Waves in SKRIPS: WaveWatch III coupling implementation and a case study of cyclone Mekunu

Sun, Rui; Cobb, Alison; Villas Bôas, Ana B.; Langodan, Sabique; Subramanian, Aneesh C.; Mazloff, Matthew R.; Cornuelle, Bruce D.; Miller, Arthur J.; Pathak, Raju; Hoteit, Ibrahim

doi:https://doi.org/10.5194/egusphere-2022-1298

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https://doi.org/10.5194/egusphere-2022-1298

Preprints

22 Nov 2022

| 22 Nov 2022

Waves in SKRIPS: WaveWatch III coupling implementation and a case study of cyclone Mekunu

Rui Sun, Alison Cobb, Ana B. Villas Bôas, Sabique Langodan, Aneesh C. Subramanian, Matthew R. Mazloff, Bruce D. Cornuelle, Arthur J. Miller, Raju Pathak, and Ibrahim Hoteit

Abstract. In this work, we integrated the WaveWatch III model into the regional coupled model SKRIPS (Scripps–KAUST Regional Integrated Prediction System). The WaveWatch III model is implemented with flexibility, meaning the coupled system can run with or without the wave component. To demonstrate the impact of coupling we performed a case study using a series of coupled and uncoupled simulations of tropical cyclone Mekunu, which occurred in the Arabian Sea in May 2018. We examined the skill of the coupled model against the stand-alone WRF model and further investigated the impact of Langmuir turbulence in the coupled system. We found that the coupled model better captures the minimum pressure and maximum wind speed compared with the stand-alone WRF model. The characteristics of the tropical cyclone do not change significantly when using different options to parameterize the influence of waves on the ocean and the atmosphere. However, in the region of the cold wake, when Langmuir turbulence is considered in the coupled system, the sea surface temperature is about 0.5 °C colder and the mixed layer is about 20 meters deeper. This indicates the ocean model is sensitive to the parameterization of Langmuir turbulence in the coupled simulations.

Received: 19 Nov 2022 – Discussion started: 22 Nov 2022

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20 Jun 2023

Waves in SKRIPS: WAVEWATCH III coupling implementation and a case study of Tropical Cyclone Mekunu

Rui Sun, Alison Cobb, Ana B. Villas Bôas, Sabique Langodan, Aneesh C. Subramanian, Matthew R. Mazloff, Bruce D. Cornuelle, Arthur J. Miller, Raju Pathak, and Ibrahim Hoteit

Geosci. Model Dev., 16, 3435–3458, https://doi.org/10.5194/gmd-16-3435-2023,https://doi.org/10.5194/gmd-16-3435-2023, 2023

Short summary

Rui Sun et al.

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2022-1298', Anonymous Referee #1, 20 Dec 2022

Review of "Waves in SKRIPS: WaveWatch III coupling implementation and a case study of cyclone Mekunu"

General comments

In this manuscript the authors assess the performance and sensitivity to different parameterizations of a regional atmosphere-ocean-wave coupled model in simulating cyclone Mekunu in Arabian Sea. The authors first compare the performance of an atmosphere-ocean-wave fully coupled simulation, an atmosphere-ocean coupled simulation, and a standalone atmosphere simulation. They conclude that both versions of the coupled simulation give better results than the standalone atmosphere simulation. They further examine the sensitivity of the coupled simulation to different options of Langmuir turbulence parameterization and found that the simulation results including the mixed layer depth and sea surface temperature are sensitive to the choice of Langmuir turbulence parameterization. The authors also report the sensitivity of the simulation results to different choices of ocean surface roughness parameterization in the appendices.

In general this is an interesting study. The results are helpful for improving our understanding of the atmosphere-ocean-wave coupling during cyclones, and are useful for the development of regional atmosphere-ocean-wave coupled models. While the manuscript is easy to read, I think it can be significantly improved by a careful revision.

One of my major concerns is that the focus of this study doesn't seem clear to me. If I understand it correctly, the focus of this study is to assess the effects of ocean surface waves by incorporating a wave model WaveWatch III into a regional atmosphere-ocean coupled model SKRIPS, using cyclone Mekunu as an example. If this is the case, the comparison with a standalone atmosphere model WRF seems to distract the readers from the focus. Also, coupled model has more skill in simulating cyclones than standalone atmosphere model may not be entirely new. I'd suggest the authors focus more on the impact of ocean surface waves by the coupling with a wave model. In this sense it would be better to examine in more detail what are the impact of including the effects of Stokes forces, Langmuir turbulence, wave modulated wind stress and ocean surface roughness seen by the atmosphere as introduced in Section 2 on simulating cyclone Mekunu. The presentation of the results in Section 4 is very brief and is not focusing on the effects of waves in my opinion, whereas Section 5 only discusses the impact of different options of Langmuir turbulence parameterization, which is only one of the wave effects included in this coupled model. So the section title of both sections are very confusing. In addition, the results of different sea state dependent surface roughness closures are presented only briefly in the appendices, which is also confusing to me why the authors choose to present these materials there.

Another major comment is on the result of VR12-MA, one of the Langmuir turbulence parameterizations tested in this study. The authors found that using VR12-MA makes the simulated mixed layer depth shallower and sea surface temperature warmer in the cyclone wake than the simulation without Langmuir turbulence parameterization. This result is not intuitive as it is expected that Langmuir turbulence enhances the vertical mixing and deepens the mixed layer. The authors provide a possible explanation in Section 5.2 by examining the regionally averaged vertical profiles, which is very interesting. This may highlight a deficiency of KPP which uses a bulk Richardson number to determine the boundary layer depth, which might be sensitive to the structure of the velocity and buoyancy profiles. I'd suggest the authors to look closer to this issue, perhaps by plotting the time evolution of these profiles in Figure 11 at a point on the cyclone track and check how these profiles change as the cyclone passes by. I guess VR12-MA would still give stronger deepening of the mixed layer depth during the cyclone, but the mixed layer depth may be shallower after the cyclone for reasons suggested by the authors.

I'd also appreciate it if the authors could provide more detailed discussion on the results. My impression is that the authors presented a lot of figures showing the results, but the corresponding description and discussion in the text are rather brief.

Specific comments

L5: Why comparing with a standalone atmosphere model? The difference would be dominated by the effect of including an active ocean model? Why not comparing with the coupled model without the wave component?

L9: Is Langmuir turbulence the only way through which the effects of waves are included? It might be helpful to mention what wave effects are included in the coupled model.

L22: "Intensity" -> "Intensity of TCs"?

L37-38: Is Langmuir turbulence the only way impact of surface waves is implemented in this study? I know this becomes clear in section 2. But it would still be helpful to discuss at least why Langmuir turbulence is emphasized here.

L68: What "surface boundary fields" are exchanged here?

L70-71: Not sure what do the authors mean here... Why online regridding is not needed? If online regrinding is not needed, why implementing it?

L73-74: Might be helpful to be specific on what inputs and outputs are included...

L81: By "Langmuir turbulence parameters" do the authors mean "Langmuir number"?

L99-100: So in addition to the surface Stokes drift mentioned on L80, the integrated Stokes transport is also needed to approximate the Stokes drift profile, right? Also, the same authors (Breivik et al) have an updated method to approximate the Stokes drift profile (Breivik et al., 2016), essentially requiring the same information from WW3. Any comments on why not using this newer method?

L117: Might be helpful to write out the equation here rather than referring the readers to an equation in Li et al., 2019?

L117: "using" -> "uses"?

L119: The entrainment flux is also affected by the enhanced turbulent velocity scale, right?

L126: Same as above, might be helpful to write out the equation here?

L147-148: Which other models are used in this study. Was there a comparison of different options?

L173-L185: Are the boundary conditions of atmosphere, ocean and waves consistent with each other?

L193: What do the authors mean by "derive skill from boundary conditions"

L208-209: I didn't follow this sentence.

Section 4: The purpose of this section is a bit confused. If the purpose is to validate the simulation results of the coupled model (which seems to be suggested by the section title "Results"), more details and discussions on the comparison among the three sets of simulations (CPL.AOW, CPL.AO, and ATM.DYN) seem appropriate. If the purpose is to provide a background information for the discussion on the wave effects, the authors might need to be explicit on that.

L250-251: Might be helpful to be specific on what is better and what is worse in CPL.AOW than CPL.AO.

L271: "Fig 5(b)" -> "Fig 5(c)"?

Figure 4 caption: What do the black and red dots mean?

L297: Switch the order of "cool the SST" and "deepen the MLD"?

L299: Nudging to what?

L300-301: Perhaps more reasoning of why nudging is necessary here deserves more clarification.

L318-321 and L325-326: Do the authors mean that the vertical mixing of momentum is too much in VR12-MA, which reduces the vertical gradient of ocean current and reduces vertical mixing of tracers like temperature? It is not clear to me why an enhanced vertical mixing of momentum coexists with a reduction in vertical mixing of temperature. It might be helpful to elaborate on why this is the case.

Figure 9, 10: The results of VR12-MA are not intuitive to me. According to Section 2.3, VR12-MA also includes the effects of Langmuir turbulence on enhancing the vertical mixing. Then why the MLD gets shallower and SST gets warmer along the cyclone track than the case without Langmuir turbulence? Are the snapshot plotted at the time indicated by the red dot?

Figure 11: Why the mixed layer depth (dashed lines) is different in panel (d) from other panels?

References

Breivik, Ø., J.-R. Bidlot, and P. A. E. M. Janssen, 2016: A Stokes drift approximation based on the Phillips spectrum. , 100, 49–56, https://doi.org/10.1016/j.ocemod.2016.01.005.

Citation: https://doi.org/10.5194/egusphere-2022-1298-RC1
- AC1: 'Reply on RC1', Rui Sun, 24 Mar 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2022/egusphere-2022-1298/egusphere-2022-1298-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2022-1298-AC1
- AC5: 'summary of our changes to the manuscript', Rui Sun, 24 Mar 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2022/egusphere-2022-1298/egusphere-2022-1298-AC5-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2022-1298-AC5
CC1:
'Comment on egusphere-2022-1298', Brandon Reichl, 18 Jan 2023
Review of “Waves in SKRIPS: WaveWatch III coupling implementation and a case study of cyclone Mekunu”
This manuscript presents a coupled atmosphere-wave-ocean model for regional studies of cyclone development including options to simulate both 2-way atmosphere-ocean and 3-way atmosphere-wave-ocean configurations. The model formulation and wave-coupling physics are briefly discussed and then a case-study is applied to analyze the impacts of the different model configurations. The text is clear and the presentation/writing is of good quality. The topic is presently significant for the ocean and wave modeling communities. However, the manuscript does not presently provide compelling arguments for new advances, capabilities, and/or findings related to wave/ocean coupled simulations under cyclones beyond what has been demonstrated in previous studies on the topic.
One main result is that the role of ocean coupling improves the simulation, but this could be investigated further to better explain why the improvement is found (see Major Concern 1). Another primary result is that Langmuir turbulence parameterizations can deepen the mixed layer and decrease the SST, but the effect of this on the coupled model is inconclusive and it is not clear if/how these conclusions would extend to other cyclone simulations. Additional analysis into the other wave processes mentioned in the model description but not analyzed would also help clarify what is learned here and what should be considered for future studies. I have several additional important technical concerns with the model and study, which are detailed below. At this point I cannot recommend the article for publication and recommend substantial revision.
Major Concerns
Adding ocean coupling to a cyclone model is expected to improve the cyclone simulation by improving SSTs, most clearly seen in cases where simulating ocean cooling in place of static SSTs reduces the cyclone intensity (e.g., Ginis & Bender, 2000: doi 10.1175/1520-0493(2000)128<0917:RCSOHO>2.0.CO;2). This study presents a downscaled model, however, and not a forecast model, so that what is called an “uncoupled” case here has time evolving SST with ocean cooling. It is unclear from the given results whether the coupling has indeed improved SST relative to the observation data (drifters or any other sources) compared to the HYCOM assimilating product SST (it might be useful to look at biases in SST, similar to Figure 8). This step and discussion would be useful to clarify that improved SST is indeed why the cyclone intensity is improved by coupling, and not, for example, other biases that are not ruled out by present analysis.

The description of model physics and equations needs some clarifications in the text. Some specific questions/issues:
The text states that the Stokes shear term in equation (1) is “parameterized” through Langmuir turbulence. L209 then implies that this term is dropped in the resolved scale model for this reason. However, there are scale separation issues here that are not discussed. Langmuir turbulence schemes are usually interpreted as representing the impacts of the WANS equations on turbulence at scales <~1km (usual LES domain, below the model resolved grid). Since the MITgcm model also solves the larger-scale “resolved” WANS equations including Stokes advection/Coriolis, including a Langmuir turbulence parameterization is not a formally consistent reason to drop the Stokes shear term in the model equations. At the relatively fine horizontal resolution of this model, the importance of the Stokes shear force should be more carefully considered and possibly retained to avoid changing the model’s resolved momentum balance (see discussions in Suzuki & Fox-Kemper, 2016, for example).

Discussions in McWilliams et al. (2014, doi: 10.1175/JPO-D-13-0122.1) and Reichl et al. (2016b) suggest from Large Eddy Simulations that the KPP bulk Richardson number and the model’s parameterized vertical momentum fluxes are improved by parameterizing with the Lagrangian current and Lagrangian current shear. I am concerned that separating between Lagrangian/Eulerian currents in some parts of the model equations and not considering this difference in parameterizations could lead to inconsistencies in how the parameterizations are applied (e.g., this seems consistent with the explanation in the text for unintuitive results when using the VR12 parameterization, perhaps using the Lagrangian current would rectify this difference).

Is the Stokes drift considered for the volume conservation equation? Wu et al. (2019) express it as a non-divergent condition on the Stokes drift vector, which presumably results in a vertical component of “Stokes drift” since the horizontal components of the Stokes drift can be divergent. It should be clarified how this is dealt with in this model.

It is not clear what is gained by including the “VR12” <w’w’> scaling as a parameterization in this study. The previous studies of LF17 (and also Reichl et al., 2016b) have shown that the ad-hoc assumption of applying <w’w’> based enhancements to the diffusivity and Vt2 in KPP are inadequate Langmuir parameterization approaches. Since the results of this scheme in the study are unintuitive, it might be best to drop this from the study (or clarify what specifically is learned by including it).

The inclusion of the LF17-ST parameterization model in the comparison is also not well motivated/discussed. Presumably the difference from the LF17 WW3 version can demonstrate what can be gained by including a wave model (sea-state dependence) for the turbulence scheme (an interesting topic), but that point is not motivated or discussed.

The implementation of the wave-budget terms and the Charnock coefficient in computing the wind stress is highlighted in the model formulation, but it is not discussed in the results. Some more analysis to clarify how including these terms impacts the simulations would be beneficial.

Minor Comments
The impression of the abstract is that the wave coupling improves the model, but this seems misleading. The “improvements” appear to come from coupling to the ocean model and the impacts of waves are less significant (but also note major concern 1).

Regarding the wave momentum flux budget terms: How are the input and dissipation source terms parameterized for this study? Are these directly from WAVEWATCH? If so, clarify which source term packages are utilized and some discussion how these source terms are validated for cyclone wind speeds.

Regarding the use of HYCOM velocities to initialize the MITgcm model: Assuming the velocities are not somehow made dynamically consistent with the regridded hydrography, are any potential implications from the initial shock/adjustment times assessed?

Is the current passed to WW3 the same as the current passed to the atmosphere (e.g., Fig 1)? Presumably the atmosphere needs the surface current, but the current appropriate for WW3 is usually assumed at a depth related to the dominant wavelength (e.g., as in Fig 1 of Fan et al., 2009). Furthermore, the relative (and neutral) 10m wind should be used to drive WW3, adjusting for the surface current. It is unclear from the text/diagram if this is done.

The time derivative should be a partial and not material derivative in equations 1&2.

L185: Why was a 19-day spin-up chosen for waves? This seems excessive for a forced regional model including boundary conditions. It would be interesting to know if this integration time was deemed necessary.

L234: both -> all

L270: HYCOM yields colder SSTs, but appears to yield shallower MLDs. Is the reason for this understood?

L290: Are the beams physical or numerical?

Figure 7: Panels are mislabeled in the caption.

Figure 8: Missing units

Figure 11 is poor quality, e.g., what is “drho/dr”, what are units, etc. Why not use the KPP boundary layer depth for mixing layer, rather than the mixed layer depth?

WaveWatch should always be capitalized WAVEWATCH (as an acronym).
Citation: https://doi.org/10.5194/egusphere-2022-1298-CC1
- AC2: 'Reply on CC1', Rui Sun, 24 Mar 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2022/egusphere-2022-1298/egusphere-2022-1298-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2022-1298-AC2
- AC5: 'summary of our changes to the manuscript', Rui Sun, 24 Mar 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2022/egusphere-2022-1298/egusphere-2022-1298-AC5-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2022-1298-AC5
RC2:
'Comment on egusphere-2022-1298', Anonymous Referee #2, 21 Jan 2023

General Comments:

            The manuscript presents a set of experiments aimed at demonstrating the impact of 1) coupling between the atmosphere/ocean, 2) further coupling the atmosphere/ocean model to a wave model and 3) various parameterizations of Langmuir turbulence using tropical cyclone Mekunu as a case study. The paper is clear, well written and provides a great level of detail on the various model formulation settings which is very useful for readers who want to explore similar experimentation.

            The authors demonstrate significant improvement to the mean and RMSE values of cyclone central pressure, wind speed and latent heat fluxes through coupling. Though as the authors point out, the CPL.AOW model does not outperform the CPL.AO model. It would have been nice to see some discussion on why this might be. There are additional figures comparing the evolution of SST and MLD in the CPL.AOW simulation and that in the HYCOM analysis; however, there is little discussion of these figures. For instance, I’m surprised the HYCOM analysis has a great decrease in SST but smaller decrease in MLD.

            The second section of “Results” (Section 5 for some reason) examines the impact different parameterizations of Langmuir turbulence have on SST and MLD. The authors demonstrate that the LF17 and LF17-ST experiments produce more accurate changes in SST with greater cooling and greater decreases in MLD relative to an experiment without Langmuir turbulence included. Interestingly, they also find that the VR12-MA experiment produces worse results than running without Langmuir turbulence. They attribute this fact to a reduction in turbulent shear and ocean mixing by the VR12-MA scheme.

            Overall, the manuscript is straightforward, easy to follow and presents some interesting results that I feel many in the scientific community will find useful. I feel the manuscript only requires minor revision with perhaps a bit more discussion in the Results section and addressing the comments below.

Specific Comments:

Sort of a general note, but I’m surprised the first mention of using an ensemble comes on Line 195. I would think this would have been mentioned in the abstract or perhaps further up in the Methodology section.

Section 2.3: It might be useful to have some description on why these three Langmuir parameterizations were selected. Especially since the Results section shows that the LF17 formulations are quite similar in their impact and the VR12-MA simulations are substantially different.

Line 196: “small random perturbations to the initial SST (<0.01 â¦C) at every grid point in the coupled model.” Are they actually random or is there some amount of spatial/temporal correlation?

Line 234-235: The language here does not reflect that there are three model being compared. Unless the purpose it to only discuss the coupled model simulations.

Figure 3: Interesting that the models are somewhat indiscernible until ~day 3. Suppose this illustrates the time necessary for the ocean/waves to begin to have a meaningful influence on these metrics.

Lines 261-263: Not sure what’s being said here.

Line 288: Please describe why this figure shows that the wave height is sensitive to the wind speed.

Line 298: Would be useful to provide more description on why spectral nudging is necessary.

Figure 9/10: Perhaps I’m missing something, but in the figures with spectral nudging the red dot isn’t centered on the largest SST/MLD differences. Should it be?

Lines 339-342: Having trouble connecting this phrase to figure 11. Perhaps demonstrate explicitly if it is seen in the figure

Technical Comments:

Line 41: “the Antarctica”?

Line 167: Maybe just personal preference, but I would rephrase to “The ocean-atmosphere model is not coupled to the wave model”. Same for #3 (Line 169).

Line 176: Perhaps linearly interpolating “to” not “between”?

Line 249: “Despite CPL.AOW better simulates…” Believe some words missing here.

Line 267: Figures 5b, c. Not 5a.

Line 271: Fig. 5c. Not 5b.

Line 277: Fig. 6b, c. Not just Fig. 6b

Figure 7 Captions don’t match the figure.

Citation: https://doi.org/10.5194/egusphere-2022-1298-RC2
- AC3: 'Reply on RC2', Rui Sun, 24 Mar 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2022/egusphere-2022-1298/egusphere-2022-1298-AC3-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2022-1298-AC3
- AC5: 'summary of our changes to the manuscript', Rui Sun, 24 Mar 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2022/egusphere-2022-1298/egusphere-2022-1298-AC5-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2022-1298-AC5
RC3:
'Comment on egusphere-2022-1298', Anonymous Referee #3, 27 Jan 2023

Review of “Waves in SKRIPS: WaveWatch III coupling implementation and a case study of cyclone Mekunu” from Sun et al.
The manuscript described the integration of the WW3 wave model into the SKRIPS couple framework. The cyclone Mekunu is chosen as study case in order to validate and compare a set of coupled and uncoupled simulations. Moreover, different Langmuir parametrizations have been implemented in order to investigate the impact of the wave to ocean coupling with an emphasis on SST and mixed layer depth analysis. Different surface roughness parametrizations are also compared to estimate the impact of waves in the coupled system. The simulations are validated against observations of the cyclone’s characteristics and SST from drifters. Overall, the coupled simulations show better skill in reproducing the cyclonic event. The sensitivity to wave coupling on the characteristics of the cyclone is not found to be significant. However, the impact on SST and mixed layer is shown with a decrease of 0.5 degC and a deepening of the mixed layer of up to 20m in the wake of the cyclone compare to the simulation without Langmuir turbulence. Although, one of the parameterizations, VR12-MA, showed counterintuitive results with weaker SST cooling and shallower MLD. This is explained looking at vertical profiles showing a reduced horizontal shear velocity when using that specific scheme.
General comments:
Overall this is a good paper and well written. Although some of the results of the study are well known (i.e. coupled model generally have better skill in modeling cyclone events) I believe the emphasis is also put on the technical implementation of the wave model into the SKRIPS framework and of the different Langmuir and roughness parameterizations. For that reason, I believe it would be a good fit for GMD journal. However, one of the major issues in my opinion is that the manuscript needs more clarity on certain aspects and more discussion of the results which would be beneficial to the overall paper. As well as maybe a reorganization/clarification between sections 4 and 5 where the goals are sometimes unclear or confusing whether it aims to compare coupled and uncoupled, the impact of the different parametrizations or some validation of background ocean state. Some findings of the study are really interesting but often times too briefly discussed or even some figures not discussed at all. Also, as an optional comment the discussion on the appendices A and C on the roughness parametrizations may fit well in the main text.
Hereafter are the details of the specific’s comments.

Specific comments:
Line 80: It could be useful to reference the section 2.4 when mentioning the “momentum flux terms due to waves”, it feels a bit too vague otherwise.
Line 86: Maybe mention here that details of the calculation of the momentum stress is given later on, otherwise one can wonder why you’re not mentioning it here.
Section 2.4: Mention that the momentum terms are output from WW3. Is the total air-side stress calculated inside MITgcm ? If yes, why don’t send the total air-side stress calculated within WRF which was calculated using wave information ? And thus what kind of bulk formulae is used in MITgcm to calculate the air-side stress (line 83-84) ? (similar remark can be made for heat flux). I think that a bit more clarity on how the surface flux are calculated and exchanged could be useful.
Line 147: In which scheme did you implemented these parametrizations ? Aren’t some of these already available in WRF ?
Line 147:150: Since you implemented several roughness parametrizations, any reasons why you didn’t implemented COARE3.5 which is more recent and as also formulation of surface roughness based on sea state variables ?
In Appendix A, line 373-374: please describe this options using the Charnock from WW3.
Table 1: Is the surface layer scheme used in WRF the same as PBL, MYNN ?
Line 207: Is a coupled frequency of 120 seconds necessary? An Hourly coupling or 30 min coupling would allow to capture the diurnal cycle as well, any reason behind the choice of the coupling frequency? Also wondering if the computation time is impacted by this high coupling frequency ?
Line 210-211: In line with my previous comment, I found it not clear how the surface stress is calculated. On Figure 1, WW3 send the surface roughness to WRF to get the stress but here it is mentioned that the Charnock coefficient is used. Is it actually the Charnock coefficient that is sent to WRF or the roughness length? Please clarify and modify Figure 1 if necessary.
Section 3.2: What are the models frequency outputs (if different from one model to another)? It would be useful to add it here.
Section 4: The name of the section, “Results”, is fairly generic. The goal of this section needs some clarification, it includes some results between coupled simulation and some more validation part, i.e. section 4.2 or 4.3 which resemble more to a validation of the wave field and no impact of waves on air-sea interaction or between simulations are discussed yet.
Line 250: Any hypothesis as why the CPL.AOW would give “worse” results than the CPL.AO ?
Line 271: “Fig. 5(c)”
Line 271: Is this a known cold bias from HYCOM ? Any reason as why HYCOM would show stronger SST cooling ?
Line 272:273: Does this mean it is in agreement with some observations ? Please clarify this statement.
Section 4.2: the results of the MLD differences in HYCOM showed in Figure 6c are not discussed, such as any hypothesis as why HYCOM would show shallower MLD while stronger cooling. Overall this section could benefit from more explanation of the figures and results showed.
Figure 7: Caption does not match the actual figure, please clarify.
Line 284: You probably meant Figure 7b or Figure 7c.
Line 291: Please precise Appendix C.
Line 304: Please precise Appendix B. Also, in the text Appendix C is cited before Appendix B so maybe the order of the appendices could be revised.
Line 389: “which are”
Line 391: Please be more explicit than “CPL” here as on the figure they all have different names and none is CPL
Line 395-396: Please specify the region, as in the Figure A4 it looks like there are some regions where the wind speed and the latent heat flux are weaker compared to CPL.CHAR.
Figure A4: “parameterization”; Also what do you mean by “without parameterization”, isn’t CPL.CHAR parameterized using the Charnock coefficient from WW3 ?
Section 5.1: The Figure 8c showing HYCOM SST compared to drifters is not discussed although it looks like the cooling along the track is the closest compare to the drifters, is that right? However, in Section 4.2 it seems like the SST cooling in HYCOM was too strong which was one of the reasons leading to a too low intensity of the cyclone simulated ATM.DYN, please clarify or comment on that. Here again, this is interesting results and a bit more discussion on these findings could be beneficial.
Figure 11: Looks like the mixed layer depths (dashed lines) are different in panel (d) than the others, please correct/clarify this.

Citation: https://doi.org/10.5194/egusphere-2022-1298-RC3
- AC4: 'Reply on RC3', Rui Sun, 24 Mar 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2022/egusphere-2022-1298/egusphere-2022-1298-AC4-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2022-1298-AC4
- AC5: 'summary of our changes to the manuscript', Rui Sun, 24 Mar 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2022/egusphere-2022-1298/egusphere-2022-1298-AC5-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2022-1298-AC5

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2022-1298', Anonymous Referee #1, 20 Dec 2022

Review of "Waves in SKRIPS: WaveWatch III coupling implementation and a case study of cyclone Mekunu"

General comments

In this manuscript the authors assess the performance and sensitivity to different parameterizations of a regional atmosphere-ocean-wave coupled model in simulating cyclone Mekunu in Arabian Sea. The authors first compare the performance of an atmosphere-ocean-wave fully coupled simulation, an atmosphere-ocean coupled simulation, and a standalone atmosphere simulation. They conclude that both versions of the coupled simulation give better results than the standalone atmosphere simulation. They further examine the sensitivity of the coupled simulation to different options of Langmuir turbulence parameterization and found that the simulation results including the mixed layer depth and sea surface temperature are sensitive to the choice of Langmuir turbulence parameterization. The authors also report the sensitivity of the simulation results to different choices of ocean surface roughness parameterization in the appendices.

In general this is an interesting study. The results are helpful for improving our understanding of the atmosphere-ocean-wave coupling during cyclones, and are useful for the development of regional atmosphere-ocean-wave coupled models. While the manuscript is easy to read, I think it can be significantly improved by a careful revision.

One of my major concerns is that the focus of this study doesn't seem clear to me. If I understand it correctly, the focus of this study is to assess the effects of ocean surface waves by incorporating a wave model WaveWatch III into a regional atmosphere-ocean coupled model SKRIPS, using cyclone Mekunu as an example. If this is the case, the comparison with a standalone atmosphere model WRF seems to distract the readers from the focus. Also, coupled model has more skill in simulating cyclones than standalone atmosphere model may not be entirely new. I'd suggest the authors focus more on the impact of ocean surface waves by the coupling with a wave model. In this sense it would be better to examine in more detail what are the impact of including the effects of Stokes forces, Langmuir turbulence, wave modulated wind stress and ocean surface roughness seen by the atmosphere as introduced in Section 2 on simulating cyclone Mekunu. The presentation of the results in Section 4 is very brief and is not focusing on the effects of waves in my opinion, whereas Section 5 only discusses the impact of different options of Langmuir turbulence parameterization, which is only one of the wave effects included in this coupled model. So the section title of both sections are very confusing. In addition, the results of different sea state dependent surface roughness closures are presented only briefly in the appendices, which is also confusing to me why the authors choose to present these materials there.

Another major comment is on the result of VR12-MA, one of the Langmuir turbulence parameterizations tested in this study. The authors found that using VR12-MA makes the simulated mixed layer depth shallower and sea surface temperature warmer in the cyclone wake than the simulation without Langmuir turbulence parameterization. This result is not intuitive as it is expected that Langmuir turbulence enhances the vertical mixing and deepens the mixed layer. The authors provide a possible explanation in Section 5.2 by examining the regionally averaged vertical profiles, which is very interesting. This may highlight a deficiency of KPP which uses a bulk Richardson number to determine the boundary layer depth, which might be sensitive to the structure of the velocity and buoyancy profiles. I'd suggest the authors to look closer to this issue, perhaps by plotting the time evolution of these profiles in Figure 11 at a point on the cyclone track and check how these profiles change as the cyclone passes by. I guess VR12-MA would still give stronger deepening of the mixed layer depth during the cyclone, but the mixed layer depth may be shallower after the cyclone for reasons suggested by the authors.

I'd also appreciate it if the authors could provide more detailed discussion on the results. My impression is that the authors presented a lot of figures showing the results, but the corresponding description and discussion in the text are rather brief.

Specific comments

L5: Why comparing with a standalone atmosphere model? The difference would be dominated by the effect of including an active ocean model? Why not comparing with the coupled model without the wave component?

L9: Is Langmuir turbulence the only way through which the effects of waves are included? It might be helpful to mention what wave effects are included in the coupled model.

L22: "Intensity" -> "Intensity of TCs"?

L37-38: Is Langmuir turbulence the only way impact of surface waves is implemented in this study? I know this becomes clear in section 2. But it would still be helpful to discuss at least why Langmuir turbulence is emphasized here.

L68: What "surface boundary fields" are exchanged here?

L70-71: Not sure what do the authors mean here... Why online regridding is not needed? If online regrinding is not needed, why implementing it?

L73-74: Might be helpful to be specific on what inputs and outputs are included...

L81: By "Langmuir turbulence parameters" do the authors mean "Langmuir number"?

L99-100: So in addition to the surface Stokes drift mentioned on L80, the integrated Stokes transport is also needed to approximate the Stokes drift profile, right? Also, the same authors (Breivik et al) have an updated method to approximate the Stokes drift profile (Breivik et al., 2016), essentially requiring the same information from WW3. Any comments on why not using this newer method?

L117: Might be helpful to write out the equation here rather than referring the readers to an equation in Li et al., 2019?

L117: "using" -> "uses"?

L119: The entrainment flux is also affected by the enhanced turbulent velocity scale, right?

L126: Same as above, might be helpful to write out the equation here?

L147-148: Which other models are used in this study. Was there a comparison of different options?

L173-L185: Are the boundary conditions of atmosphere, ocean and waves consistent with each other?

L193: What do the authors mean by "derive skill from boundary conditions"

L208-209: I didn't follow this sentence.

Section 4: The purpose of this section is a bit confused. If the purpose is to validate the simulation results of the coupled model (which seems to be suggested by the section title "Results"), more details and discussions on the comparison among the three sets of simulations (CPL.AOW, CPL.AO, and ATM.DYN) seem appropriate. If the purpose is to provide a background information for the discussion on the wave effects, the authors might need to be explicit on that.

L250-251: Might be helpful to be specific on what is better and what is worse in CPL.AOW than CPL.AO.

L271: "Fig 5(b)" -> "Fig 5(c)"?

Figure 4 caption: What do the black and red dots mean?

L297: Switch the order of "cool the SST" and "deepen the MLD"?

L299: Nudging to what?

L300-301: Perhaps more reasoning of why nudging is necessary here deserves more clarification.

L318-321 and L325-326: Do the authors mean that the vertical mixing of momentum is too much in VR12-MA, which reduces the vertical gradient of ocean current and reduces vertical mixing of tracers like temperature? It is not clear to me why an enhanced vertical mixing of momentum coexists with a reduction in vertical mixing of temperature. It might be helpful to elaborate on why this is the case.

Figure 9, 10: The results of VR12-MA are not intuitive to me. According to Section 2.3, VR12-MA also includes the effects of Langmuir turbulence on enhancing the vertical mixing. Then why the MLD gets shallower and SST gets warmer along the cyclone track than the case without Langmuir turbulence? Are the snapshot plotted at the time indicated by the red dot?

Figure 11: Why the mixed layer depth (dashed lines) is different in panel (d) from other panels?

References

Breivik, Ø., J.-R. Bidlot, and P. A. E. M. Janssen, 2016: A Stokes drift approximation based on the Phillips spectrum. , 100, 49–56, https://doi.org/10.1016/j.ocemod.2016.01.005.

Citation: https://doi.org/10.5194/egusphere-2022-1298-RC1
- AC1: 'Reply on RC1', Rui Sun, 24 Mar 2023
  
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  Citation: https://doi.org/10.5194/egusphere-2022-1298-AC1
- AC5: 'summary of our changes to the manuscript', Rui Sun, 24 Mar 2023
  
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CC1:
'Comment on egusphere-2022-1298', Brandon Reichl, 18 Jan 2023
Review of “Waves in SKRIPS: WaveWatch III coupling implementation and a case study of cyclone Mekunu”
This manuscript presents a coupled atmosphere-wave-ocean model for regional studies of cyclone development including options to simulate both 2-way atmosphere-ocean and 3-way atmosphere-wave-ocean configurations. The model formulation and wave-coupling physics are briefly discussed and then a case-study is applied to analyze the impacts of the different model configurations. The text is clear and the presentation/writing is of good quality. The topic is presently significant for the ocean and wave modeling communities. However, the manuscript does not presently provide compelling arguments for new advances, capabilities, and/or findings related to wave/ocean coupled simulations under cyclones beyond what has been demonstrated in previous studies on the topic.
One main result is that the role of ocean coupling improves the simulation, but this could be investigated further to better explain why the improvement is found (see Major Concern 1). Another primary result is that Langmuir turbulence parameterizations can deepen the mixed layer and decrease the SST, but the effect of this on the coupled model is inconclusive and it is not clear if/how these conclusions would extend to other cyclone simulations. Additional analysis into the other wave processes mentioned in the model description but not analyzed would also help clarify what is learned here and what should be considered for future studies. I have several additional important technical concerns with the model and study, which are detailed below. At this point I cannot recommend the article for publication and recommend substantial revision.
Major Concerns
Adding ocean coupling to a cyclone model is expected to improve the cyclone simulation by improving SSTs, most clearly seen in cases where simulating ocean cooling in place of static SSTs reduces the cyclone intensity (e.g., Ginis & Bender, 2000: doi 10.1175/1520-0493(2000)128<0917:RCSOHO>2.0.CO;2). This study presents a downscaled model, however, and not a forecast model, so that what is called an “uncoupled” case here has time evolving SST with ocean cooling. It is unclear from the given results whether the coupling has indeed improved SST relative to the observation data (drifters or any other sources) compared to the HYCOM assimilating product SST (it might be useful to look at biases in SST, similar to Figure 8). This step and discussion would be useful to clarify that improved SST is indeed why the cyclone intensity is improved by coupling, and not, for example, other biases that are not ruled out by present analysis.

The description of model physics and equations needs some clarifications in the text. Some specific questions/issues:
The text states that the Stokes shear term in equation (1) is “parameterized” through Langmuir turbulence. L209 then implies that this term is dropped in the resolved scale model for this reason. However, there are scale separation issues here that are not discussed. Langmuir turbulence schemes are usually interpreted as representing the impacts of the WANS equations on turbulence at scales <~1km (usual LES domain, below the model resolved grid). Since the MITgcm model also solves the larger-scale “resolved” WANS equations including Stokes advection/Coriolis, including a Langmuir turbulence parameterization is not a formally consistent reason to drop the Stokes shear term in the model equations. At the relatively fine horizontal resolution of this model, the importance of the Stokes shear force should be more carefully considered and possibly retained to avoid changing the model’s resolved momentum balance (see discussions in Suzuki & Fox-Kemper, 2016, for example).

Discussions in McWilliams et al. (2014, doi: 10.1175/JPO-D-13-0122.1) and Reichl et al. (2016b) suggest from Large Eddy Simulations that the KPP bulk Richardson number and the model’s parameterized vertical momentum fluxes are improved by parameterizing with the Lagrangian current and Lagrangian current shear. I am concerned that separating between Lagrangian/Eulerian currents in some parts of the model equations and not considering this difference in parameterizations could lead to inconsistencies in how the parameterizations are applied (e.g., this seems consistent with the explanation in the text for unintuitive results when using the VR12 parameterization, perhaps using the Lagrangian current would rectify this difference).

Is the Stokes drift considered for the volume conservation equation? Wu et al. (2019) express it as a non-divergent condition on the Stokes drift vector, which presumably results in a vertical component of “Stokes drift” since the horizontal components of the Stokes drift can be divergent. It should be clarified how this is dealt with in this model.

It is not clear what is gained by including the “VR12” <w’w’> scaling as a parameterization in this study. The previous studies of LF17 (and also Reichl et al., 2016b) have shown that the ad-hoc assumption of applying <w’w’> based enhancements to the diffusivity and Vt2 in KPP are inadequate Langmuir parameterization approaches. Since the results of this scheme in the study are unintuitive, it might be best to drop this from the study (or clarify what specifically is learned by including it).

The inclusion of the LF17-ST parameterization model in the comparison is also not well motivated/discussed. Presumably the difference from the LF17 WW3 version can demonstrate what can be gained by including a wave model (sea-state dependence) for the turbulence scheme (an interesting topic), but that point is not motivated or discussed.

The implementation of the wave-budget terms and the Charnock coefficient in computing the wind stress is highlighted in the model formulation, but it is not discussed in the results. Some more analysis to clarify how including these terms impacts the simulations would be beneficial.

Minor Comments
The impression of the abstract is that the wave coupling improves the model, but this seems misleading. The “improvements” appear to come from coupling to the ocean model and the impacts of waves are less significant (but also note major concern 1).

Regarding the wave momentum flux budget terms: How are the input and dissipation source terms parameterized for this study? Are these directly from WAVEWATCH? If so, clarify which source term packages are utilized and some discussion how these source terms are validated for cyclone wind speeds.

Regarding the use of HYCOM velocities to initialize the MITgcm model: Assuming the velocities are not somehow made dynamically consistent with the regridded hydrography, are any potential implications from the initial shock/adjustment times assessed?

Is the current passed to WW3 the same as the current passed to the atmosphere (e.g., Fig 1)? Presumably the atmosphere needs the surface current, but the current appropriate for WW3 is usually assumed at a depth related to the dominant wavelength (e.g., as in Fig 1 of Fan et al., 2009). Furthermore, the relative (and neutral) 10m wind should be used to drive WW3, adjusting for the surface current. It is unclear from the text/diagram if this is done.

The time derivative should be a partial and not material derivative in equations 1&2.

L185: Why was a 19-day spin-up chosen for waves? This seems excessive for a forced regional model including boundary conditions. It would be interesting to know if this integration time was deemed necessary.

L234: both -> all

L270: HYCOM yields colder SSTs, but appears to yield shallower MLDs. Is the reason for this understood?

L290: Are the beams physical or numerical?

Figure 7: Panels are mislabeled in the caption.

Figure 8: Missing units

Figure 11 is poor quality, e.g., what is “drho/dr”, what are units, etc. Why not use the KPP boundary layer depth for mixing layer, rather than the mixed layer depth?

WaveWatch should always be capitalized WAVEWATCH (as an acronym).
Citation: https://doi.org/10.5194/egusphere-2022-1298-CC1
- AC2: 'Reply on CC1', Rui Sun, 24 Mar 2023
  
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  Citation: https://doi.org/10.5194/egusphere-2022-1298-AC2
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  Citation: https://doi.org/10.5194/egusphere-2022-1298-AC5
RC2:
'Comment on egusphere-2022-1298', Anonymous Referee #2, 21 Jan 2023

General Comments:

            The manuscript presents a set of experiments aimed at demonstrating the impact of 1) coupling between the atmosphere/ocean, 2) further coupling the atmosphere/ocean model to a wave model and 3) various parameterizations of Langmuir turbulence using tropical cyclone Mekunu as a case study. The paper is clear, well written and provides a great level of detail on the various model formulation settings which is very useful for readers who want to explore similar experimentation.

            The authors demonstrate significant improvement to the mean and RMSE values of cyclone central pressure, wind speed and latent heat fluxes through coupling. Though as the authors point out, the CPL.AOW model does not outperform the CPL.AO model. It would have been nice to see some discussion on why this might be. There are additional figures comparing the evolution of SST and MLD in the CPL.AOW simulation and that in the HYCOM analysis; however, there is little discussion of these figures. For instance, I’m surprised the HYCOM analysis has a great decrease in SST but smaller decrease in MLD.

            The second section of “Results” (Section 5 for some reason) examines the impact different parameterizations of Langmuir turbulence have on SST and MLD. The authors demonstrate that the LF17 and LF17-ST experiments produce more accurate changes in SST with greater cooling and greater decreases in MLD relative to an experiment without Langmuir turbulence included. Interestingly, they also find that the VR12-MA experiment produces worse results than running without Langmuir turbulence. They attribute this fact to a reduction in turbulent shear and ocean mixing by the VR12-MA scheme.

            Overall, the manuscript is straightforward, easy to follow and presents some interesting results that I feel many in the scientific community will find useful. I feel the manuscript only requires minor revision with perhaps a bit more discussion in the Results section and addressing the comments below.

Specific Comments:

Sort of a general note, but I’m surprised the first mention of using an ensemble comes on Line 195. I would think this would have been mentioned in the abstract or perhaps further up in the Methodology section.

Section 2.3: It might be useful to have some description on why these three Langmuir parameterizations were selected. Especially since the Results section shows that the LF17 formulations are quite similar in their impact and the VR12-MA simulations are substantially different.

Line 196: “small random perturbations to the initial SST (<0.01 â¦C) at every grid point in the coupled model.” Are they actually random or is there some amount of spatial/temporal correlation?

Line 234-235: The language here does not reflect that there are three model being compared. Unless the purpose it to only discuss the coupled model simulations.

Figure 3: Interesting that the models are somewhat indiscernible until ~day 3. Suppose this illustrates the time necessary for the ocean/waves to begin to have a meaningful influence on these metrics.

Lines 261-263: Not sure what’s being said here.

Line 288: Please describe why this figure shows that the wave height is sensitive to the wind speed.

Line 298: Would be useful to provide more description on why spectral nudging is necessary.

Figure 9/10: Perhaps I’m missing something, but in the figures with spectral nudging the red dot isn’t centered on the largest SST/MLD differences. Should it be?

Lines 339-342: Having trouble connecting this phrase to figure 11. Perhaps demonstrate explicitly if it is seen in the figure

Technical Comments:

Line 41: “the Antarctica”?

Line 167: Maybe just personal preference, but I would rephrase to “The ocean-atmosphere model is not coupled to the wave model”. Same for #3 (Line 169).

Line 176: Perhaps linearly interpolating “to” not “between”?

Line 249: “Despite CPL.AOW better simulates…” Believe some words missing here.

Line 267: Figures 5b, c. Not 5a.

Line 271: Fig. 5c. Not 5b.

Line 277: Fig. 6b, c. Not just Fig. 6b

Figure 7 Captions don’t match the figure.

Citation: https://doi.org/10.5194/egusphere-2022-1298-RC2
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  Citation: https://doi.org/10.5194/egusphere-2022-1298-AC3
- AC5: 'summary of our changes to the manuscript', Rui Sun, 24 Mar 2023
  
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  Citation: https://doi.org/10.5194/egusphere-2022-1298-AC5
RC3:
'Comment on egusphere-2022-1298', Anonymous Referee #3, 27 Jan 2023

Review of “Waves in SKRIPS: WaveWatch III coupling implementation and a case study of cyclone Mekunu” from Sun et al.
The manuscript described the integration of the WW3 wave model into the SKRIPS couple framework. The cyclone Mekunu is chosen as study case in order to validate and compare a set of coupled and uncoupled simulations. Moreover, different Langmuir parametrizations have been implemented in order to investigate the impact of the wave to ocean coupling with an emphasis on SST and mixed layer depth analysis. Different surface roughness parametrizations are also compared to estimate the impact of waves in the coupled system. The simulations are validated against observations of the cyclone’s characteristics and SST from drifters. Overall, the coupled simulations show better skill in reproducing the cyclonic event. The sensitivity to wave coupling on the characteristics of the cyclone is not found to be significant. However, the impact on SST and mixed layer is shown with a decrease of 0.5 degC and a deepening of the mixed layer of up to 20m in the wake of the cyclone compare to the simulation without Langmuir turbulence. Although, one of the parameterizations, VR12-MA, showed counterintuitive results with weaker SST cooling and shallower MLD. This is explained looking at vertical profiles showing a reduced horizontal shear velocity when using that specific scheme.
General comments:
Overall this is a good paper and well written. Although some of the results of the study are well known (i.e. coupled model generally have better skill in modeling cyclone events) I believe the emphasis is also put on the technical implementation of the wave model into the SKRIPS framework and of the different Langmuir and roughness parameterizations. For that reason, I believe it would be a good fit for GMD journal. However, one of the major issues in my opinion is that the manuscript needs more clarity on certain aspects and more discussion of the results which would be beneficial to the overall paper. As well as maybe a reorganization/clarification between sections 4 and 5 where the goals are sometimes unclear or confusing whether it aims to compare coupled and uncoupled, the impact of the different parametrizations or some validation of background ocean state. Some findings of the study are really interesting but often times too briefly discussed or even some figures not discussed at all. Also, as an optional comment the discussion on the appendices A and C on the roughness parametrizations may fit well in the main text.
Hereafter are the details of the specific’s comments.

Specific comments:
Line 80: It could be useful to reference the section 2.4 when mentioning the “momentum flux terms due to waves”, it feels a bit too vague otherwise.
Line 86: Maybe mention here that details of the calculation of the momentum stress is given later on, otherwise one can wonder why you’re not mentioning it here.
Section 2.4: Mention that the momentum terms are output from WW3. Is the total air-side stress calculated inside MITgcm ? If yes, why don’t send the total air-side stress calculated within WRF which was calculated using wave information ? And thus what kind of bulk formulae is used in MITgcm to calculate the air-side stress (line 83-84) ? (similar remark can be made for heat flux). I think that a bit more clarity on how the surface flux are calculated and exchanged could be useful.
Line 147: In which scheme did you implemented these parametrizations ? Aren’t some of these already available in WRF ?
Line 147:150: Since you implemented several roughness parametrizations, any reasons why you didn’t implemented COARE3.5 which is more recent and as also formulation of surface roughness based on sea state variables ?
In Appendix A, line 373-374: please describe this options using the Charnock from WW3.
Table 1: Is the surface layer scheme used in WRF the same as PBL, MYNN ?
Line 207: Is a coupled frequency of 120 seconds necessary? An Hourly coupling or 30 min coupling would allow to capture the diurnal cycle as well, any reason behind the choice of the coupling frequency? Also wondering if the computation time is impacted by this high coupling frequency ?
Line 210-211: In line with my previous comment, I found it not clear how the surface stress is calculated. On Figure 1, WW3 send the surface roughness to WRF to get the stress but here it is mentioned that the Charnock coefficient is used. Is it actually the Charnock coefficient that is sent to WRF or the roughness length? Please clarify and modify Figure 1 if necessary.
Section 3.2: What are the models frequency outputs (if different from one model to another)? It would be useful to add it here.
Section 4: The name of the section, “Results”, is fairly generic. The goal of this section needs some clarification, it includes some results between coupled simulation and some more validation part, i.e. section 4.2 or 4.3 which resemble more to a validation of the wave field and no impact of waves on air-sea interaction or between simulations are discussed yet.
Line 250: Any hypothesis as why the CPL.AOW would give “worse” results than the CPL.AO ?
Line 271: “Fig. 5(c)”
Line 271: Is this a known cold bias from HYCOM ? Any reason as why HYCOM would show stronger SST cooling ?
Line 272:273: Does this mean it is in agreement with some observations ? Please clarify this statement.
Section 4.2: the results of the MLD differences in HYCOM showed in Figure 6c are not discussed, such as any hypothesis as why HYCOM would show shallower MLD while stronger cooling. Overall this section could benefit from more explanation of the figures and results showed.
Figure 7: Caption does not match the actual figure, please clarify.
Line 284: You probably meant Figure 7b or Figure 7c.
Line 291: Please precise Appendix C.
Line 304: Please precise Appendix B. Also, in the text Appendix C is cited before Appendix B so maybe the order of the appendices could be revised.
Line 389: “which are”
Line 391: Please be more explicit than “CPL” here as on the figure they all have different names and none is CPL
Line 395-396: Please specify the region, as in the Figure A4 it looks like there are some regions where the wind speed and the latent heat flux are weaker compared to CPL.CHAR.
Figure A4: “parameterization”; Also what do you mean by “without parameterization”, isn’t CPL.CHAR parameterized using the Charnock coefficient from WW3 ?
Section 5.1: The Figure 8c showing HYCOM SST compared to drifters is not discussed although it looks like the cooling along the track is the closest compare to the drifters, is that right? However, in Section 4.2 it seems like the SST cooling in HYCOM was too strong which was one of the reasons leading to a too low intensity of the cyclone simulated ATM.DYN, please clarify or comment on that. Here again, this is interesting results and a bit more discussion on these findings could be beneficial.
Figure 11: Looks like the mixed layer depths (dashed lines) are different in panel (d) than the others, please correct/clarify this.

Citation: https://doi.org/10.5194/egusphere-2022-1298-RC3
- AC4: 'Reply on RC3', Rui Sun, 24 Mar 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2022/egusphere-2022-1298/egusphere-2022-1298-AC4-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2022-1298-AC4
- AC5: 'summary of our changes to the manuscript', Rui Sun, 24 Mar 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2022/egusphere-2022-1298/egusphere-2022-1298-AC5-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2022-1298-AC5

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

AR by Rui Sun on behalf of the Authors (24 Mar 2023) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (28 Mar 2023) by Deepak Subramani

RR by Anonymous Referee #3 (08 Apr 2023)

RR by Anonymous Referee #1 (10 Apr 2023)

Suggestions for revision or reasons for rejection

I think this revised version of the manuscript has been improved in clarity as compared to the original version. However, I'm still not quite satisfied with the authors' response to my two major concerns in my previous review.

1. I still feel that the inclusion of the uncoupled WRF run in the comparison is a bit confusing and distracting. Since the focus is on the effects of including waves in SKRIPS (based on the title, abstract and the two goals described in the introduction), it may make more sense to start from the atmosphere-ocean coupled version of SKRIPS. The standalone WRF simulation may be used as a reference. But it would be good to focus on the comparison between CPL.AO and CPL.AOW in the main results and conclusions. If the authors also want to highlight the changes due to air-sea coupling (as mentioned in their response to my previous comments), it might be helpful to discuss in more detail what is the role of air-sea coupling here. But I think such discussion may be quite distracting.

2. The added explanation of the unintuitive results of VR12-MA in Section 5.2 is not satisfactory. In fact, the argument of reduced velocity shear due to enhanced diffusivity of momentum also applies to LF17 -- the same Langmuir enhancement factor in VR12-MA is applied in LF17 as well. So the increased bulk Richardson number due to reduced |u_r-u|^2 should also occur in LF17 -- indeed in Fig 11(d) the same reduction in surface velocity shear can be seen for LF17 too. I think the suggestion from the Community Comments 1 about the inconsistency of the Lagrangian versus Eulerian currents in the momentum equation and Langmuir turbulence parameterization may explain this unintuitive result of VR12-MA. In both Li et al., 2016 and Li et al., 2019, VR12-MA (also LF17) was not used together with Stokes Coriolis. Their assumption (see the notes in Table 1 of Li et al., 2019) was that the simulated velocity is Lagrangian so implicitly it is the Lagrangian shear that is used to compute |u_r-u|^2 in KPP. Since the vertical shear of Stokes drift is strongest near the surface and often roughly aligns with the wind, the Eulerian velocity shear is much reduced when Stokes-Coriolis force is included in the momentum equation. This is consistent with what the authors saw in their simulations in Fig. 11(d) with reduced surface velocity shear in both VR12-MA and LF17 as compared to NoLT. I'd suggest the authors run two more simulations without Stokes-Coriolis and Stokes-advection terms (i.e., only Langmuir turbulence parameterization of VR12-MA and LF17). I guess one may see deeper MLD and cooler SST along the cyclone track in both cases, but more MLD deepening and SST cooling in LF17 than shown here.

Specific comments:

L8-10: Given that the focus of this study is on including waves in SKRIPS (suggested by the title), I still feel it is not necessary and is really confusing to put any emphasize on the difference between coupled model and standalone WRF. Perhaps rephrase to use the difference between the stand-alone WRF and ocean-atmosphere coupled simulations as a reference to describe how large the impact of including surface waves in these experiment is?

L49-54: These discussions seem to divert the purpose of this manuscript to test the coupled model? I think it is already widely accepted that accurately modeling tropical cyclones is important but challenging.

L80-84: I understand that the added text here is to address one of the reviewer's comments. But it seems a bit confusing and distracting. Perhaps rephrase? For example, by "overestimate the strength of surface currents" on L81, do the authors mean overestimating the currents that are dynamically important for waves in WW3, which shouldn't be the surface currents?

L89-90: What do the authors mean by "surface Stokes drift forces"?

L95: What do the authors mean by "surface boundary fields"?

L100-102: So the three components are all on the same grid? Perhaps mention briefly the possible uses in the future?

L116: By using Breivik et al., 2014, it means the total Stokes transport (vertically integrated Stokes drift) is also needed to be passed from WW3 to MITgcm to reconstruct the Stokes drift profile, not only the surface Stokes drift as mentioned on L77?

L115-121: It is not entirely clear from this discussion why approximation is necessary for the Stokes drift profile as it can be computed in WW3. The problem is passing the Stokes drift profile from WW3 to MITgcm, right?

L127-132: I didn't follow the reasoning here...

L133-134: Didn't follow this sentence as well...

L137: It is not very clear what "based on the waves" refer to?

L143-144: Not clear why this case is necessary to validate LF17

L144-145: True only if it works...

L186: Not the appropriate citation for computing the wind stress?

L417-421: These same arguments apply to LF17 as well -- in LF17 the same Langmuir enhancement factor is applied to amplify the KPP diffusivity term. In fact, the same reduction in near surface velocity shear can be seen for LF17 in Fig. 11(d).

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ED: Publish subject to minor revisions (review by editor) (17 Apr 2023) by Deepak Subramani

AR by Rui Sun on behalf of the Authors (12 May 2023) Author's response Author's tracked changes Manuscript

ED: Publish as is (15 May 2023) by Deepak Subramani

AR by Rui Sun on behalf of the Authors (16 May 2023)

Journal article(s) based on this preprint

20 Jun 2023

Waves in SKRIPS: WAVEWATCH III coupling implementation and a case study of Tropical Cyclone Mekunu

Rui Sun, Alison Cobb, Ana B. Villas Bôas, Sabique Langodan, Aneesh C. Subramanian, Matthew R. Mazloff, Bruce D. Cornuelle, Arthur J. Miller, Raju Pathak, and Ibrahim Hoteit

Geosci. Model Dev., 16, 3435–3458, https://doi.org/10.5194/gmd-16-3435-2023,https://doi.org/10.5194/gmd-16-3435-2023, 2023

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Rui Sun et al.

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In this work, we integrated the WaveWatch III model into the regional coupled model SKRIPS. We then performed a case study using the newly implemented model to study the tropical cyclone Mekunu, which occurred in the Arabian Sea. We found that the coupled model better simulates the cyclone than the uncoupled model, but the impact of waves on the cyclone is not significant. However, the waves change the sea surface temperature and mixed layer, especially in the cold wave due to the cyclone.


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