Waves in SKRIPS: WaveWatch III coupling implementation and a case study of cyclone Mekunu
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.
Rui Sun et al.
Status: final response (author comments only)
- RC1: 'Comment on egusphere-2022-1298', Anonymous Referee #1, 20 Dec 2022
- CC1: 'Comment on egusphere-2022-1298', Brandon Reichl, 18 Jan 2023
- RC2: 'Comment on egusphere-2022-1298', Anonymous Referee #2, 21 Jan 2023
- RC3: 'Comment on egusphere-2022-1298', Anonymous Referee #3, 27 Jan 2023
Rui Sun et al.
Rui Sun et al.
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Review of "Waves in SKRIPS: WaveWatch III coupling implementation and a case study of cyclone Mekunu"
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.
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?
Breivik, Ø., J.-R. Bidlot, and P. A. E. M. Janssen, 2016: A Stokes drift approximation based on the Phillips spectrum. Ocean Modelling, 100, 49–56, https://doi.org/10.1016/j.ocemod.2016.01.005.