This is an interesting paper and the results are very promising. There are a few things that need improvement though, for instance parts of the manuscript should have more mathematical rigor instead of attempting to describe in words certain formulations. A more detailed description on computational times should also be included, since the focus is almost entirely on the comparison between numerical predictions of SWAN and DELWAVE. This is certainly important, but your initial argument was that DELWAVE was a mean to save computational time, and I did not find a convincing argument as to whether you have shown this in your paper. Please find in the attached document a complete list of minor and major points that I would like the authors to address before I can reconsider this manuscript for publication.

This study presents an emulator for wind-waves in the Adriatic Sea that is claimed to be capable to produce “numerically cheap large-ensemble predictions over synoptic to climate time scales”. While the procedure following which the emulator was built, trained and tested is well-described and seems sound (this should be evaluated by an expert in machine learning techniques), the setup and evaluation of the SWAN wave model is lacking. Further, no discussion about the choices made to build the emulator is present. Consequently, although the scientific significance and quality of this manuscript for the understanding of future wind-wave climate in the Adriatic Sea has the potential to be high, it is, in the current stage, not convincing. I list below some major comments to support the authors with the resubmission of their manuscript.

Major comments:

The main issue with the article is the relatively low resolution used in the COSMO-CLM and SWAN models. In particular, the SWAN model horizontal resolutions reach at best 2 km for the AA station but up to 8-9 km for the MB station and about 6 km for the remaining stations. In the southern Adriatic the resolutions are close to the Med-CORDEX regional climate models covering the full Mediterranean Sea at about 12 km of resolution. Further, from Bonaldo et al. (2020), it seems that “the minimum water depth in the model grid equals approximately 8 m”. In my opinion this defy the purpose of using an emulator for the wind-waves at only 6 locations of interest. Such an approach should, in fact, allow to reach a resolution of few (maybe hundred) meters at locations of interest (where bathymetry should be updated with observations; e.g., multi-beam or LiDAR) in order to properly resolve refraction, diffraction, shoaling, reflection, etc. of the waves along the coastline.

The choice of the locations where the emulators were built is also a bit puzzling and unexplained. Why the stations OB2 and OB3 are of any interest? Why the emulator results are only presented for AA, OB and MB and not at Grado, for example? Why emulators were not built for all the major coastal cities along the Adriatic coast and/or the Adriatic LNG terminal, the major commercial harbors like Koper, etc.? I understand the choice to include the wave buoy stations but not to limit the emulators to it.

This leads to another important point. The evaluation of the SWAN model against observations is not presented. The COSMO-CLM model has been evaluated for the (EURO-?) CORDEX domain by forcing its boundaries with reanalysis (i.e., ERA-Interim). The evaluation of the SWAN model should thus be performed during the period of this control run for extreme events (and not as a climatology like done in Bonaldo et al., 2020) and compare with the available observations in order to assess the capacity of the COSMO-CLM and SWAN models to reproduce bora/sirocco winds and wave parameters, respectively. Without such an evaluation for extreme events, the skills of the COSMO-CLM and SWAN models during sirocco/bora events, and, hence, of the emulator, cannot be thoroughly assessed and no conclusion about the quality of the results presented in the manuscript can be reached (i.e., an emulator can only be as good as the geoscientific models it is built with).

In terms of the technical implementation of the emulator, I would recommend the article to be reviewed by an expert in machine learning. The article is presenting a lot of details about the way the emulator was built that only such a specialist can accurately review.

Another important question not discussed in this paper is whether or not the emulator can be used with other regional atmospheric forcing than COSMO-CLM. In terms of producing robust ensembles to cover the climate uncertainty, it is crucial to use as many different atmospheric climate models as possible. As nevertheless the COSMO-CLM model is corrected with the ERA5 reanalysis, one can even ask, why not using directly the CMIP6 100-km resolution ensemble of atmospheric models (obviously corrected with ERA5 in order to catch extreme events) to implement, test and train the emulator. As it seems that the authors limit their study to wave buoy locations, it could even be envisioned to directly use the wave observations to build such an emulator (and maybe even skip the SWAN modelling). I am not suggesting that it is a better method but I am just highlighting that, over all, there is a lack of discussion concerning both the choices made to build the emulator and the practical use of the emulator for climate studies.    

Finally, the last major point is that the authors did not prove that their emulator was cheaper than, for example, look-up tables that are commonly used for wind-waves. In combination with the relatively low resolution of the SWAN model and the lack of evaluation of the COSMO-CLM and SWAN models during storm events, the manuscript thus fails to prove the added value of the DELWAVE model.

Overall, even if the mathematical exercise of setting up an emulator for wind-waves in the Adriatic Sea is interesting as such, I am not convinced that, the DELWAVE emulator can really be used for the intended purposes stated in the introduction: study “morphodynamic processes”, the “safety and durability of human infrastructures, along the coast and offshore” and assess “the feasibility and improving the design of wave energy converter facilities”.

Reference:

Bonaldo, D., Bucchignani, E., Pomaro, A., Ricchi, A., Sclavo, M., and Carniel, S.: Wind waves in the Adriatic Sea under a severe climate change scenario and implications for the coasts, International Journal of Climatology, 40, 5389–5406, https://doi.org/10.1002/joc.6524, 2020.