Round 1

Reviewer 1 Report

The authors compared the integrated water vapor (IWV) product from TROPOspheric Monitoring Instrument (TROPOMI) with Global Ozone Monitoring Instrument - 2 (GOME-2) standard product against reference ground-based Global Navigation Satellite Systems (GNSS) IWV data. The overall article is meaningful and in a good present form. I recommend it for publication after addressing some minor comments.

1. Too many long sentences, I suggest using short sentences in the context.

Author Response

Response to Reviewer 1

Responses are marked in bold

The authors compared the integrated water vapor (IWV) product from TROPOspheric Monitoring Instrument (TROPOMI) with Global Ozone Monitoring Instrument - 2 (GOME-2) standard product against reference ground-based Global Navigation Satellite Systems (GNSS) IWV data. The overall article is meaningful and in a good present form. I recommend it for publication after addressing some minor comments.

Thank you.

1. Too many long sentences, I suggest using short sentences in the context.

  We have changed the longest sentences to divide them into two or more sentences. Please, see the new version with tracked changes to see the sentences that we have changed. We think the manuscript readability has improved with these changes. Thank you.

Reviewer 2 Report

Summary:

Javier et al. (2022) evaluated water vapor products from TROPOMI and GOME-2 satellites against ground-based GNSS data over Europe. Their results show that TROPOMI exhibits less bias than GOME-2 and better results in the dispersion and regression parameters. In summary, I think the logic of this paper is clear and simple and it has satisfied the requirements for publication in Atmosphere. I would like to recommend the publication of the paper in Atmosphere after some minor revisions. The following are two suggestions that I hope will be helpful to the authors.

Ÿ   Firstly, the authors just used one-year data to evaluate the performance of TROPOMI and GOME-2. Actually, the samples are relatively less but I know it is limited by the covering period of data. I mean the GOME-2 maybe also have less bias when using another period of data. So, I suggest the authors should give a small discussion in the final section.

Ÿ   Secondly, the authors focus on the general performances of two products in Europe. I suggest you should give more details on the evaluation. That is, in which region of Europe does the performance of TROPOMI / GOME-2 show good/bad, and what difference of their performances (TROPOMI & GOME-2) over Europe's different regions. It will be very helpful and meaningful to the data users.

Author Response

Response to Reviewer 2

Author responses are marked in bold

 

Javier et al. (2022) evaluated water vapor products from TROPOMI and GOME-2 satellites against ground-based GNSS data over Europe. Their results show that TROPOMI exhibits less bias than GOME-2 and better results in the dispersion and regression parameters. In summary, I think the logic of this paper is clear and simple and it has satisfied the requirements for publication in Atmosphere. I would like to recommend the publication of the paper in Atmosphere after some minor revisions. The following are two suggestions that I hope will be helpful to the authors.

   Firstly, the authors just used one-year data to evaluate the performance of TROPOMI and GOME-2. Actually, the samples are relatively less but I know it is limited by the covering period of data. I mean the GOME-2 maybe also have less bias when using another period of data. So, I suggest the authors should give a small discussion in the final section.
   
   It must be noted that Garane et al. (2022), using 2.5 years of time span and sun-photometers as reference, have obtained very similar values for TROPOMI statistics (we have added some discussion in the section 3.1 about this). We have also added the phrase (Lines 307-310): "The period considered, May 2018 to May 2019, allows us to put in context the quality of the new TROPOMI product.
However, it needs to be extended in further work to be able to generalize the conclusions drawn from the present work."

   Secondly, the authors focus on the general performances of two products in Europe. I suggest you should give more details on the evaluation. That is, in which region of Europe does the performance of TROPOMI / GOME-2 show good/bad, and what difference of their performances (TROPOMI & GOME-2) over Europe's different regions. It will be very helpful and meaningful to the data users.
   
   We have extendend the paragraph where Figure 3 is commented, in agreement to your comment. We have written (lines 250-260):
   
   The distribution is relative uniform.It is interesting to note that some coastal sites, specially in the Baltic Sea, with overestimations over 40 % of rMBE in GOME-2 are not so overestimated by TROPOMI (all of them in the range ±20 %. TROPOMI also tends to overestimation in the coastal stations, while to underestimation in the inland stations. In  fact, some inland stations around the mediterranean basin show more underestimation than in the rest of Europe, as well as two stations in the north of Norway. rSD is also generally lower in TROPOMI, with most stations below 20 %, with only two stations over 40 % in North-Eastern Europe. GOME-2 seems to show smaller values in the South-West of Europe than in the North-East. R2 is also improved in TROPOMI with respect to GOME-2 in most stations, except one in Iceland and two in the east of Norway. GOME-2 also shows lower values of R2 in some coastal stations, specially in the Iberian Peninsula.
   

Reviewer 3 Report

Manuscript ID: atmosphere-1768386

Title: Evaluation of water vapor product from TROPOMI and GOME-2 satellites against ground-based GNSS data over Europe 

Author: Javier Vaquero-Martinez, Manuel Anton, Ka Lok Chan and Diego Loyola

General Comments:

This study used one-year data of GOME-2 and TROPOMI IWV to compare with the ground-based GNSS data, and both GOME-2 and TROPOMI IWV were retrieved by the same algorithm. Several parameters are computed and compared for the two data (GOME-2 and TROPOMI). The study takes GNSS data as true for the verification; however, the retrieved GNSS IWV could involve bias, as indicated in Vaquero-Martínez et al. (2019), that made comparisons between GNSS data and radiosondes. Observations from radiosondes are more accurate and straightforward. Thus, I would recommend including verifications against radiosonde, which is more intuitive. Or, can the authors elaborate and discuss further about the differences/correlations in TROPOMI, GNSS, and radiosonde? 

Also, the TROPOMI MBE and GOME-2 MBE show different signs compared to GNSS. What do the different patterns indicate? Could the authors elaborate in addition?

 

Vaquero-Martínez, J.; Antón, M.; Ortiz de Galisteo, J.P.; Román, R.; Cachorro, V.E.; Mateos, D. Comparison of integrated water vapor from GNSS and radiosounding at four GRUAN stations. Science of The Total Environment 2019, 648, 1639-1648.

 

Minor 

Line 181: than GOME-2’s, so In the case … -> than GOME-2’s, so in the case ...

Author Response

Reviewer 3

Authors response are marked in bold

 

Manuscript ID: atmosphere-1768386

Title: Evaluation of water vapor product from TROPOMI and GOME-2 satellites against ground-based GNSS data over Europe 

Author: Javier Vaquero-Martinez, Manuel Anton, Ka Lok Chan and Diego Loyola

General Comments:

This study used one-year data of GOME-2 and TROPOMI IWV to compare with the ground-based GNSS data, and both GOME-2 and TROPOMI IWV were retrieved by the same algorithm. Several parameters are computed and compared for the two data (GOME-2 and TROPOMI). The study takes GNSS data as true for the verification; however, the retrieved GNSS IWV could involve bias, as indicated in Vaquero-Martínez et al. (2019), that made comparisons between GNSS data and radiosondes. Observations from radiosondes are more accurate and straightforward. Thus, I would recommend including verifications against radiosonde, which is more intuitive. Or, can the authors elaborate and discuss further about the differences/correlations in TROPOMI, GNSS, and radiosonde? 

  It is out of the scope of this paper to use a second reference to compare with GOME-2 and TROPOMI. Morever, Vaquero-Martinez et al. (2019) showed that, despiste the possible bias, the agreement between radiosondes and GNSS was excellent and that GNSS could be use as reference against other instruments for validation. Radiosonde stations are relatively scarce and their measurements only occur a few times a day (1-2 usually), which are not always going to be coincident with the time at which TROPOMI overpasses the station. Therefore, the small number of data pairs would prevent us from carrying out a proper validation.
  
Also, the TROPOMI MBE and GOME-2 MBE show different signs compared to GNSS. What do the different patterns indicate? Could the authors elaborate in addition?

  We cannot give a full explanation for this, but it could be due to the use of different wavelength windows (614-683 nm for GOME-2 and 435-455 nm for TROPOMI) for the water vapor retrieval.
  GOME-2 and TROPOMI data sets are retrieved in completely different spectral bands with different techniques. Chan et al. (2020) showed that, for GOME-2, the bias between blue and red retrieval could be up to 2 mm. In addition, GOME-2 and TROPOMI are two instruments, and some small bias between them is expected. Cloud filtering might also play a role here, as GNSS data are more sensitive to water vapour in cloud. Including cloudy data in the retrieval would also cause the negative bias for measurements in the blue band. Finally, the different passing times of both satellite instruments (GOME-2 in the morning, TROPOMI in the afternoon) can cause some differences in bias as well.

Chan, K. L., Valks, P., Slijkhuis, S., Köhler, C., and Loyola, D.: Total column water vapor retrieval for Global Ozone Monitoring Experience-2 (GOME-2) visible blue observations, Atmos. Meas. Tech., 13, 4169–4193, https://doi.org/10.5194/amt-13-4169-2020, 2020.

Vaquero-Martínez, J.; Antón, M.; Ortiz de Galisteo, J.P.; Román, R.; Cachorro, V.E.; Mateos, D. Comparison of integrated water vapor from GNSS and radiosounding at four GRUAN stations. Science of The Total Environment 2019, 648, 1639-1648.

 

Minor 

Line 181: than GOME-2’s, so In the case … -> than GOME-2’s, so in the case ...

 Thank you, we have corrected this

Round 2

Reviewer 3 Report

1.     I understand some limitations to using radiosonde for the comparison. Although the manuscript had listed some references in the early version, I do think it is better to put some descriptions about the data quality of the GNSS IWV in magnitude, which could refer to other studies, that would be clear for readers to catch the difference.

2.     Recommend the authors discuss the possible reasons for the different signs (TROPOMI MBE and GOME-2 MBE) in the revised manuscript.

Author Response

Reviewer #3

1.     I understand some limitations to using radiosonde for the comparison. Although the manuscript had listed some references in the early version, I do think it is better to put some descriptions about the data quality of the GNSS IWV in magnitude, which could refer to other studies, that would be clear for readers to catch the difference.

We have added the following sentences to the introduction (lines 36-41):

Guerova et al. [12], in a review including several inter-technique comparisons concluded that ground-based GNSS has root mean squared errors corresponding to 0.4-0.6 mm in integrated water vapor (IWV).The presence of systematic errors can be due to satellite and receiver instrumentation effects, in situ environmental effects, and modelling approximations. Therefore, they are very dependent on the specifics of the station and period of study.

2.     Recommend the authors discuss the possible reasons for the different signs (TROPOMI MBE and GOME-2 MBE) in the revised manuscript.

We have added the following sentences to the discussion (lines 232-238):

The different bias sign in both satellite instruments could be due to the use of different wavelength windows (614-683 nm for GOME-2 and 435-455 nm for TROPOMI) for the water vapor retrieval. GOME-2 and TROPOMI data sets are retrieved in completely different spectral bands with different techniques. Chan et al. [19] showed that the bias between blue and red retrieval in the case of GOME-2 could be up to 2 mm. In addition, GOME-2 and TROPOMI measure water vapor at different passing  times, and this could also have some influence in the reported bias.