the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 Sep 2024
Extreme Mediterranean cyclones and associated variables in an atmosphere-only vs an ocean-coupled regional model
Abstract. Complex air-sea interactions play a major role in both the variability and the extremes of the Mediterranean climate. This study investigates the differences between an atmosphere-only and an ocean-coupled model in reproducing Mediterranean cyclones and their associated atmospheric fields. To this end, two simulations are performed using the ENEA-REG regional Earth system model at 12 km atmospheric horizontal resolution over the Med-CORDEX domain, both driven by ERA5 reanalysis, for a common 33-year period (1982–2014). The atmosphere stand-alone simulation uses the WRF model with prescribed ERA5 Sea Surface Temperature (SST), while in the second WRF is coupled to the MITgcm ocean model at horizontal resolution of 1/12°. A cyclone track method, based on sea level pressure, is applied to both simulations and to the ERA5 reanalysis to assess the model capability to reproduce the climatology of intense, potentially most impactful, cyclones. Results show that the seasonal and spatial distribution of the 500 most intense cyclones is similarly reproduced between WRF and ERA5, regardless the use of the coupling. The two simulations are then compared in terms of sub-daily fields at the cyclones' maximum intensity. Differences in SST distribution between the models primarily control variations in atmospheric variables, not only at the surface, but throughout the planet boundary layer, due to the mixing of the turbulent processes, enhanced during intense cyclones. Additionally, the research investigates the cyclone effects on ocean properties in the coupled simulation, revealing that strong winds enhance surface heat fluxes and upper ocean mixing, while lowering SST. The analysis shows the effectiveness of the coupled model in representing dynamic and thermodynamic processes associated with extreme cyclones across both the atmosphere and the ocean.
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Status: open (until 08 Nov 2024)
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RC1: 'Comment on egusphere-2024-2829', Anonymous Referee #1, 14 Oct 2024
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This manuscript uses a cyclone tracking algorithm to track the most intense Mediterranean cyclones in atmosphere-only and coupled atmosphere-ocean simulations. It is shown that both STD and CPL simulations represent the climatology of storms in the Mediterranean with no notable advantage to CPL. Also, it is shown that CPL has an SST bias relative to STD, which affects various fields in the PBL. CPL can be used to understand dynamical mechanisms in the ocean mixed layer in the presence of atmospheric cyclones.
Overall, the work performed for this study is impressive – a combination of coupled model simulations with cyclone tracking algorithms and the subsequent analysis. The presentation of the results and the discussions are interesting and well-structured. Yet, I struggle to see the innovative part of this paper. Instead, I see a nice comparison between two simulations – coupled and uncoupled. Ultimately, the primary distinction between simulations is the SST bias in CPL, which imprints on various fields in the PBL. The relevancy of the coupling vs. non-coupling is only demonstrated in Figure 10, Panels a and b. There, the advantage of coupling is clear.
I think it would be helpful to use more careful language that does not attribute the difference of various fields to the coupling (or the explicitly resolved SST). These suggested changes perhaps mean that the conclusions are more relaxed. Still, considering the large effort made by the authors, after addressing the above critics and the more specific comments below, I would recommend the paper for publication.
Specific comments:
Lines 103-104: The authors try to answer the question, “To which extent in the vertical column, and through which physical mechanisms, the explicitly resolved SST distribution and sea surface fluxes impact the precipitation, and the wind speed during extreme cyclones?”, but the designed simulations can’t really separate the effect of explicitly resolved SST when the SST in the Western Mediterranean is about 1.5 degrees warmer. In that case, I think the only thing that can be done is to downgrade the question to something that fits the analysis in the paper.
Line 167-168: “Two cyclones are considered the same event if their minimum of SLP is within a 500 km distance and within a time range of 12 hours.” – this stage removes from the analysis all cases in which CPL is different from STD (~30%). This difference by itself sounds very large, suggesting that many cyclones are represented very differently in the CPL simulation. The authors do not compare their tracks with ERA5-based tracks (e.g., by calculating the RMSE of the distance between observed and simulated cyclone location at maximum intensity), and it is hard to tell which simulation is better. Therefore, it may lead to an unverified conclusion that CPL represents the cyclones well (although it has a large SST bias) and that CPL does not have an added value.
Line 172-173: does this mean that out of 341, 199 cyclones occur in DJF and SON? Please clarify this.
Line 176: can you explain or provide a reference for why this field at the specific level was chosen?
Figure 3: It is not clear how this figure was made. Is the percentage calculated from all days in the specific grid cell or is it a percentage from all cyclones in the region? Could you please clarify this? Also, while the spatial variability is well represented in the models, there is still a very large difference in the percentage relative to observations. The authors should discuss this, at least by providing some information about the source of this large difference.
Line 307: “This is explained by the higher Θ gradient of the CPL (Fig. 7d), that makes the PBL less stratified and higher” – what is exactly explained by the higher Θ gradient, and does this gradient is the reason why PBL is less stratified and higher? I would say that this is because of the higher SST, as mentioned in the previous sentence. This reasoning is not clear to me. I would say that, in general, the PBL should be well mixed, and differences between STD and CPL should be pretty small in terms of the temperature gradients inside the PBL. I would attribute the difference only to the SST difference.
Figure 10: It is unclear which region is considered when calculating the SST difference. Is it one grid cell where maximum cyclone intensity occurred, or is it a regional average?
Minor:
Line 21: “plant” -> “planetary”
Line 27: “because is” -> “because it is”?
Line 240: “(Fig. 3a and b)” -> “(Fig. 3b and c)”?
Line 286: “CLP” -> “CPL”
Line 288: “Fig. S6” - > “Fig. 6”
Line 291: “CLP” -> “CPL”
Figure S2: need to correct the reference to the panels in the caption
Line 351: “CLP” -> “CPL”
Figures: can you explain what the deltas at the top of the panels mean? is it a simple domain average?
Citation: https://doi.org/10.5194/egusphere-2024-2829-RC1
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