A robust framework for comparing Lagrangian and gridded Eulerian velocity fields: an example application to surface drifters and altimetry-derived surface currents
Abstract. A novel framework for comparing Lagrangian and gridded Eulerian velocity fields is proposed. The method involves three steps that individually leverage the strengths of each reference frame to provide robust intercomparisons, while also allowing the user to adapt the individual steps to the process of interest. The utility of the methodology is then demonstrated by applying it to a comparison between a set of 34 Lagrangian surface drifters deployed on the continental shelf around the southern tip of Greenland and Eulerian altimetry-derived surface currents with and without Ekman velocities. We conclude that the methodology sufficiently addresses differences between the reference frames and accurately identifies errors when they exist. This result enables us to conclude that the altimetry-derived surface currents accurately resolve the important components of the shelf circulation around the southern tip of Greenland and can be used to track pathways of fresh water around Greenland.
Arthur Coquereau and Nicholas P. Foukal
Status: final response (author comments only)
RC1: 'Comment on egusphere-2022-1450', Anonymous Referee #1, 09 Feb 2023
- AC1: 'Reply on RC1', Arthur Coquereau, 28 May 2023
RC2: 'Comment on egusphere-2022-1450', Anonymous Referee #2, 06 Apr 2023
- AC2: 'Reply on RC2', Arthur Coquereau, 28 May 2023
Arthur Coquereau and Nicholas P. Foukal
Global Ocean Gridded L4 Sea Surface Heights And Derived Variables Nrt https://doi.org/10.48670/moi-00149
Global Total Surface and 15m Current (COPERNICUS-GLOBCURRENT) from Altimetric Geostrophic Current and Modeled Ekman Current Processing https://doi.org/10.48670/moi-00049
Arctic Ocean Physics Analysis and Forecast https://doi.org/10.48670/moi-00001
Global Drifter Program quality-controlled 6-hour interpolated data https://doi.org/10.25921/7ntx-z961
Global Ocean Gridded L 4 Sea Surface Heights And Derived Variables Reprocessed 1993 Ongoing https://doi.org/10.48670/moi-00148
Global Total Surface and 15m Current (COPERNICUS-GLOBCURRENT) from Altimetric Geostrophic Current and Modeled Ekman 440 Current Reprocessing https://doi.org/10.48670/moi-00050
Arthur Coquereau and Nicholas P. Foukal
Viewed (geographical distribution)
The authors propose a more comprehensive approach for evaluating and intercomparing different Eulerian gridded surface current products against Lagrangian drifting data. It consists in evaluating gridded Eulerian products in 3 independent steps: a direct comparison of model velocities to drifter velocities, a comparison in the Eulerian frame of reference by transforming Lagrangian drifting velocities onto a Eulerian grid, and finally a comparison in the Lagrangian framework through the generation of model trajectories from eulerian velocities. These approaches are not novel as suggested in the title as each one is widely applied in the community but not necessarily altogether in one same study as done here. A novelty however is the complementary assessment of products using along and across shore velocities.
The authors illustrate the approach with a case study around Greenland comparing two altimetry-derived surface velocity products (geostrophy and geostophy+ekman), and conclude that both products reproduce well the shelf circulation of the region without entering too much in the detail as to why. The tone of the paper is ambitious, yet the manuscript does not contain major advances, and I feel that the manuscript would require significant modification for reaching the objectives it sets itself.
A concluding sentence of the introduction highlights the model errors in the region as a justification for such study, yet no model have been included in the investigation, solely altimetry derived products. The regional analysis system TOPAZ4 was included in step1 as a reference point, but not carried across in the following steps which is regrettable. One main purpose of carrying out such intercomparison work is to assess the skill of operational systems. Previous studies have evaluated altimetry products with operationnel systems (e.g. Dagestad and Rohrs, 2019). It would therefore be very valuable to assess TOPAZ4 in all 3 steps.
The manuscript would benefit from a more exhaustive review of work carried out on the topic (and in the region), so to highlight the state of the art, present their limitation to justify the proposed approach.
For step1, the methodology suggests to select the closest gridded product point to compare with the observation (daily average). I disagree with this approach and would recommend to interpolate the product value at the point of observation. It is not clear what resolution TOPAZ4 is, and for an approach to be robust a methodology should also be provided when comparing products of different resolution.
It is not clear how across and along shelf velocity are calculated, (this could be illustrated in figure1). The author mention smoothing the bathymetry, but without further detail (which bathymetry product, is the bathymetry common to all products?), how is the distance to the coast calculated? The authors introduce frequency spectrum analysis but do not carry it out in this study.
Comments for step2:
Lagrangian velocity is transformed onto Eulerian grid for comparison. The result is a map of average velocity for each gridcell wherethrough drifter travelled for the period that the drifter covered. The spatial averaging is well explained and clear, what is not so clear is how the eulerian gridded product velocities have been selected to confront with each transformed lagrangian velocity. Let say if velocity values for a drifter travelling a given cell covers the temporal window not centered on the time window of the daily average gridded product, how is this addressed?
A map of mean velocity vectors for each product, together with differences in magnitude and direction are proposed to evaluate the skill of products for the different subregions of the shelf. It would be interesting to measure second order statistics so to get a better feel of the product behaviour, and put uncertainties on the values (e.g. on skill score like Revelard et al. 2021).
Comments for step3:
Statistics are done for the Liu skill score. A score that sets to 0 the negative values. Mean values of this skill score do not take into account the negative values, and may not be so robust when intercomparing models. Other studies have suggested alternative statistics for the skill score distribution such as the proportion of score > 0.6 (Revelard et al. 2021), which may be more informative.
No information is given for the particle tracking software used.
Overall point for the case study:
This study investigates altimetry derived products for the Greenland shelf, with an assessment of Ekman component on improving the representation of shelf circulation. The study would benefit from a more in depth study of the processes. For example, evaluating the altimetry tracks, in particular their angle relative to meridional/zonal component, which could explain the better resolution of one component relative to the other. Relative to Ekman, questions are raised to explain why it may perform better than other regions. The Ekman component of the product could be assessed by investigating wind patterns of the regions, and see if a better parameterisation would improve results.
A paragraph is missing in the discussion wherein the 3 steps are brought together and their complementarity illustrated, as such it feels like 3 independent steps.
The study does not address approaches raised in the community such as for example particle ensemble releases with diffusion terms to account uncertainties.
I feel that the manuscript would be better pitched if the authors stated that they used a complementary approach to validate surface current (here geo and geo + ekman) illustrated with their Greenland case study, rather than presenting it as a new framework.