Sea Surface Currents Estimated from Spaceborne Infrared Images Validated against Reanalysis Data and Drifters in the Mediterranean Sea
Abstract
:1. Introduction
2. Data and Methods
2.1. Satellite, Drifter, and Reanalysis Data
2.2. Sea Surface Current Retrieval with the Maximum Cross Correlation Algorithm
- the size of the template, that we here define as a box of 5, 10 (default), or 20 km per side;
- the size of the search window, here defined by setting the maximum velocity that the retrieved field cannot exceed to 1.0 m (default), 1.3 m , or 1.8 m ;
- the maximum cross-correlation threshold, which means that velocity values are returned only if the maximum correlation is larger than 0.3, 0.6 (default), or 0.9.
2.3. Methods for Comparison
- we discard both MCC and drifter points with a speed equal to zero, for which a direction cannot be estimated;
- we discard the MCC points that are further than 20 km away from a drifter (Figure 1). We tested different thresholds between 10 and 50 km, and found that the results were not significantly affected by this choice of threshold;
- we discard the MCC points that are within 20 km of each other and whose directions differ by more than . For such points, the uncertainty attached to the MCC result is so high that an agreement with in-situ observation is meaningless (and due to luck).
3. Results and Discussion
3.1. MCC Performance—Sensitivity Experiment
- template window of 10 km by 10 km;
- maximum speed of 1.0 m ;
- correlation threshold R = 0.6;
3.2. MCC Performance—Characteristics of the Images
3.3. MCC and Drifters vs. Reanalysis
4. Summary, Limitations, and Conclusions
- the size of the template window, on the first image;
- the maximum velocity allowed, which determines the size of the search window, on the second image;
- the cutoff threshold in correlation.
Acknowledgments
Author Contributions
Conflicts of Interest
References
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Template Size | 10 km | 5 km | 20 km | 10 km | 10 km | 10 km | 10 km |
Maximum Velocity | 1.0 m | 1.0 m | 1.0 m | 1.3 m | 1.8 m | 1.0 m | 1.0 m |
Correlation | R = 0.6 | R = 0.6 | R = 0.6 | R = 0.6 | R = 0.6 | R = 0.3 | R = 0.9 |
Speed Only | 56% | 63% | 45% | 45% | 40% | 51% | 53% |
Direction Only | 28% | 23% | 29% | 28% | 30% | 32% | 24% |
Speed and Direction | 16% | 12% | 13% | 14% | 11% | 17% | 12% |
Total Points | 351 | 293 | 176 | 353 | 359 | 317 | 337 |
MCC-drifter | MCC-Reanalysis | Drifter-Reanalysis | ||
---|---|---|---|---|
Speed | mean | m | 0.08 m | 0.31 m |
RMSE | 0.51 m | 0.19 m | 1.09 m | |
Direction | mean | |||
RMSE |
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Heuzé, C.; Carvajal, G.K.; Eriksson, L.E.B.; Soja-Woźniak, M. Sea Surface Currents Estimated from Spaceborne Infrared Images Validated against Reanalysis Data and Drifters in the Mediterranean Sea. Remote Sens. 2017, 9, 422. https://doi.org/10.3390/rs9050422
Heuzé C, Carvajal GK, Eriksson LEB, Soja-Woźniak M. Sea Surface Currents Estimated from Spaceborne Infrared Images Validated against Reanalysis Data and Drifters in the Mediterranean Sea. Remote Sensing. 2017; 9(5):422. https://doi.org/10.3390/rs9050422
Chicago/Turabian StyleHeuzé, Céline, Gisela K. Carvajal, Leif E. B. Eriksson, and Monika Soja-Woźniak. 2017. "Sea Surface Currents Estimated from Spaceborne Infrared Images Validated against Reanalysis Data and Drifters in the Mediterranean Sea" Remote Sensing 9, no. 5: 422. https://doi.org/10.3390/rs9050422