the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Fusion of Lagrangian drifter data and numerical model outputs for improved assessment of turbulent dispersion
Abstract. Transport and dispersion processes in the ocean are crucial, as they determine the lifetime and fate of biological and chemical quantities drifting with ocean currents. Due to the complexity of the coastal ocean environment, numerical circulation models have difficulties to accurately simulate highly turbulent flows and dispersion processes, especially in highly energetic tidal basins such as the eastern English Channel. A method of improving the results of coastal circulation modeling and tracer dispersion in the Dover Strait is proposed. Surface current velocities derived from Lagrangian drifter measurements in November 2020 and May 2021 were optimally interpolated in time and space to constrain a high-resolution coastal circulation MARS model, with careful attention given to selecting ensemble members composing the model covariance matrix. The space-time velocity covariances derived from model simulations were utilized by the Optimal Interpolation algorithm to determine the most likely evolution of the velocity field under constraints provided by Lagrangian observations and their error statistics. The accuracy of the velocity field reconstruction was evaluated at each time step. The results of the fusion of model outputs with surface drifter velocity measurements show a significant improvement (by ~50 %) of the model capability to simulate the drift of passive tracers in the Dover Strait. Optimized velocity fields were used to quantify the absolute dispersion in the study area. The implications of these results are important, as they can be used to improve existing decision-making support tool or design new tools for monitoring the transport and dispersion in coastal ocean environment.
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Status: open (until 10 May 2024)
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RC1: 'Comment on egusphere-2024-176', Anonymous Referee #1, 26 Feb 2024
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The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-176/egusphere-2024-176-RC1-supplement.pdf
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AC1: 'Reply on RC1', Sloane Bertin, 15 Mar 2024
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Dear reviewer,
Thank you very much for dedicating time to review our manuscript, and for your clear and pertinent remarks. Please find our point-by-point response concerning the major comments. Concerning the minor comments, all of them have been considered and clarified in the revised version of the manuscript (given in red).
Major comments
- Line 125 in page 5, authors just present the tidal conditions for model’s boundary. What are the other boundary conditions for simulation? Such as gradient, Clamped, Flather or other boundary conditions?
The numerical model utilizes nested configurations with progressive resolutions: (i) 2 km covering the Northeast Atlantic (level 0), (ii) 700 m at the regional scale, encompassing the English Channel (level 1), and (iii) 250 m for the Eastern English Channel (level 2). This nesting technique enables the accurate capture of interactions between large-scale and small-scale processes. This enables the transfer of all resolved fields from lower resolution levels to the open boundaries of higher resolution levels.
The model accounts for kinematic free-surface and bottom boundary conditions, contingent upon friction terms (Lazure and Dumas, 2008). The turbulence closure employed in the model follows the approach described in Gaspar et al. (1990).
All these details will be added to the part 2.3 ‘Current velocity from numerical model’.
- Two one-year long model runs are implemented in this research. Can authors provide more detailed modeling settings to make model stable in such long simulation.
Comprehensive information regarding model equations, the coupling of barotropic and baroclinic modes, model discretization, solving methods, computational stability according to CFL criterion (table 1, Lazure and Dumas, 2008), and costs are meticulously outlined in Lazure and Dumas (2008). To maintain CFL stability, the modeling timestep was set to 30 seconds for the level 2 model.
All these details will be added to the part 2.3 ‘Current velocity from numerical model’.
- In Figure 5, authors just present the model absolute errors at S2-4. May authors present the absolute errors using “Box-Whisker” plot over all drifters of S2?
Thank you for this advice. I modified Figure 5 and used box-whiskers to present the model absolute error for all the drifters during S1 (Fig. 5a) and S2 (Fig. 5b). Consequently, I modified the descriptive paragraph L266-280 and legend.
- Line 280-281, authors describe that the drifter S2-4 is well reproduced. Can authors plot all drifters of S1and S2 in Figure 6 to make reader directly understand the simulation results. There is similar problem in Figure 7. In Figure 7, authors use drifters S1-2 and S2-1, why is it different from Figure 6? Can authors also present all drifters of S1 and S2 in Figure 7? Or using “Box-Whisker” plot to present separation distance in Figure 7? Furthermore, how about the wind-corrected trajectories of other drifters in S1 and S2? Can present other drifters in Figure 8? If other OI- or corrected trajectories are similar to S1-1 and S2-1, please describe the related statement of other trajectories in S1 and S2.
I modified Figures 6 and 8 by presenting all the drifters’ trajectories (2 for S1 and 4 for S2) in order to make the reader directly understand the simulation results. I also modified Figure 7 by using box-whiskers to present the separation distance results for all the drifters simultaneously. Consequently, the paragraphs and legends concerning these three figures have been modified.
Thank you,
Best regards.
Sloane Bertin
Citation: https://doi.org/10.5194/egusphere-2024-176-AC1
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AC1: 'Reply on RC1', Sloane Bertin, 15 Mar 2024
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