See attached pdf.

Do not hesitate to write if clarifications are needed.

General appraisal

The manuscript by Loisel and co-workers presents a synthetic data set of so-called apparent optical properties (AOPs) generated through simulation of the radiative transfer (RT) in oceanic waters. The goal is to help the development and test of inversion algorithms that use these AOPs as input (generally the remote-sensing reflectance, Rrs) and attempt to derive quantities such as the inherent optical properties (IOPs, here the absorption and backscattering coefficients) or biogeochemical / ecological properties such as the phytoplankton chlorophyll concentration (Chl). The Rrs are generally derived from satellite ocean colour observations, although the inversion can also be applied to Rrs derived from in-situ radiometry measurements.

Numerous such data sets exist and the justification for proposing this new one is that the IOPs used as inputs to the RT simulations are made more representative of the real world by using probability distribution functions that are consistent with what global in-situ or satellite data sets reveal.

I guess this is an important feature if the data set is then used in its entirety to, e.g., train ML-based algorithms such as neural networks. In such a case, it is indeed critical that the training data set is as realistic as possible. If the data set is used by expert users who can appropriately select what they need in this simulated data (e.g., they can pick up whatever range of the various input parameters that they think is relevant to their development), then the justification for this new data set is a bit weaker.

In any case, I think it is a very useful data set. I appreciate that the authors have included not only the Rrs spectra but also most apparent optical properties as well as the full radiance distributions. This will undoubtedly make this dataset of a broader interest.

The effort to make the synthetic data set better representative of the real global ocean, which is largely made of clear and moderately clear waters, is also a very good one. As clearly shown by Fig. 6, previous synthetic data sets did not match at all what the global ocean is. They were largely skewed towards high values of most IOPs, which is rather typical of coastal waters. Still, they were often used to develop and test algorithms that are then applied globally. To my opinion, this situation has led to frequent overstatement about the performance of inversion algorithms applied to satellite ocean colour.

The use of the Hydrolight RT code is a relevant choice, in particular because it allows Raman scattering and Chl fluorescence to be simulated. I would however recommend that the authors make clearer in the paper that modelling Chl fluorescence includes several assumptions so that any algorithm development using the part of the spectrum affected by Chl fluorescence will be largely depending on those assumptions. They allude to this on lines 577-578 (by mentioning that the quantum efficiency of Chl fluorescence was set to a default value) but a better description of this part and of the associated limitations should be included.

The selection of possibly realistic combinations of absorption by phytoplankton, coloured dissolved organic matter (CDOM) and non-algal particles is important if users of the data set aim at retrieving these quantities. However, the authors know that RT in the ocean, for given boundary conditions, is entirely driven by only two inherent optical properties: the total absorption coefficient and the total volume scattering function (VSF). It does not matter for RT whether a given total absorption at a given wavelength is generated by various proportions of phytoplankton or water or anything else. The important parameters are therefore the ratio of particulate to molecular scattering (defining then the total VSF) and the ratio of total absorption to total scattering or total attenuation. Therefore, although I recognise that it is useful to see IOP distributions for components like phytoplankton and CDOM (Fig. 2), it would be equally useful to see the distributions of the parameters mentioned above. With what is presented, we cannot be sure that they are well covered.

A few more detailed comments

• Title: a data base of what? Maybe that could be said (Although the title is already quite long)
• The entire section 2 is quite dense and not that easy to read. Some sub-headings might help better figure out the various steps followed in generating the IOP set. I guess many details could also be moved into a supplemental section, at least if the Journal allows it.
• Line 115: the use of Zhang et al to calculate the seawater scattering coefficient implies that a temperature and a salinity have been chosen. This should be indicated, and it should be made clear that this data set cannot be used for freshwater, for instance. The manuscript title says “global ocean” but we know that not all users might be that careful.
• Paragraph 199-214: the in-situ data bases that have been used should be listed and acknowledged. I know it can be a bit cumbersome because there are many, but a Table would make this efficiently. Citing papers that have previously used these data bases is not the correct way of doing it.
• The spectral values of the particulate backscattering coefficient resulting from summing up the phytoplankton and non-algal particle backscattering coefficients derived through Eqs. 8 and 11 should be displayed for some Chl values (or the spectral slope as a function of Chl could be displayed). That is an important parameter and it is unclear how it looks like.
• Pages 12-13: not sure I have got the rationale for the P1, 2, 3, 4 parameters and corresponding equations in Table 1. This should be better explained.
• Pages 5-9: this is quite long for describing how absorption is modelled. And, there is conversely little about scattering (discussing the phase function only comes at page 14).
• Fig. 9 would be advantageously completed by a similar plot of Rrs490/Rrs555 vs. Chl.
• Another very important parameter to show would be the ratio of CDOM absorption to total non-water absorption vs. Chl. I guess what I am trying to say here is that a more comprehensive assessment of how this data set compares to previous ones and to established Case 1 water bio-optical models would be really useful.

Review of “A synthetic database generated by radiative transfer simulations in support of studies in ocean optics and optical remote sensing of the global ocean” by H. Loisel, D.S.F. Jorge, R.A. Reynolds, and D. Stramski

The Authors present a new synthetic dataset for use in satellite ocean color algorithm development and refinement activities.  In my opinion, this dataset improves upon previous versions (that also focused on the global ocean) in its attention to realistic distributions of bio-optical inputs, inclusion of various flavors of inelastic scattering, and the utility of including depth-resolved parameters as output.  Overall, I found the manuscript to be mostly clear and otherwise very well written.  I have no major concerns regarding its acceptance and publication. I have several minor editorial comments provided below that might be addressed.

Line 49:  Suggest providing units the first time a variable is introduced.

Line 59:  Suggest expanding the SeaBASS acronym and providing a URL or reference.

Line 78:  Suggest changing “the publicly available” to “a widely-used publicly available”.

Lines 83, 126, 131, 141, elsewhere (please check throughout the manuscript):  which -> that

Line 93:  By “driven” do you mean “described”?

Line 98:  No “and” in the PACE acronym.

Line 246:  Suggest changing “a couple of” to “several”.

Lines 242-264:  So, I think I understand what was done, but it took me several reads.  The paragraph reads clumsily to me – wondering if there’s a way to tighten it up (make it more concise and clear) and/or add a flow chart? Also, how many aph’s were originally hyperspectral and how many fit into each of the two multispectral categories?

Lines 275-277:  I’m wondering if it’s unequivocally ok to assume that just because two spectra match in the 400-750 nm region that they will also match in the 350-400 nm region.  Could you comment on this? 

Line 472:  Suggest adding a reference for the 0.01 value.

Line 486:  Suggest adding a reference for the 0.018 value.

Table 1:  Please add references for the expressions or reiterate in the caption that these expressions are those used in previous studies (e.g., IOCCG 2006, Craig et al. 2020).