest reserving "significant" for contexts in which statistical significance is indicated. In these lines it would be better to use large, substantial or important.
L32: I would add Samrat et al (2025) to the list of references demonstrating the importance of GNSS-RO.
Samrat, N.H., Candy, B., Lewis, O., Cotton, J., Smith, F., Halloran, G., et al. (2025) Observation impact evaluation through data denial experiments in the Met Office global numerical weather prediction system. Quarterly Journal of the Royal Meteorological Society, 151(771), e5002. Available from: https://doi.org/10.1002/qj.5002
L41: It should be 6000 COSMIC-2 profiles. I suggest also mentioning that this is a daily number "with the addition of 6,000 COSMIC-2 refractivity profiles per day."
L43: The CGMS is not part of the WMO. Formally, the WMO is a founding member of the CGMS. Otherwise they are unrelated.
L54: 30,000 to 40,000 occultations per day is no longer three times the number of RO observations used in NWP, approximately 20,000 occultations are currently available daily.
L64: Since the data assimilation uses hybrid covariances, it would be useful to note whether the ensemble prediction system is rerun with the additional observations to provide updated covariances with the experiments. Therefore, please can this information be added to the description.
L87: The formatting of this equation makes it difficult to follow how the piecewise linear function is constructed. For instance, the third line is "linearly varies from -0.01% to 0.2% Â 18 < h < 23 km". I would normally read that as -0.01% at 18km and 0.2% at 23km. In reality it is the other way around in order to be continuous with the adjacent layers. I would suggest a fundamental reformat of this equation to make the presentation unambiguous. I also note that an em-dash is used in place of a minus sign.
L89: It would be helpful to be clear about whether y' is added to or subtracted from the observation.
L111: The text indicates that the innovations are normalised by the error estimates. Figure 2 however, also normalises by the background bending angles. Therefore, I would suggest replacing "error estimates" with "the background bending angles and by the estimated observation uncertainties".
L125 and L136: I suggest replacing "error" with "uncertainties", since they are not observation errors.
L135: Please remove "more physically-based". The authors have not presented any arguments for a physical basis for adjusting the refractivity coefficients.
L170: Please state what the change is relative to (the ROMEX control presumably).
L293: It would be worth noting that the RMSE performance at 700 hPa is worse for romex_bc and (to a lesser extent) romex_nl than for romex. This is despite the specific humidity biases (relative to ECMWF) being smaller at this height.
L330: The manuscript makes no mention of the possibility of adjusting other coefficients in the refractivity formulation, or the possibility of using more accurate refractivity equations than that of Smith and Weintraub. I feel that these topics should be mentioned, even if they are not addressed directly by the experiments.
Typographical corrections
L9: OSE is normally written as "observing system experiments" rather than "observation".
L18: Please change "treatment of the observation" to "treatment of observations".
L25: Please change "low-Earth orbit" to "low-Earth orbiting".
L152: Please delete "the most".
L199: Presumably you are using the operational analyses from ECMWF. Therefore, please insert "operational" after "ECMWF".
L229: Please insert a space between "Fig." and "6".
L241: Please delete "change to".
L277: Please change "leads to" to "demonstrates" as this reads more clearly.
L298: Please add "analyses" after ECMWF and change "observation" to "observations".
L313: Please add "for dry air" after "refractivity coefficient".