|The manuscript by Zoppetti et al. has improved in clarity for some of the aspects remarked by the reviewers. However, major revisions are still needed, and the manuscript cannot be published in the current state. My major concern is that there are mistakes in the characteristics of the instruments onboard MTG-S and EPS-SG together with the instruments Sentinel 4 and 5, whose measurements are simulated for demonstrating the performance of the Fusion algorithm. These instrumental characteristics are greatly important in this work. They must be corrected and all the results using these simulated measurements (Figures 3, 4, 6 and 7) should be done again with the correct configuration. These major errors are the following:|
1) IRS onboard MTG-S (called in the paper S4-TIR): this sensor is NOT IASI-NG (nor IRS2b) which is described by Crevoisier et al. (2014). Crevoisier et al. (2014) do NOT describe the instrument that will be onboard the geostationary satellite MTG-S together with UVN/Sentinel 4, but they only describe IASI-NG which will be onboard the low orbit EPS-SG satellite together with Sentinel 5/UVNS. IASI-NG is supposed to be S5-TIR in the paper. EUMETSAT describes IRS onboard MTG-S here: https://www.eumetsat.int/website/home/Satellites/FutureSatellites/MeteosatThirdGeneration/MTGDesign/index.html
“The Infrared Sounder (IRS) on MTG-S … with a hyperspectral resolution of 0.625 cm-1 wave-number...”
This means that IRS onboard MTG-S (in your paper S4-TIR) will have an even coarser spectral resolution than IASI (0.5 cm-1) and much coarser resolution than IASI-NG (0.25 cm-1). The SNR for IRS onboard MTG-S will be much lower than that for IASI-NG and similar to IASI. IASI, with a finer spectral resolution than IRS/MTG-S, enables the retrieval of ozone with 3,5 DOFs at most. Therefore, it is not possible to obtain 5 degrees of freedom for the retrieval of the ozone total column derived from IRS onboard MTG-S, as illustrated in Figure 3 of the paper.
In consequence, all simulations and retrievals using S4-TIR must be done again with the correct instrumental characteristics of this instrument. Figures 3, 4, 6 and 7 should be revised with correct simulations of IRS/MTG-S measurements.
2) IASI-NG (called in the paper S5-TIR): the spatial sampling for IASI-NG (called in your paper S5-TIR) is not correct. The spatial sampling for IASI-NG is the following (the same as for IASI): circular pixels of 12 km of diameters whose centers are spaced by 25 km at nadir. Figure 1 of the paper shows 5 pixels within 0.5 ° of latitude and that is not possible. One may have at best 3 pixels and typically 2 pixels within 0.5 ° (which is roughly 55 km) in both directions (along and across the satellite track). Therefore, it is not possible to have so many IASI-NG pixels within a box of 0.5° x 0.625°, but half of them. Therefore, the number of measurements used for the fusion for the given box is not correct. The spacing of the IASI-NG pixels must be corrected and the results should be revised.
3) UVNS (called in the paper S5-UV): The horizontal sampling of UVNS pixels is not correct either. As announced by ESA https://earth.esa.int/web/eoportal/satellite-missions/c-missions/copernicus-sentinel-5, the horizontal sampling of UVNS is 7 km x 7 km which is much smaller than the box of 0.5 x 0.625° in Fig. 1. Therefore, it is not possible that no pixel is available for UVNS (as indicated in the current version of the manuscript).
For avoiding all these errors, I strongly recommend using the real names of the instruments: UVNS/Sentinel 5, UVN/Sentinel 4, IASI-NG/EPS-SG and IRS/MTG-S, or at least part of their real names. In my opinion, creating new names that are only used in this paper only add confusion. In addition, “Sentinel” only refers to the UV-VIS instruments and not the TIR. Of course, the spectral resolution and horizontal sampling for each of them must be corrected.
Another major point concerns the explanation of the results of Figure 3 of the manuscript. In the revision, a new paragraph explains the gain of sensitivity in the Fused product with respect to the L2 retrievals. However, they do not explain two key points: At the lowest point (1000 hPa) and near the highest (0.5 hPa) the AK for the fused product is much stronger than the ones of the L2 products. At 1000 hPa, a local minimum is seen for the AK of all the L2 products (S4-TIR, S5-TIR and S4-UV), however the AK shows a relative maximum more than twice larger than the best case of the L2 products. At 0.5 hPa, the AK of the Fused product is 5 or 6 times larger than the individual L2 products. Elsewhere, the AK enhances by about 30 or 50 %, but not by a factor 2 or 6. Could you explain in the detail these features? Is this only a numerical feature of this single profile? What are the physical reasons for such an extremely large enhancement of sensitivity? and where would the information come from? Moreover, why vertical resolution in Fig. 3 right panel (FUS product) is the finest at 1000 hPa (above it is coarser)? This is never the case for any L2 product of ozone derived by optimal estimation or Tikhonov-Philips regularization. I cannot understand such a performance of the Fusion algorithm, which is not consistent with those of the original products. This should be clearly and thoroughly demonstrated.
1) Title: it is written “the Copernicus Atmospheric Sentinel missions” but it is not precise. The paper only refers to EPS-SG and MTG-S missions. There are other Sentinel missions that are used for the atmosphere which are not analyzed in the paper: Sentinel 5 Precusor/TROPOMI, Sentinel 3 SLSTR and there will probably be other atmospheric sentinel missions in the future. Therefore, the title should be revised: “the Copernicus Atmospheric Sentinel missions” should be replaced by EPS-SG and MTG-S missions.
2) Section 3.2: Please correct “contribute of the measures” … by “contribution of measurements”
3) Keep the same nomenclature for averaging kernels: AK or AKM, but do not use both.
4) Although requested in the previous review, authors provided only scarce additional information on “Atmospheric scenario” in section 2.1. A more detailed description of the used models should be provided.
5) This sentence and others are not correct “… ozone profiles measured in the UV region” Ozone is not measured in a spectral region. Please replace by “ozone profiles derived from measurements in the UV region”
6) In the reply, please cite every change in the response to the reviewer’s documents and the lines where they are. Otherwise, it is very difficult to track the changes done.
7) Last sentence of abstract may be cut into two.