see attachement

Review of Nimac et al.: The added value and potential of long-term radio occultation data for climatological wind field monitoring”, submitted to Atmospheric Measurement Techniques.

The paper reports on a study of global wind-field monitoring from the middle troposphere to the middle stratosphere using the satellite-based Radio Occultation (RO) technique. The focus is on climate applications, hence monthly means and a rather coarse 2.5x2.5 degree latitude-longitude resolution. The latter is also a consequence of the limitations of the RO observing system.

Winds are derived from the gridded RO data by means of the geostrophic and gradient wind approximations. The idea is to use ERA5 reanalysis data as a reference in two ways: by computing wind fields from ERA5 using the same approximation as for RO, and by using the original ERA5 winds. In this way the approximation itself can be evaluated (ERA approximation vs original), RO data can be evaluated (RO vs ERA approximation), and the general ability of RO-derived wind fields to represent the real winds can be evaluated (RO vs ERA original). Key questions addressed are where and under what circumstances the approximation holds and RO can be expected to provide useful information on the wind.

Scientifically, the study presented is perhaps not a major leap forward, but it is a well-designed study, it fills a gap in the litterature, it is well-written and easy to follow and it provides practically useful information. For anyone interested in generating atmospheric wind fields from RO data, or that is interested in the validity of the geostrophic and gradient wind approximations, this paper will provide highly valuable information.  

The manuscript is well worth to be published. I mainly have one set of questions that I would like to see clarified. That relates to sampling and data retrievals. See the comments and questions below. I believe that addressing these questions and issues will improve the manuscript. 

Comments and questions

In Section 2.2, it is described how the monthly 2.5x2.5 degree grids are derived for the RO satellite data. For a calendar month, all RO profiles within 600 km from the center of a grid box are averaged with data weighted less with increasing distance from the center, following a Gaussian.

1) What is the width of the Gaussian?  

2) Doesn't this mean that there are more data points per monthly mean at high latitudes compared to low latitudes, given the polar orbits of the satellites? Does this have any implications for the sampling errors of the monthly means?

3) Are the monthly means adjusted for sampling errors? It appears to be used in other studies using RO satellite data for climate studies.

RO data retrievals: The key variable used in the study is the geopotential height of isobaric surfaces, i.e. geopotential height as a function of pressure. How is pressure retrieved? It is mentioned that "background information (re)analys data" is used in the retrieval. As an alternative, the pressure can be retrieved from the refractivity without such background information ("dry" retrieval).

4) Which data are used as background? If ECMWF reanalysis is used, does it have any consequences for the comparison between RO and ERA5?

5) Is the "dry" retrieval used in this study, e.g., at altitudes where humidity is small? If so, describe this.

In Section 2.1, it is briefly described that the geostrophic and gradient winds are computed from isobaric geopotential height data on a monthly 2.5x2.5 degree grid.

6) To avoid that sampling errors have an impact on the results, wouldn't it be an idea to extract ERA5 profiles co-located with the RO profiles and then compute the ERA5 wind fields from that sampled data set? Or is that the way it has been done? If not, could you comment on that, e.g., why sampling issues are not likely to change the conclusions of the study. 

You also mention that you apply a 5-point Gaussian filter to the geopotential fields before computing the geostrophic winds.

7) What is the reason for this additional smoothing? Do you apply this smoothing to both RO and ERA5, or only to RO? 

To do before acceptance

- Provide answers and explanations to the above questions and comments.

- Provide a bit more information in the manuscript (preferably in Section 2) as to how the RO-based geopotential heights where actually computed from the observed refractivity profiles.

- Provide some mentioning of sampling errors in Section 2.