Surface circulation characterization along the middle-south coastal region of Vietnam from high-frequency radar and numerical modelling

Tran, Thanh Huyen; Sentchev, Alexei; To, Duy Thai; Herrmann, Marine; Ouillon, Sylvain; Nguyen, Kim Cuong

doi:https://doi.org/10.5194/egusphere-2024-2323

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https://doi.org/10.5194/egusphere-2024-2323

Preprints

08 Aug 2024

| 08 Aug 2024

Surface circulation characterization along the middle-south coastal region of Vietnam from high-frequency radar and numerical modelling

Thanh Huyen Tran, Alexei Sentchev, Duy Thai To, Marine Herrmann, Sylvain Ouillon, and Kim Cuong Nguyen

Abstract. Coastal water dynamics along the Vietnam Middle-South Coastal region (VMSC), part of the South China Sea, is highly complex with large spatio-temporal variability whose drivers are not yet well understood. For the first time, high-resolution surface current data from high-frequency radar (HFR) measurements were obtained in this region during the early (transition) phase of the Asian summer monsoon. The data were used to compare with simulation results from a circulation model, SYMPHONIE, and ultimately to optimise the wind forcing in the model. Both modelling and HFR were able to show the spatial and temporal evolution of the surface circulation, but some discrepancies were found between model and HFR data on some days, coinciding with the evolution of the wind. Two methods were used to optimise the wind forcing, namely the Ensemble Perturbation Smoother (EnPS) and the wind correction method using wind-driven surface currents (EkW). Both methods achieved a significant reduction (~36–40 %) in the error of the surface current velocity fields compared to the measured data. Optimised winds obtained from the two methods were compared with satellite wind data for validation. The results show that both optimisation methods performed better in the far field, where topography no longer affects the coastal surface circulation. The optimisation results revealed that the surface circulation is not only driven by winds but also by other factors such as intrinsic ocean variability which is not entirely controlled by boundary conditions. This indicates the potential usefulness of large velocity datasets and other data fusion methods to effectively improve modelling results.

Received: 23 Jul 2024 – Discussion started: 08 Aug 2024

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Thanh Huyen Tran, Alexei Sentchev, Duy Thai To, Marine Herrmann, Sylvain Ouillon, and Kim Cuong Nguyen

Status: closed

RC1:
'Comment on egusphere-2024-2323', Anonymous Referee #1, 20 Sep 2024

“the spatial and temporal evolution of the surface circulation, but some discrepancies were found between model and HFR data on some days, coinciding with the evolution of the wind. Two methods were used to optimise the wind forcing, namely the Ensemble Perturbation Smoother (EnPS) and the wind correction method using wind-driven surface currents (EkW)” this is assuming that the model is not following wind forcing, or wind forcing is not accurate enough. Do you have independent wind data from meteorologic stations nearby to further confirm this?
“The optimisation results revealed that the surface circulation is not only driven by winds but also by other factors such as intrinsic ocean variability which is not entirely controlled by boundary condition“ what are these processes and factors?
L33, “and fully“ “and is fully”
L40:”coastal dynamics along the VMSC is strongly influenced by ocean intrinsic variability” such as?
L90: remove “performed”
L104-108: explain more in details how data analysis is performed: what depth the current meter data refer to, what grid point is used the the radar data, what data QC is used for the current meter,…and so forth. Data quality from the current meter seems to be failry poo if compared to HFR currents. what is the casue for that difference? is it maybe using the closest bin to surface which may be contaminated? Provide units for MAE and RMSE. The current meter is at the oundary of the V2 station and fairly close to the baseline so errors and differences may be explained by a combination of factors
L205: “VMSC_ref’s time-series associated with wind time-series, but not for HFR measurements.” the inertial peaks are also found on the model simulations. Interestingly, model and HFR spectra share same structure in the low freq band as the wind suggesting that this band is relatively well mateched however this is not found in the high-frequency tails. I remember seeing this elsewhere where models and HFR data were compared and that was explained by the poor wind energy in these frequency band, the model restart or the lack of stratification in the model. how does this fit within the context of the region of interest here?

Citation: https://doi.org/10.5194/egusphere-2024-2323-RC1
- AC3:
  'Reply on RC1', Thanh Huyen Tran, 14 Oct 2024
  Dear Reviewer,
  We would like to thank you for your comprehensive feedback. We would like to respond to your questions as follow:
  1. Comment#1: “the spatial and temporal evolution of the surface circulation, but some discrepancies were found between model and HFR data on some days, coinciding with the evolution of the wind. Two methods were used to optimise the wind forcing, namely the Ensemble Perturbation Smoother (EnPS) and the wind correction method using wind-driven surface currents (EkW)” this is assuming that the model is not following wind forcing, or wind forcing is not accurate enough. Do you have independent wind data from meteorologic stations nearby to further confirm this?
  Our response: Yes, there are two meteorological stations nearby. However, they are not representative for the wind in the open sea since they are located far inland. The additional text to explain this was elaborated in lines 405-409 in Section Discussion of the revised manuscript.
  2. Comment#2: “The optimisation results revealed that the surface circulation is not only driven by winds but also by other factors such as intrinsic ocean variability which is not entirely controlled by boundary condition”. what are these processes and factors?
  Our response:
  The main factor was a nonlinear behavior of the ocean that has been explained by a concept of ocean intrinsic variability, explained in lines 366-375 in the Section “Discussion” of the revised manuscript.
  Another factor was the interaction of a powerful flow with headlands. This was added in the “Conclusion and future work” Section, in lines 446-448, of the revised manuscript.
  3. Comment#3: L33, “and fully“ “and is fully”
  Our response: The text was modified in the revised manuscript.
  4. Comment#4: L40:”coastal dynamics along the VMSC is strongly influenced by ocean intrinsic variability” such as?
  Our response: Such as: producing variations of upwelling expansion, location of sub-mesoscale eddies and current jets, and the intensity and size of eddies. The text in lines 40-41 of the revised manuscript was modified to meet the Reviewer’s recommendation.
  5. Comment#5: L90: remove “performed”
  Our response: The text was modified in the revised manuscript.
  6. Comment#6: L104-108: explain more in details how data analysis is performed: what depth the current meter data refer to, what grid point is used the the radar data, what data QC is used for the current meter,…and so forth. Data quality from the current meter seems to be failry poo if compared to HFR currents. what is the casue for that difference? is it maybe using the closest bin to surface which may be contaminated? Provide units for MAE and RMSE. The current meter is at the oundary of the V2 station and fairly close to the baseline so errors and differences may be explained by a combination of factors.
  Our response:
  The fairly poor quality of AWAC measurements was due to the impact of waves since the data was taken from the surface layer depth (1.5m bin size).
  
  The discrepancy between AWAC and HFR velocity time-series can be attributed to two primary factors: the difference in measurement depths and the wave-induced effects on AWAC measurements in the surface layer depth.
  
  Regarding the baseline, the surface current used in all analyses have been reconstructed by using 2DVar method, a non-local interpolation technique, providing good-quality vector maps, also in areas with high GDOP (geometric dilution of precision).
  
  The text in lines 105-115 was modified to meet the Reviewer’s recommendation
  7. Comment#7: L205: “VMSC_ref’s time-series associated with wind time-series, but not for HFR measurements.” the inertial peaks are also found on the model simulations. Interestingly, model and HFR spectra share same structure in the low freq band as the wind suggesting that this band is relatively well mateched however this is not found in the high-frequency tails. I remember seeing this elsewhere where models and HFR data were compared and that was explained by the poor wind energy in these frequency band, the model restart or the lack of stratification in the model. how does this fit within the context of the region of interest here?
  Our response:
  Regarding the high-frequency tails, two reasons have been explained in lines 224-229 of the revised manuscript.
  The explanations for the coherence between the model and observation within sub-tidal bands, as well as the impact of low-resolution forcing data on the model's ability to reconstruct higher-frequency variabilities of surface circulation, have been elaborated in the revised manuscript, which includes a text and two new references (lines 229-234).
  
  Citation: https://doi.org/10.5194/egusphere-2024-2323-AC3
RC2:
'Comment on egusphere-2024-2323', Anonymous Referee #2, 26 Sep 2024

\title{Rewiev Ms. {\bf {``Surface circulation characterization along the middle-south coastal

region of Vietnam from high-frequency radar and numerical

modeling'' }}}

The Ms. presents the use of HFR data in the Vietnam Middle South coastal region to optimise wind forcing for model simulation during the the transition phase of the Asian summer monsoon. Wind forcing was modified using 2 different methods; the Ensemble Perturbation Smoother and correction from wind driven currents showing a notable reduction in the error compared with measurements from satellite.
The study of great interest either for modelling studies where the interest is to improve as much as possible the forcing data as well as for operational research where the availability of detailed surface currents from the HFR might provide a large improvement in the outputs of coastal numerical models.
The Ms is well presented and it follows a rigorous presentation. However I think that two issues have

to be addressed or commented at a certain point.
\begin{itemize}

\item While the use of HFR data to correct winds seems to provide a very promising approach for models, HFR measures the total velocity including the stokes drift from waves. I think that in this case, Eq 7 is not valid. Can the authors explain this point?.
\item The simulation experiments for the wind reduction (from the 5th -19th May) is too short to infer some conclusion about the general improvement of the methodology. Are they just resulting from the specific wind and mesoescale conditions present during the transition phase?.
\item I don't see the necessity of including Section 3.4 in this manuscript.
\ Page 19. line 351. Remove ``non linear chaos''

\end{itemize}

Citation: https://doi.org/10.5194/egusphere-2024-2323-RC2
- AC1: 'Reply on RC2', Thanh Huyen Tran, 01 Oct 2024
  
  Dear Reviewer (RC2),
  On behalf of all Co-Authors, I would like to thank you for your helpful and comprehensive reviewing report.
  I would like to make clear about the point "\item I don't see the necessity of including Section 3.4 in this manuscript.". In the manuscript, there is no Section 3.4. Could you clarify in details which Section you mentioned about, and whether we should remove it or should we modify it in a proper way?
  Regarding your other comments, we will synthesize them in one document and upload in the next days with the revised version of the manuscript.
  Thank you with best regards,
  Tran et. al
  
  Citation: https://doi.org/10.5194/egusphere-2024-2323-AC1
- AC2: 'Reply on RC2', Thanh Huyen Tran, 14 Oct 2024
  
  Dear Reviewer,
  Thank you for your precious comments. We would like to respond to your comments and questions as follow:
  1. Comment#1: "While the use of HFR data to correct winds seems to provide a very promising approach for models, HFR measures the total velocity including the stokes drift from waves. I think that in this case, Eq 7 is not valid. Can the authors explain this point?"
  Our response:
  We quantified the contribution of sea states (waves) to the surface current obtained from HF radar using equations A4 and A5 in the paper of “A. Sentchev, P. Forget, Y. Barbin, M. Yaremchuk, Surface circulation in the Iroise Sea (W. Brittany) from high resolution HF radar mapping, Journal of Marine Systems 109-110 (2013) S153–S168"
  The space-averaged wind speed from the ECMWF did not exceed 6-7 ms^-1, and the significant wave height (H_s, space-averaged) did not exceed 0.5 m. With this information, we proceeded to quantify the velocity of wave-induced currents, whereby we determined that the contribution of Stokes-drift to the total surface currents measured by HF radar was estimated at 0.02 m/s, representing approximately 4-5% of the total surface current velocity.
  This illustrates that the Stokes' impact on the present velocity estimation from the EkW method can be neglected. We're assuming that Eq. 7 applies.
  2. Comment#2: "The simulation experiments for the wind reduction (from the 5th -19th May) is too short to infer some conclusion about the general improvement of the methodology. Are they just resulting from the specific wind and mesoscale conditions present during the transition phase?"
  Our response:
  We have applied the methods to different periods (both April and May) but only selected the specific time period from May 5 to May 14 (10 days). We agree with the Reviewer that the selected period was short. However, during this period, the current velocity maps from HFR demonstrated large variability of circulation patterns (please see Fig. 6a,b,c), which were not consistent with the evolution of the ECMWF wind. Additionally, a significant discrepancy was observed between the model and observations in V component of the surface currents during this period (Fig. 7e,f).
  With all those evidences, our hypothesis was that the wind forcing was the main cause of errors in model simulations. During this period, there was a significant shift in the wind direction, which could potentially explain the identified errors.
  3. Comment#3: I don't see the necessity of including Section 3.4 in this manuscript.
  Our response: We could not meet the Reviewer’s recommendation because we do not have the Section 3.4 in our manuscript.
  4. Comment#4: Page 19. line 351. Remove “nonlinear chaos''
  Our response: The text in lines 361-366 in the revised version of the manuscript was modified toward the comment of the Reviewer.
  
  Citation: https://doi.org/10.5194/egusphere-2024-2323-AC2

Status: closed

RC1:
'Comment on egusphere-2024-2323', Anonymous Referee #1, 20 Sep 2024

“the spatial and temporal evolution of the surface circulation, but some discrepancies were found between model and HFR data on some days, coinciding with the evolution of the wind. Two methods were used to optimise the wind forcing, namely the Ensemble Perturbation Smoother (EnPS) and the wind correction method using wind-driven surface currents (EkW)” this is assuming that the model is not following wind forcing, or wind forcing is not accurate enough. Do you have independent wind data from meteorologic stations nearby to further confirm this?
“The optimisation results revealed that the surface circulation is not only driven by winds but also by other factors such as intrinsic ocean variability which is not entirely controlled by boundary condition“ what are these processes and factors?
L33, “and fully“ “and is fully”
L40:”coastal dynamics along the VMSC is strongly influenced by ocean intrinsic variability” such as?
L90: remove “performed”
L104-108: explain more in details how data analysis is performed: what depth the current meter data refer to, what grid point is used the the radar data, what data QC is used for the current meter,…and so forth. Data quality from the current meter seems to be failry poo if compared to HFR currents. what is the casue for that difference? is it maybe using the closest bin to surface which may be contaminated? Provide units for MAE and RMSE. The current meter is at the oundary of the V2 station and fairly close to the baseline so errors and differences may be explained by a combination of factors
L205: “VMSC_ref’s time-series associated with wind time-series, but not for HFR measurements.” the inertial peaks are also found on the model simulations. Interestingly, model and HFR spectra share same structure in the low freq band as the wind suggesting that this band is relatively well mateched however this is not found in the high-frequency tails. I remember seeing this elsewhere where models and HFR data were compared and that was explained by the poor wind energy in these frequency band, the model restart or the lack of stratification in the model. how does this fit within the context of the region of interest here?

Citation: https://doi.org/10.5194/egusphere-2024-2323-RC1
- AC3:
  'Reply on RC1', Thanh Huyen Tran, 14 Oct 2024
  Dear Reviewer,
  We would like to thank you for your comprehensive feedback. We would like to respond to your questions as follow:
  1. Comment#1: “the spatial and temporal evolution of the surface circulation, but some discrepancies were found between model and HFR data on some days, coinciding with the evolution of the wind. Two methods were used to optimise the wind forcing, namely the Ensemble Perturbation Smoother (EnPS) and the wind correction method using wind-driven surface currents (EkW)” this is assuming that the model is not following wind forcing, or wind forcing is not accurate enough. Do you have independent wind data from meteorologic stations nearby to further confirm this?
  Our response: Yes, there are two meteorological stations nearby. However, they are not representative for the wind in the open sea since they are located far inland. The additional text to explain this was elaborated in lines 405-409 in Section Discussion of the revised manuscript.
  2. Comment#2: “The optimisation results revealed that the surface circulation is not only driven by winds but also by other factors such as intrinsic ocean variability which is not entirely controlled by boundary condition”. what are these processes and factors?
  Our response:
  The main factor was a nonlinear behavior of the ocean that has been explained by a concept of ocean intrinsic variability, explained in lines 366-375 in the Section “Discussion” of the revised manuscript.
  Another factor was the interaction of a powerful flow with headlands. This was added in the “Conclusion and future work” Section, in lines 446-448, of the revised manuscript.
  3. Comment#3: L33, “and fully“ “and is fully”
  Our response: The text was modified in the revised manuscript.
  4. Comment#4: L40:”coastal dynamics along the VMSC is strongly influenced by ocean intrinsic variability” such as?
  Our response: Such as: producing variations of upwelling expansion, location of sub-mesoscale eddies and current jets, and the intensity and size of eddies. The text in lines 40-41 of the revised manuscript was modified to meet the Reviewer’s recommendation.
  5. Comment#5: L90: remove “performed”
  Our response: The text was modified in the revised manuscript.
  6. Comment#6: L104-108: explain more in details how data analysis is performed: what depth the current meter data refer to, what grid point is used the the radar data, what data QC is used for the current meter,…and so forth. Data quality from the current meter seems to be failry poo if compared to HFR currents. what is the casue for that difference? is it maybe using the closest bin to surface which may be contaminated? Provide units for MAE and RMSE. The current meter is at the oundary of the V2 station and fairly close to the baseline so errors and differences may be explained by a combination of factors.
  Our response:
  The fairly poor quality of AWAC measurements was due to the impact of waves since the data was taken from the surface layer depth (1.5m bin size).
  
  The discrepancy between AWAC and HFR velocity time-series can be attributed to two primary factors: the difference in measurement depths and the wave-induced effects on AWAC measurements in the surface layer depth.
  
  Regarding the baseline, the surface current used in all analyses have been reconstructed by using 2DVar method, a non-local interpolation technique, providing good-quality vector maps, also in areas with high GDOP (geometric dilution of precision).
  
  The text in lines 105-115 was modified to meet the Reviewer’s recommendation
  7. Comment#7: L205: “VMSC_ref’s time-series associated with wind time-series, but not for HFR measurements.” the inertial peaks are also found on the model simulations. Interestingly, model and HFR spectra share same structure in the low freq band as the wind suggesting that this band is relatively well mateched however this is not found in the high-frequency tails. I remember seeing this elsewhere where models and HFR data were compared and that was explained by the poor wind energy in these frequency band, the model restart or the lack of stratification in the model. how does this fit within the context of the region of interest here?
  Our response:
  Regarding the high-frequency tails, two reasons have been explained in lines 224-229 of the revised manuscript.
  The explanations for the coherence between the model and observation within sub-tidal bands, as well as the impact of low-resolution forcing data on the model's ability to reconstruct higher-frequency variabilities of surface circulation, have been elaborated in the revised manuscript, which includes a text and two new references (lines 229-234).
  
  Citation: https://doi.org/10.5194/egusphere-2024-2323-AC3
RC2:
'Comment on egusphere-2024-2323', Anonymous Referee #2, 26 Sep 2024

\title{Rewiev Ms. {\bf {``Surface circulation characterization along the middle-south coastal

region of Vietnam from high-frequency radar and numerical

modeling'' }}}

The Ms. presents the use of HFR data in the Vietnam Middle South coastal region to optimise wind forcing for model simulation during the the transition phase of the Asian summer monsoon. Wind forcing was modified using 2 different methods; the Ensemble Perturbation Smoother and correction from wind driven currents showing a notable reduction in the error compared with measurements from satellite.
The study of great interest either for modelling studies where the interest is to improve as much as possible the forcing data as well as for operational research where the availability of detailed surface currents from the HFR might provide a large improvement in the outputs of coastal numerical models.
The Ms is well presented and it follows a rigorous presentation. However I think that two issues have

to be addressed or commented at a certain point.
\begin{itemize}

\item While the use of HFR data to correct winds seems to provide a very promising approach for models, HFR measures the total velocity including the stokes drift from waves. I think that in this case, Eq 7 is not valid. Can the authors explain this point?.
\item The simulation experiments for the wind reduction (from the 5th -19th May) is too short to infer some conclusion about the general improvement of the methodology. Are they just resulting from the specific wind and mesoescale conditions present during the transition phase?.
\item I don't see the necessity of including Section 3.4 in this manuscript.
\ Page 19. line 351. Remove ``non linear chaos''

\end{itemize}

Citation: https://doi.org/10.5194/egusphere-2024-2323-RC2
- AC1: 'Reply on RC2', Thanh Huyen Tran, 01 Oct 2024
  
  Dear Reviewer (RC2),
  On behalf of all Co-Authors, I would like to thank you for your helpful and comprehensive reviewing report.
  I would like to make clear about the point "\item I don't see the necessity of including Section 3.4 in this manuscript.". In the manuscript, there is no Section 3.4. Could you clarify in details which Section you mentioned about, and whether we should remove it or should we modify it in a proper way?
  Regarding your other comments, we will synthesize them in one document and upload in the next days with the revised version of the manuscript.
  Thank you with best regards,
  Tran et. al
  
  Citation: https://doi.org/10.5194/egusphere-2024-2323-AC1
- AC2: 'Reply on RC2', Thanh Huyen Tran, 14 Oct 2024
  
  Dear Reviewer,
  Thank you for your precious comments. We would like to respond to your comments and questions as follow:
  1. Comment#1: "While the use of HFR data to correct winds seems to provide a very promising approach for models, HFR measures the total velocity including the stokes drift from waves. I think that in this case, Eq 7 is not valid. Can the authors explain this point?"
  Our response:
  We quantified the contribution of sea states (waves) to the surface current obtained from HF radar using equations A4 and A5 in the paper of “A. Sentchev, P. Forget, Y. Barbin, M. Yaremchuk, Surface circulation in the Iroise Sea (W. Brittany) from high resolution HF radar mapping, Journal of Marine Systems 109-110 (2013) S153–S168"
  The space-averaged wind speed from the ECMWF did not exceed 6-7 ms^-1, and the significant wave height (H_s, space-averaged) did not exceed 0.5 m. With this information, we proceeded to quantify the velocity of wave-induced currents, whereby we determined that the contribution of Stokes-drift to the total surface currents measured by HF radar was estimated at 0.02 m/s, representing approximately 4-5% of the total surface current velocity.
  This illustrates that the Stokes' impact on the present velocity estimation from the EkW method can be neglected. We're assuming that Eq. 7 applies.
  2. Comment#2: "The simulation experiments for the wind reduction (from the 5th -19th May) is too short to infer some conclusion about the general improvement of the methodology. Are they just resulting from the specific wind and mesoscale conditions present during the transition phase?"
  Our response:
  We have applied the methods to different periods (both April and May) but only selected the specific time period from May 5 to May 14 (10 days). We agree with the Reviewer that the selected period was short. However, during this period, the current velocity maps from HFR demonstrated large variability of circulation patterns (please see Fig. 6a,b,c), which were not consistent with the evolution of the ECMWF wind. Additionally, a significant discrepancy was observed between the model and observations in V component of the surface currents during this period (Fig. 7e,f).
  With all those evidences, our hypothesis was that the wind forcing was the main cause of errors in model simulations. During this period, there was a significant shift in the wind direction, which could potentially explain the identified errors.
  3. Comment#3: I don't see the necessity of including Section 3.4 in this manuscript.
  Our response: We could not meet the Reviewer’s recommendation because we do not have the Section 3.4 in our manuscript.
  4. Comment#4: Page 19. line 351. Remove “nonlinear chaos''
  Our response: The text in lines 361-366 in the revised version of the manuscript was modified toward the comment of the Reviewer.
  
  Citation: https://doi.org/10.5194/egusphere-2024-2323-AC2

Thanh Huyen Tran, Alexei Sentchev, Duy Thai To, Marine Herrmann, Sylvain Ouillon, and Kim Cuong Nguyen

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Short summary

For the first time, high-resolution surface current data from high-frequency radar have been obtained along the central and southern coasts of Vietnam, and combined with a modelling approach, this is helping scientists to understand coastal processes. The research showed that the surface circulation is not only driven by winds, but also by other factors. This can enrich public knowledge of the coastal dynamics that govern other environmental impacts along the coasts.


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