This study does an impressive job of consolidating the information in a wide variety of data sets, and manages to nicely balance depth of analysis with breadth of topic.  The results are relevant and interesting, and reveal several important insights.  For these reasons, the study is appropriate for publication.  I recommend some minor changes to the text.

The first recommended change is to revamp the discussion of diurnal cycle in around line 465 and elsewhere.  The text mentions potential biases from a lack of ‘full day’ observations without being specific about what this means.  Sun-synchronous satellite measurements of the true diurnal cycle (exactly 24 hour period) taken every 12 hours will average to the diurnal mean.  Consequently, the long-term mean will be unbiased.  In contrast, sampling once daily will introduce a bias since the average is over only a single phase of the diurnal cycle.  This is the case for the daytime-only satellite observations.  The distinction between these two situations isn’t clear from the text, but some minor editing should fix this.  (A semidiurnal cycle in TCWV or the effects of diurnally varying clouds on sampling further complicate the picture.  This is worth mentioning, but beyond the scope of the study.)  Because of the potential for the diurnal cycle to introduce a bias, the study should mention the times of day of all satellite data sets and whether the observations are obtained only during day or night.  Some information about orbit local times and day-only sampling is provided in the current text, but not for every data set.  Consistent statements for all satellite data sets would be helpful. A table may even be appropriate, though that is at the authors’ discretion.

A discussion of HOAPS data set is needed, especially since it is the reference data set over ocean.  (The first reference on p. 9 does not even spell out the acronym.)  Even a short paragraph would be very helpful.

The text would also benefit from a brief discussion of the use of ocean surface temperature and near-surface air temperature over land in calculating the temperature-TCWV regression.  The use of different reference temperatures has implications for interpreting the temperature-humidity relationship over land versus over ocean, and intercomparison of land and ocean relationships.

Here are more specific comments:

Line 38 forward.  How are these goals different from the first phase of the GVAP assessment?

Line 70.  It’s not obvious that Aqua is included in this study.  Words like “now includes both Terra and the Aqua” would be helpful.

Section 2.1.1.  The local time of the Aqua spacecraft is needed here.  Also, the reference to Manning et al. on line 83 should be Susskind et al.  Here is the first link on a web search of the title: https://docserver.gesdisc.eosdis.nasa.gov/public/project/AIRS/L2_ATBD.pdf

Sections 2.1.2.  Please mention the local time of the satellite observations, and/or the drifting orbits, as appropriate.

Section 2.1.3.  Are the GOME observations obtained day and night, or 10:00 daytime only and not 22:00?

Section 2.1.4.  Some mention is needed of which satellite data sets considered in the study are also assimilated into ERA5.  Even a statement like “nearly all” would be helpful.

Section 2.1.5.  The local times of Terra and Aqua overpasses are needed, and it should be stated that the retrievals are available both day and night (if so).

Section 2.1.6. Are MPIC daytime-only observations?

Line 175 forward.  Is the lack of SSM/I etc., data different from ERA5?  That seems to be the major distinction, but it’s not stated.

Line 195.  Is this bilinear interpolation in latitude and longitude.  This is worth mentioning because bilinear interpolation is actually a quadratic function, so has a specific (if confusing) definition.

Line 224.  This paragraph on comparison temperatures is worthy of its own short subsection.

Line 237.  As discussed above, this is the first mention of HOAPS.

Line 284.  The region described as California is almost entirely along the coast of Mexico. (The Baja California Peninsula is in the northern part of the region of interest, but is within Mexico and mostly outside of the California climate region.)

Line 305.  Is this the first use of Delta-TCWV?  It does not appear to be defined earlier (or else I can’t find it).

Line 317.  We noted the wet biases in stratiform regions in Fetzer et al. (2006) and attributed them to a combination of subsidence-induced extensive cloudiness and low TCWV, while clearer conditions have higher TCWV.  Interestingly, that study considered only a few weeks of observations.

Line 339.  Fix Figure ??.

Line 347.  The ‘also’ can be deleted since it’s redundant with ‘In addition’.  A similar argument can be made for ‘also’ and its relationship to ‘include’ in the next sentence.

Line 352.  The sentence starting ‘The UWHIRS…’ should start a new paragraph since the HIRS data sets obviously deviate from the other data sets.  This is worth be highlighting.

Line 358.  Change “range of regression coefficients are” to “…coefficients is” since ‘range’ is the subject.

Line 361.  Is the ‘theoretical range’ the one set by the Clausius-Clapeyron relationship?  It’s not obvious from the text.

Line 365.  Should “noise” be “large, and likely anomalous, high variability” or something like that?  A noise mechanism hasn’t been clearly established.

Line 388.  Change to “Also noteworthy are…”

Line 405.  Change the sentence ending slightly to “…trend estimates discussed earlier.”

Line 411.  That should be “at least 50% and 100%” since ‘or’ implies either one or the other, but not both.

Line 415.  Are the “expected regions” those mentioned in Table B1?  The text is ambiguous here.

Line 416.  Changing “observed” to “observe” will made this statement present tense, like others in the section.

Line 425.  Delete “partly” or else explain the differences more fully.

Line 430. State the reference data sets used to generated the metrics since they are fundamental to the results.

Line 440.  Are ‘expected ranges’ determined by the Clausius-Clapeyron relationship?  In any case, the reason should be given.

Line 444.  Shi (2018) should be Shi et al.

Line 445.  Not clear what is meant by ‘that time.’

Line 465.  See earlier comments about diurnal sampling with sun-synchronous satellites.

Thank you for the all the careful work!

Eric Fetzer

I thoroughly enjoyed reviewing this manuscript. The authors work with an interesting dataset of global total column water vapour measurements from satellite observations and reanalysis within the GEWEX water vapour assessment. The evaluation was conducted through multiple approaches, some discrepancies and biases are observed, particularly in regions with complex topography or under certain meteorological conditions.

I believe that the manuscript addresses a relevant topic and includes a timely discussion. This is a well-written manuscript that only needs to undergo a few minor changes in addition to the other reviewers' comments:

1. L193: please justify the reason for conducting this evaluation based on the monthly mean at 2° x 2°, since the coarse resolution may overlook some discrepancies among the datasets.
2. L296: please briefly explain the reasons that there is a “significant disagreement” between the datasets for dry atmosphere.
3. L339: missing cross-reference for the figure.