Future Changes in Wave Conditions at the German Baltic Sea Coast Based on a Hybrid Approach Using an Ensemble of Regional Climate Change Projections

Round 1

Reviewer 1 Report

The paper touches an interesting topic but it needs an in depth analysis of the obtained results moving from a descriptive presentation of the statistical results to an explanation based on physical principles discussing:

• What are the actual advances with respect to the state of the art?

• How these advances can be applied to the same Baltic region or to other regions improving the understanding on how wave average and extreme values will evolve under climate change projections?

• More specifically, when mentioning that the obtained results are suited to coastal nearshore processes (line 42) this should be presented in much milder terms since the analysis does not include typical wave nearshore propagation processes nor the expected bathymetric change.

• When presenting the types of uncertainty (lines 49 to 53) mention is made of five main types of uncertainty, which is all right. However, in other sections of the paper (e.g. lines 92 and 93 and others) a much more simplified analysis is presented. The suggestion is to carry on with the initial presentation of five types of uncertainty thorough the paper.

• When discussing the results, the paper should go beyond the present descriptive presentation (e.g. in lines 57 and 58) when it says that “no agreement has been found … regarding … changes of extreme wave events in the Baltic Sea). Here it is just an introduction but the suggestion is to use those limitations in the present state of the art to show what is the actual advances of the proposed work.

• The paper should also incorporate more references to the state of the art, increasing the number of citations from 2015 onwards.

• When discussing the future changes on average and extreme waves a more in depth discussion would benefit the paper, distinguishing between physical processes (for instance the different behavior of waves models for average and extreme conditions) and statistical properties (for instance, considering if they belong to two different populations).

• When presenting the time and space resolutions mention is made of the one day resolution for EUROCORDEX. However, there are alternative products (for instance those of the European center (ECMWF) with resolutions of 6 hours and down to 1 hour). The effect of spatial and temporal resolution should also be discussed with reference to physical processes such as for instance the duration of the storm or the topo-bathymetric gradients (lines 96 to 103 and following).

• When presenting the runs driven by ERA-Interim data, it would be important to justify what is the expected achievement of the “evaluation”. In other words, what will be achieved and what are the differences between CMIP5 and ERA-Interim for this analysis (lines 118 and following).

• When discussing the lack of accuracy of point values (lines 128 and following) some more discussion of what is the error introduced by the horizontal resolution and by the spatial averaging should be included, linking that to the sensitivity analysis (mentioned in section 2.2).

• Regarding the sensitivity analysis, the paper would benefit from a more quantitative discussion on how the sensitivity and error levels vary depending on the variable and the processes considered, not only the resolution. Here would be a good opportunity to discuss what is the actual advancement with respect to the state of the art.

• When presenting the four locations at the German Baltic Sea coast it would be important to discuss how those four locations are representative of Baltic conditions and the physical rationale for selecting them. While the availability of data may justify that selection it would be important also to present the selection in more physical terms, particularly when extrapolating to future climate and distinguishing between eastern and western conditions as mentioned in the paper.

• When presenting (lines 159 and following) the bathymetry and its discretization it would be important to discuss what type of errors are present even for deep water unless they do not affect any of the wave analysis. However, parameters like fetch, not mentioned in the paper, should depend on the domain geometry and are important for the analysis.

• When discussing the hybrid approach a more in depth analysis of how the accuracy is improved with respect to the state of the art would benefit the paper since this hybrid approach lies at the core of the analysis. In particular, since the wave model is used in non-stationary mode and that is associated to a higher accuracy, this should be discussed considering what would be the performance for non-stationary simulations, such as for instance, under rapid storm development associated to future climate scenarios.

• When presenting the results (line 210 and following) mention is made of larger biases over 30 years seasonal averages. However, they are labeled only as possible. This should be discussed more in depth and the misinterpretation mentioned in that same paragraph should be discussed with specific ways proposed to correct it.

• When presenting the biases which are assumed to be negligible because of changes in futures wind and waves, that should be also put in context since under future climate evolution that doesn’t need to be necessarily so and the presented conclusions will depend on the relative level of the variable and the type of variable, including the possibility of coupling between variables and therefore affecting the conclusions.

• When presenting tables 2 and following to indicate which variables present “future changes” I think it would be useful to include in these tables some sort of “quantification”, for instance with colours or percentages. Otherwise, it remains too qualitative and at least in my opinion it does not clearly show any advance that can be extrapolated to other areas.

• When presenting the future changes (end of section 3.1, lines 266 and following) it is conclude that climate change signals are more frequent for RCP 8.5 and by the end of this century. That is quite obvious and it should be presented in more quantitative terms and making reference to the results from the performed simulations.

• When discussing the intra-annual viability (lines 230 and following) this is a point that should also be related to the physical processes typical of transitional seasons. A discussion on how those processes are expected to change with climate projections would be in order here.

• When presenting the results in the following lines (e.g. 284 and following) the results are very qualitative mentioning that only “few changes” of the wind and wave variables can be linked together. This is somewhat unexpected since wind and waves for the Baltic Sea where there is no swell in significant energy, should be more clearly linked. The discussion would also benefit by considering more wave field details such as the frequency bands or the spectral shape.

• When presenting the future changes for extreme waves some more discussion on the stability and transient character of the time series should be included, linking that to the type model to be used.

• When discussing the probability distribution for extremes (lines 315 and following) the log normal distribution is selected because of previous studies. Here, a due justification to this selection showing that this log normal distribution is also the one providing a better fit for this data set should be included.

• When presenting the extreme results, not only the average trends should be discussed but also the width of the confidence interval since that would help to quantify error bars.

• When presenting the results for the westerly wind expose locations (line 338 and following) and this also applies to other result presentations, some more in depth analysis of why there are no trends on extreme wave projections should be included. This should be related to the physics, the conditions of the exchanges between a semi-enclosed sea and the atmosphere and the presence or evolution of stability in the air sea boundary layers.

• When comparing the results (section 4.3) mention is made of the REMO/MPI-OM model with a resolution of 50km and 3h time steps. The paper should discuss how this is consistent with a previous selection of time and space scales and why the changes of extremes have not been assessed for this data set because of the “cost temporal resolution”. A 3 hour temporal resolution is not to coarse depending on the type of analysis to be performed, and this should be better justified.

• Finally the conclusions section should be more closely linked to the research work presented in previous sections, in particularly considering how the fact that the Baltic Sea is a semi-enclosed domain with limited swell, affects the projections. In the same way as for previous sections, the results should be assessed in more physical and explanatory terms, going beyond the present statistical description included in the paper.

• In general terms the paper should also improve the English grammar, introduce some missing words (e.g. “and” in line 146) and delete the “error references source not found” so often found throughout the text.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Paper presents look at the impact of climate change on wave conditions along coast of Baltic Sea and thus makes a contribution to understanding the impact of CO2 emissions on the coastal dynamics.

Some items of concern.

• The paper uses lots of acronyms most of which are never explained. It makes the paper really hard to read and understand.
  * "Error! Reference source not found" is found repeatedly throughout the paper. Needs to be fixed.
  * The climate change cases investigated are never described in any detail.
  * The paper focuses on the change in wave conditions along the Baltic coast due to the impact of climate change. What seems unusual is that there is no summary of the impact of winds, which are the underlying cause of the changes in waves.
  * Found the paper really hard to read and follow.

Extensive comments found on attached 

Comments for author File: Comments.pdf

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Add a list of acronyms at the end of the paper