In this study COSMIC radio occultation and ECMWF reanalyses data is used to estimate trends in tropospheric water vapour. The trend analyses is performed globally for specific latitude bands, separating the globe into latitude bins of 20° and on three pressure levels, namely 300, 500 and 850 hPa. Before the trends are estimated the data sets are inter-compared.

General comments:

The study itself is worth to be published, however needs major revisions before publication in ACP. Since in this study first a detailed inter-comparison is performed followed by a quite detailed trend analyses, the paper becomes very long and hard to follow. The current version of the manuscript gives the impression that actually two manuscripts have been combined. I would suggest to significantly shorten the paper, especially the inter-comparison part. The most important results of the inter-comparison should, however, be provided in an appendix or supplement to this manuscript because knowing the differences between data sets is important for interpreting the derived trends.

A motivation for why a separation into 20° latitude bins has been performed. Accordingly, a motivation for why the three pressure levels 300, 500 and 850 hPa have been used is missing. Why do you look at the seasonal cycle before calculating the trends?

Further, the manuscripts need significant improvements in writing and presentation of the results. Most of the figures and all tables need to be improved. 

Some figures are not really concise and use to small fonts.

Instead of water vapour variation you should clearly state “seasonal cycle and trend”. The time period considered could also be mentioned in the title.

Specific comments:

P2, L38-41: This paragraph is too general and too broad and thus a bit out of the context of the study and thus not useful at all. The whole paragraph should be removed.

P2, L46-47: I would suggest to put the references at the end of sentence.

P2, L58: Please rephrase the sentence. “such as” is not correct here. It should rather read “and”.

P6, L196: Which selected months? Do you mean January and July? Why have these two months been selected?

P7, Figure 1:  Why is the comparison done for ERA-5 and ERA-interim? Why not only ERA-5?

P9, Figure 2: Put the labels of the panels at the top left of each panel.

P9, Figure 2: I am surprised by the good agreement between the two data sets and was wondering if COSMIC data is assimilated into ERA-5. If yes, this needs to be considered in the interpretation and discussion of the results.

Figure 1-2 and according text could be moved to an appendix/supplement.

P10, L255ff: Is this shift in the NH/SH due to the ITCZ? If yes, then I assume this figure would look different for other months? Which month actually is shown here?

P11, Figure 3: The time period that has been considered should be added. Has an average over the 2007-2018 period been considered? For which month is shown in this figure? Or is here an average over all months/years shown?

Figure 4-6: I am not happy with these figures. In my opinion these are two overloaded and hardly readable. I am not yet convinced the figure 4b, 5b, 6b. These could be moved in an appendix/supplement. Or consider rearranging the results presented. See the following comment.

P12-14, Figure 4-6: My suggestion would be to completely change the way of presenting the results for the seasonal cycle. Wouldn’t it be better to show the NH and SH separately and then use one figure for each hemisphere showing the results for the three pressure levels. You then could have additionally one figure showing the differences for the three pressure levels (and as now with differences for both hemispheres).

P15, Figure 7: I am also not happy with this figure. Is it really worth to who three pressure levels? The results are quite similar and thus there is no need to show in all figures all three levels. Also I would suggest to improve the figure so that the hemispheres can be better compared. One way of doing this would be to add a vertical line in the middle of the plot separating the NH and SH bars.

P18, L451: Add references.

P19, L472: Which latitude bin? 0-20°, thus tropics?  What trend do you derive for the other regions?

P22, Figure 10: Add a vertical line at 0° to visually separate NH and SH. You could also write in the plot SH and NH, respectively.

P23, Table 2: The table should be improved. In it’s current form it is hard to read and thus not really useful.

P26, Figure 12: I don’t understand this figure. What exactly has been done? Why is this kind of analyses useful? I think this analyses does not need to be shown in the main part of the paper and could be moved to the appendix.

P28, Figure 13: Why is here only 500 and 850 hPa shown and not 300 hPa?

P29, Figure 14 and corresponding text: It should be motivated how these sites have been selected.

P30-31, Tables 3-5: These tables are hard to read. You should find a way to present the results in a readable way. All additional information could be put into the appendix. E.g. in table 3 columns 2 and 3 could be combined.

P33, L762: ERA-5 is the latest version of reanalyses and has been significantly improved compared to ERA-interim. Thus, it is not astonishing that the agreement between COSMIC and ERA-5 is better than the agreement between COSMIC and ERA-interim. It would be maybe useful to check and discuss the results from the SPARC reananlyses comparison project (https://www.sparc-climate.org/activities/reanalysis-intercomparison/).

Technical corrections:

P1, L24: sub-tropics → sub-tropical

P1, L28: are around → “are found around” or “are observed around”

P1, L28: delete “at sites”

P3, L75: in → of

P5, L139: Put “in this study” at the begin of the sentence.

P5, L141:  delete “for global environment and weather studies”.

P7, Figure 1: Write “ECMWF” in the x-label instead of “ERA”.

P8, L220: tropics → tropical

P11, Figure 3: Put labels at the top left of each panel.

P12, L294-195: same month …..same latitude zones → each month and each latitude zone

P15, L373: “RO” obsolete → delete

P17,  Figure 8: Put labels at the top left of each panel (should be aligned).

P18, L425: “~ “should be “- “

P18, L433: Same here.

P19, Figure 9: Put labels at the top left of each panel.

P22, Figure 10: Put labels at the top left of each panel.

P22, Figure 10 caption: What do you mean with “zone—mean”? Zonal mean or do you mean the latitude bins?

P22, L535: add “tropical” after “northern” so that it reads “northern tropical 0° to 20° latitude bins”.

P24, L69: Write either “no data points” or “missing data points”.

P24, L584: [2020] → (2021)

P25, L589: [2020] → (2021)

P25, Figure 11 caption: either “no monthly data” or “ missing data”.

P26, Figure 12: Put labels at the top left of each panel.

P27, Figure 13: Put labels at the top left of each panel.

P32, L717: low latitude → tropical regions

P32, L720 and L727: trending → trending

P32, L739: tropics → tropical,  subtropics → subtropical

P33, L751: trending → trends

P34, L778: from trending → from estimating the trend for 2007-2018 from

P34, L792: tropics and sub-tropics region → tropical and subtropical regions

P35, L803 and 810: degree sign misplaced.

P35, L805: difficulty → difficult

P35, L813: having → have

P35, L814: trending → trending

P36, L829: trending → trend estimation

P36, L846: [2021] → (2021)

P38, L912: Co-authors are missing.