Variability in coastal downwelling circulation in response to high-resolution regional atmospheric forcing off the Pearl River estuary

Lai, Wenfeng; Gan, Jianping

doi:https://doi.org/10.5194/os-19-1107-2023

Articles | Volume 19, issue 4

https://doi.org/10.5194/os-19-1107-2023

© Author(s) 2023. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/os-19-1107-2023

© Author(s) 2023. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 19, issue 4

Research article

|

19 Jul 2023

Research article |

| 19 Jul 2023

Variability in coastal downwelling circulation in response to high-resolution regional atmospheric forcing off the Pearl River estuary

Wenfeng Lai and Jianping Gan

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Final revised paper (published on 19 Jul 2023)
Preprint (discussion started on 17 Mar 2023)

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-318', Anonymous Referee #1, 19 Apr 2023

The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-318/egusphere-2023-318-RC1-supplement.pdf

Citation: https://doi.org/10.5194/egusphere-2023-318-RC1
- AC1: 'Reply on RC1', Wenfeng Lai, 29 Apr 2023
  
  Review of “Variability of coastal downwelling circulation in response to high-resolution regional atmospheric forcing off the Pearl River Estuary”
  
  This paper by Lai and Gan investigated the variabilities of coastal circulation and dynamics in response to spatiotemporally variable high-resolution atmospheric forcing off the Pearl River Estuary during the downwelling wind. The authors conducted three numerical experiments based on (1) single station observation, (2) global reanalysis data and (3) high-resolution regional atmospheric model. Results show that the model with high-resolution atmospheric forcing significantly improved the temperature-salinity profiles and ocean current simulation. In addition, the model with high atmo-forcing improved the estimation of cross-isobath transport. This paper is well-written and organized, and the results are beneficial for ocean modeler to improve their model results and associated studies. I would suggest a minor revision for this paper before publishing it.
  
  Reply: Thank you for your constructive comments. We appreciate the positive feedback and are pleased to know that our work is beneficial for ocean modelers. We hope that our revisions have adequately addressed the reviewer’s concerns.
  Major Comments:
  
  (1) One of the comment problems in ocean model is the over-heating in the surface layer. Usually the ocean model needs surface temperature nudging to the reanalysis or climatology SST data, such as GHRSST and MODIS, to avoid over-heating in the surface layer. In this paper, the author mentioned that ROMS were forced by high-resolution wind stress and heat fluxes. I wonder whether ROMS model in this paper only driven by high-resolution heat fluxes without SST nudging, or only driven by high-resolution SST. Please clarify it in the discussion.
  Reply: We understand that the surface temperature nudging to reanalysis or climatology SST data in the ocean model is important. However, in this study, we did not apply additional surface temperature nudging to the reanalysis or climatology SST data to avoid response of the model to artificial forcing which could twist internal physical response of the model and smear our analysis. We conducted a short-term simulation, with the main driving forces being wind stress and bulk heat fluxes. Applying high-frequency SST nudging could potentially affect the physical dynamics.
  
  Furthermore, in this coastal region, the accuracy of GHRSST and MODIS SST is highly questionable, and the variability of SST in this region is large, making it difficult to find high-quality SST data for the coastal area.
  (2) The author used the ERA data with 75 km resolution as the “coarse” resolution product to compared with the WRF 1 km production. Actually, in nowadays, the 12-15 km resolution and 0.2 degree (approximately 22 km) resolution products are quite common, and provided by ECWMF and CFSV2 (from NECP), respectively. Ocean model driven by this 10-20 km resolution products may be closed to that driven by WRF 1 km product. The author may give some comment on this.
  Reply: We acknowledge that other relatively high-resolution datasets are available, such as the latest ERA5 data from ECMWF with a 0.25-degree resolution and the 0.2-degree products from CFSv2 from NCEP. However, different reanalysis datasets can have differences in spatial and temporal resolution, as well as in the way they assimilate observations and model data, leading to different influences on ocean model performance. Thankaswamy et al. (2022) investigated the sensitivity of different reanalysis data (ERA-Interim and NCEP-CFSv2) on WRF dynamic downscaling for the South China Sea and found that the model forced with ERA-Interim data provides the best simulation of surface wind speed characteristics in the region.
  
  In this study, we also used the latest ECMWF ERA5 data to drive the ocean model and found comparable results to those driven by the ERA-Interim data. This could be because they use similar physics schemes.
  
  The primary focus of this study was to compare the response of the high-resolution coastal ocean model (less than 1 km horizontal resolution) results driven by the high-resolution WRF forcing with that driven by the widely used global reanalysis data, to demonstrate the benefits of using high-resolution atmospheric forcing. Therefore, we chose to compare the ocean response forced by our ultra-high-resolution (1 km horizontal resolution) WRF forcing with that by the relatively coarser ERA-Interim data.
  Reference:
  
  Thankaswamy, A.; Xian, T.; Ma, Y.-F.; Wang, L.-P. Sensitivity to Different Reanalysis Data on WRF Dynamic Downscaling for South China Sea Wind Resource Estimations. Atmosphere 2022, 13, 771. https://doi.org/10.3390/atmos13050771
  
  Citation: https://doi.org/10.5194/egusphere-2023-318-AC1
- AC3: 'Reply on RC1', Wenfeng Lai, 09 Jun 2023
  
  The response was uploaded in the form of a supplement
  
  Citation: https://doi.org/10.5194/egusphere-2023-318-AC3
- AC5: 'Reply on RC1', Wenfeng Lai, 09 Jun 2023
  
  Please check this link: https://editor.copernicus.org/index.php?_mdl=msover_md&_jrl=778&_lcm=oc108lcm109w&_acm=get_comm_sup_file&_ms=109814&c=245596&salt=13365367471420842352
  
  Citation: https://doi.org/10.5194/egusphere-2023-318-AC5
RC2:
'Comment on egusphere-2023-318', Anonymous Referee #2, 09 May 2023

Lai and Gan 2023 analyse the variability of coastal circulation and dynamics in response to different atmospheric forcing during a period of downwelling-favorable winds. The analysis is similar to the one by the same authors (i.e. Lai and Gan 2022, cited) for upwelling winds (10-28^th July 2015), while in this case they analyse a period of downwelling-favorable winds (5-23 July 2017). They analyse the sensitivity of the results to different spatio-temporal variable atmospheric forcing, namely 1) single station observation (WL-OBS), 2) global reanalysis data (LR-ERAI) and 3) high resolution regional atmospheric forcing (HR-WRFW with heatflux from ERAI and WRFA with heatflux from WRF) while in Lai and Gan 2022 they used 1) global reanalysis data, 2) high resolution regional atmospheric forcing and 3) air-sea coupled model. The results show that the model with high resolution forcing (and hence better representation of near-surface wind and air temperature) improved the simulation of coastal ocean currents, water temperature and salinity, and estimates of the across-isobath transport. The paper is well written and logically organized. The results are probably of interest for the community. I would suggest publication after minor revison.
Minor comments
It is not clear from the text whether heat flux is used in the simulations forced with observational data. In The methods it only mentions the wind, however on L324 it states ”…the heat flux forcing being the same in the WL-OBS, LR-ERAI, and HR-WRFW experiments…”
The authors used ERAI that has ~75km resolution and 6h resolution as forcing for the LR-ERAI case and as boundary and initial conditions for the production of the WRF 1km that is then used as forcing for the high-resolution cases. ERA5 is available at ECMWF for the period of interest and has higher temporal and spatial resolution (~30km and hourly resolution). A model forced with this higher resolution dataset (ERA5) may provide more accurate results.
Please rephrase L338 “A positive (negative) value represented an onshore (offshore) transport of the shelf water perpendicular to the isobaths”. Parenthesis are used to add extra information. The way the authors use them in the sentence can save a little of space but is confusing.
Please revise the colour schemes you use. The ones in figures 7, 8, 9 and 11 are not colour-blind friendly.

Citation: https://doi.org/10.5194/egusphere-2023-318-RC2
- AC2: 'Reply on RC2', Wenfeng Lai, 09 Jun 2023
  
  Please see the supplement pdf file
  
  Citation: https://doi.org/10.5194/egusphere-2023-318-AC2
- AC4: 'Reply on RC2', Wenfeng Lai, 09 Jun 2023
  
  Please check this link: https://editor.copernicus.org/index.php?_mdl=msover_md&_jrl=778&_lcm=oc108lcm109w&_acm=get_comm_sup_file&_ms=109814&c=245594&salt=267814852887592822
  
  Citation: https://doi.org/10.5194/egusphere-2023-318-AC4

Peer review completion

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

AR by Wenfeng Lai on behalf of the Authors (09 Jun 2023) Author's response Author's tracked changes Manuscript

ED: Publish subject to technical corrections (12 Jun 2023) by John M. Huthnance

Dear Authors
Thank-you for your response to referees’ comments and the revised manuscript. Please consider the comments below in preparing your final manuscript. These are “Technical Corrections” meaning that I do not need to see the manuscript again before you upload it for the final processes leading to publication. However, please note that (i) all the reviewer and editor comments (including those below) will be available to readers of the published paper; they will be able to see how you have responded to the comments; (ii) the manuscript will be copy-edited by Copernicus and you should check that your intended meaning remains.
Thank-you for publishing in Ocean Science
Yours sincerely
John Huthnance (editor).

Comments
Many times you refer to “high resolution”. I prefer “fine resolution” in the same sense that you use “finer” and “fine” in lines 51, 52. But I don’t insist on this change.
Line 27. “moisture fluxes transports” – omit “fluxes” or “transports” or do you want “fluxes and transports”?
Line 28. “parameter” –> “variable” or omit.
Line 57. “over a 36-km spacing” is unclear. If it refers to the atmospheric forcing resolution being compared with 4 km, then “over” –> “compared with”. If 36 km refers to the ocean model, then what is the comparison implied by “more accurate and skillful”?
Line 80. “processes” tends to mean some ocean phenomena. Maybe “procedures” or omit?

Line 93. Omit “quality” [it does not seem to add meaning]?

Section 2.2 last paragraph. Referee 1 suggested nudging but you responded “we did not apply additional surface temperature nudging . . because we conducted a short-term simulation, with the main driving forces being wind stress and heat fluxes. Applying high-frequency SST nudging could potentially twist the physical dynamics.” I accept your response but the referee and any new reader of the revised manuscript might again raise the same question. I think you should include a short version of your response here.

Line 156. “rotated by 23 degrees” from what orientation? As it stands, this whole sentence adds very little meaning., e.g. “cross-shore component being approximately perpendicular to the coastline” is to be expected.
Line 162. “erroneously weaker variability” refers to spatial variability?

Line 253. “21, 2017.”
Figures 6 and 7 differences. The captions should make clear which experiment result is subtracted from which.

Line 398. “domain-averaged” –> “domain-average”
Line 493. Better “. . Ekman transport east of the PRE but weakened it in the west.”?
Line 500. Better “. . exchange west of the PRE.”
Data Availability. “Model and observational data is available on request.” This is not satisfactory for publications generally in 2023. For Ocean Science, please see https://www.ocean-science.net/policies/data_policy.html and the “Statement on the availability of underlying data” there.

Line 517. “This work was supported . .”; at CORE or by CORE? (word missing)

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AR by Wenfeng Lai on behalf of the Authors (20 Jun 2023) Author's response Manuscript

Short summary

Our study shows a high-resolution regional atmospheric model improves near-surface wind and air temperature simulation, resulting in improved circulation and hydrographic simulations in the Pearl River estuary. High-resolution wind forcing is critical for coastal circulation and cross-isobath transport, while high-resolution heat forcing greatly improves sea surface temperature simulation. High-resolution atmospheric forcing is essential for accurately simulating dynamic coastal ocean processes.