On Evolution of Young Wind Waves in Time and Space

Round 1

Reviewer 1 Report

The paper presents results based on a large amount of work. The experiment designs are sound and descriptions of experiment results are sufficient. Overall, I think the manuscript is publishable after addressing my following concerns. It is not clear what is new for this work compared with other work. For example, there is quite some literature on wave generation by a steady wind in lab results. The author may want to highlight the differences in this work. In the abstract, Line 17, add descriptions of the results and conclusions such as what theoretical approaches have been validated or invalidated? The major problem for this paper is that the main point is not clear. As a review paper, the author may want to provide recommendations in terms of approaches or theories by the end.

Author Response

Reply to Reviewer 1

The paper presents results based on a large amount of work. The experiment designs are sound and descriptions of experiment results are sufficient. Overall, I think the manuscript is publishable after addressing my following concerns.

I appreciate very much the positive attitude of the reviewer to my work.

It is not clear what is new for this work compared with other work. For example, there is quite some literature on wave generation by a steady wind in lab results. The author may want to highlight the differences in this work.

I certainly agree with the reviewer that numerous studies on waves generated in laboratory facilities by steady wind have been published over the years. Most those papers are cited in the present manuscript. On the other hand, the number of studies on waves excited by wind that varies with time is much more limited. However, even for the steady forcing case, the author is unaware of any study where the spatial evolution of wind waves and of the airflow above them was investigated in such a detail and for a wide range of wind velocities. Since the results accumulated over the years are spread over numerous publications, it seems appropriate to concentrate the major findings in a single review paper. Section 5 of the manuscript summarizes and discusses the major understandings gained in the course of the whole project. These points are now highlighted in the Introduction (1^st paragraph, p. 4, lines 148-158 that replaced lines 147-151 in the previous version).

In the abstract, Line 17, add descriptions of the results and conclusions such as what theoretical approaches have been validated or invalidated? The major problem for this paper is that the main point is not clear. As a review paper, the author may want to provide recommendations in terms of approaches or theories by the end.

In view of the Reviewer’s comment, the Abstract has been revised with the goal to emphasize new findings. The Abstract, however, is limited to 200 words. This limit prevents giving a more detailed description of the novelty of findings. The recommendations regarding the possible venues for modeling wind waves are presented in Section 5.4.

Reviewer 2 Report

The present manuscript includes state of the art measurements of different observables in a wind wave experiment. Accurate measurements, using different techniques, are reported and discussed. The manuscript discuss a very complete data set including space time measurements of the surface elevation and wind under very different forcing conditions. An extensive discussion on the results is then reported and the limitations of these kinds of experiments for the interpretation of ocean waves is discussed. A review of the state of the art experimental and theoretical results is also included.

I find the present manuscript impressive for the quality and quantity of the the results presented. Sometimes there is a lack of explanation of the observed results but I guess that it is not possible to properly understand what is going on in such a complex dynamics. Just to give an example, the author finds a value of the kurtosis which is well below the Gaussian expectation; I have already seen this effect in data recorded from wind wave facilities but hardly the kurtosis of teh surface elevation in real ocean data is below 3 (at least for deep water conditions). A deeper understanding of this issue would be relevant but probably it is out of the scope of the present manuscript. I also do not understand the directional spectra: it seems to me that waves propagate with the same energy along the wind and against the wind. The asymmetry issues is also not clear at all. I would expect some more careful statement on the description of the figures or maybe a reanalysis of the data.

Despite these minor remarks, I find the present manuscript a nice peace of work and acceptable for publication. The data could really serve as benchmark for future studies and comparison with models.

Author Response

Reply to Reviewer 2

The present manuscript includes state of the art measurements of different observables in a wind wave experiment. Accurate measurements, using different techniques, are reported and discussed. The manuscript discuss a very complete data set including space time measurements of the surface elevation and wind under very different forcing conditions. An extensive discussion on the results is then reported and the limitations of these kinds of experiments for the interpretation of ocean waves is discussed. A review of the state of the art experimental and theoretical results is also included.

I find the present manuscript impressive for the quality and quantity of the results presented.

The author thanks the reviewer for his strong support of this publication

Sometimes there is a lack of explanation of the observed results but I guess that it is not possible to properly understand what is going on in such a complex dynamics. Just to give an example, the author finds a value of the kurtosis which is well below the Gaussian expectation; I have already seen this effect in data recorded from wind wave facilities but hardly the kurtosis of the surface elevation in real ocean data is below 3 (at least for deep water conditions). A deeper understanding of this issue would be relevant but probably it is out of the scope of the present manuscript. … The asymmetry issues is also not clear at all. I would expect some more careful statement on the description of the figures or maybe a reanalysis of the data.

The author appreciates the understanding of the reviewer of the limitations of this review paper. Indeed, the issue of the higher moments is not central to this manuscript, and presentation containing more details would probably deviate from its scope. I would like to add that the present results on kurtosis variation with fetch agree with Huang & Long, J. Fluid Mech. (1980), 101, p. 179. Similarly, analysis of asymmetry, although important, is somewhat outside the scope of this manuscript. To clarify those points that indeed were not discussed in detail, and in order to deal with the reviewer’s comment, a sentence is now added at the end of Section 3 (lines 1011-1012) that addresses the reader to publications where those issues have been considered in greater detail.

I also do not understand the directional spectra: it seems to me that waves propagate with the same energy along the wind and against the wind.

The 180⁰  directional ambiguity naturally appears in directional spectra computed applying 2D Fourier transform to real-valued instantaneous wave field images, acquired by optical, radar or other means (see, e. g. ref. [109] where directional spectra of ocean waves from Interferometric SAR images are presented). In view of the reviewer’s comment, lines 895-897 were modified.