# Evaluation of Sentinel-3A Wave Height Observations Near the Coast of Southwest England

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## Abstract

**:**

## 1. Introduction

## 2. Study Area and Datasets

#### 2.1. Southwest England

#### 2.2. Sentinel-3A Observations

#### 2.3. Buoy Observations

#### 2.4. Numerical Model Simulations

## 3. Methods

#### 3.1. S3A and Buoy Correlations

#### 3.2. Areas of Correlation

## 4. Results

#### 4.1. S3A and Buoy Correlations

#### 4.2. Areas of Correlation

#### 4.3. Evaluation of S3A Performance in the Coastal Region

## 5. Discussion

## 6. Conclusions

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 2.**SWH along track 128 for cycle 6: (

**a**) PLRM; and (

**b**) SAR mode. The gap in both tracks is due to land. In orange are the data averaged with a moving Gaussian window of 50 samples at FWHM (corresponding to $\sigma \sim $21 samples).

**Figure 3.**(

**a**,

**d**,

**g**) Locations of the Hub, WBy and Pnz buoys (red circles). Each displayed area extends for ∼15 km on all sides of the relevant buoy. (

**b**,

**e**,

**h**) Time series of SWH for the three buoys. (

**c**,

**f**,

**i**) Polar distribution of observed principal wave direction and period for the three buoys.

**Figure 4.**Example of significant wave height (colour) and swell direction (streamlines) from the WWIII-AMM7 model for 15 February 2018 at 01:00.

**Figure 5.**(

**a**) Map showing the Hub buoy position (magenta star), the two nearest S3A tracks to the buoy (tracks 094 and 071, both in blue) and the various locations along each track at which the correlations between satellite and in-situ observations were computed (coloured circles and triangles). (

**b**,

**c**) Scatter plots between S3A SAR and in-situ observations at each location (symbols and colours are the same as in panel (

**a**)) and corresponding regression lines. The gray dashed line indicates the 1:1 relationship.

**Figure 6.**Same as Figure 5 but for the Pnz buoy and S3A tracks 071 and 094.

**Figure 7.**Same as Figure 5 but for the WBy buoy and S3A tracks 265 and 299.

**Figure 8.**Variation of the regression slope as function of the distance from the buoy for the Hub, Pnz and WBy buoys. The regression slopes were computed at varying locations along the two closest S3A tracks to each buoy.

**Figure 9.**Example of spatial distribution of the correlation coefficient (

**a**), regression slope (

**b**), regression intercept (

**c**) and RMSE (

**d**) computed between the time series of the WWIII-AMM7 model simulation nearest to the Hub buoy and those at each of the grid points of the model domain (see Section 3.2 for more details). The red circle marks the position of the Hub buoy. The thicker contours mark the thresholds for each parameter used to define the areas of correlation around each buoy shown in Figure 10: correlation coefficient (${r}^{2}$) $\ge 0.95$, $0.8\phantom{\rule{3.33333pt}{0ex}}\le $ slope$\phantom{\rule{3.33333pt}{0ex}}\le \phantom{\rule{3.33333pt}{0ex}}1.2$, $-0.1$ m $\le \phantom{\rule{3.33333pt}{0ex}}$intercept$\phantom{\rule{3.33333pt}{0ex}}\le \phantom{\rule{3.33333pt}{0ex}}0.1$ m and RMSE $\le 0.25$ m.

**Figure 10.**Examples of areas of correlation retrieved from the model output for the Hub (

**a**), Pnz (

**b**) and WBy (

**c**) buoys.

**Figure 12.**Correlation slope (

**a**) and RMSE (

**b**) between S3A and in-situ observations as function of distance between along-track location and buoy position. Only the satellite–buoy pairings from Table 2 are included. Both panels are for SAR observations.

**Figure 15.**SWH difference between S3A SAR and in-situ observations as a function of swell period. Only the pairing from Table 2 are included. Each point is the bias computed for a given S3A cycle at the location closest to the coast for a specific track–buoy pairing.

**Figure 16.**Polar plots with SWH difference between S3A SAR and in-situ observations as a function of swell direction for the WBy, E1, and Prp buoys. The bias varies along the radial direction from −2 m at the center to 2 m along the outermost circle. The thicker circle marks zero bias.

**Table 1.**List of the buoys used in our analysis. For each buoy general characteristics and the two closest S3A tracks are provided. Buoy locations and depth are from the latest reports available for each buoy.

Buoy | Full Name | Lat | Lon | Depth (m) | S3A Tracks | |
---|---|---|---|---|---|---|

Bdf | Bideford Bay | 51${}^{\circ}$03.48${}^{\prime}$N | 04${}^{\circ}$16.62${}^{\prime}$W | 11 | 151 | 185 |

Csl | Chesil | 50${}^{\circ}$36.13${}^{\prime}$N | 02${}^{\circ}$31.37${}^{\prime}$W | 12 | 265 | 299 |

Dwl | Dawlish | 50${}^{\circ}$34.80${}^{\prime}$N | 03${}^{\circ}$25.04${}^{\prime}$W | 11 | 242 | 265 |

E1 | E1 Station | 50${}^{\circ}$01.56${}^{\prime}$N | 04${}^{\circ}$13.50${}^{\prime}$W | 75 | 185 | 208 |

Hub | Wave Hub | 50${}^{\circ}$20.84${}^{\prime}$N | 05${}^{\circ}$36.84${}^{\prime}$W | 50 | 071 | 094 |

LoB | Looe Bay | 50${}^{\circ}$20.33${}^{\prime}$N | 04${}^{\circ}$24.65${}^{\prime}$W | 10 | 185 | 208 |

Mhd | Minehead | 51${}^{\circ}$13.68${}^{\prime}$N | 03${}^{\circ}$28.15${}^{\prime}$W | 10 | 208 | 242 |

Plv | Porthleven | 50${}^{\circ}$03.76${}^{\prime}$N | 05${}^{\circ}$18.44${}^{\prime}$W | 15 | 128 | 151 |

Pnz | Penzance | 50${}^{\circ}$06.86${}^{\prime}$N | 05${}^{\circ}$30.18${}^{\prime}$W | 10 | 071 | 094 |

Prp | Perranporth | 50${}^{\circ}$21.19${}^{\prime}$N | 05${}^{\circ}$10.48${}^{\prime}$W | 14 | 128 | 151 |

PtI | Port Isaac | 50${}^{\circ}$35.65${}^{\prime}$N | 04${}^{\circ}$50.07${}^{\prime}$W | N/A | 128 | 151 |

SMS | St Mary’s Sound | 49${}^{\circ}$53.53${}^{\prime}$N | 06${}^{\circ}$18.77${}^{\prime}$W | 53 | 014 | 094 |

StB | Start Bay | 50${}^{\circ}$17.53${}^{\prime}$N | 03${}^{\circ}$36.99${}^{\prime}$W | 10 | 185 | 208 |

Tor | Torbay | 50${}^{\circ}$26.02${}^{\prime}$N | 03${}^{\circ}$31.08${}^{\prime}$W | 11 | 242 | 265 |

WBy | West Bay | 50${}^{\circ}$41.63${}^{\prime}$N | 02${}^{\circ}$45.06${}^{\prime}$W | 10 | 265 | 299 |

Wey | Weymouth | 50${}^{\circ}$37.36${}^{\prime}$N | 02${}^{\circ}$24.85${}^{\prime}$W | 11 | 208 | 299 |

Wst | Weston Bay | 51${}^{\circ}$21.13${}^{\prime}$N | 03${}^{\circ}$01.23${}^{\prime}$W | 13 | 299 | 322 |

**Table 2.**List of buoy–track parings used in Section 4.3 to evaluate S3A performance in the coastal zone. In bold are the tracks identified using the area of correlation described in Section 4.2.

Buoy | S3A Tracks | |
---|---|---|

Bdf | 185 | – |

Csl | 265 | 299 |

Dwl | 242 | – |

E1 | 185 | 208 |

LoB | 128 | – |

Prp | 128 | 151 |

StB | 242 | – |

WBy | 265 | 299 |

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## Share and Cite

**MDPI and ACS Style**

Nencioli, F.; Quartly, G.D.
Evaluation of Sentinel-3A Wave Height Observations Near the Coast of Southwest England. *Remote Sens.* **2019**, *11*, 2998.
https://doi.org/10.3390/rs11242998

**AMA Style**

Nencioli F, Quartly GD.
Evaluation of Sentinel-3A Wave Height Observations Near the Coast of Southwest England. *Remote Sensing*. 2019; 11(24):2998.
https://doi.org/10.3390/rs11242998

**Chicago/Turabian Style**

Nencioli, Francesco, and Graham D. Quartly.
2019. "Evaluation of Sentinel-3A Wave Height Observations Near the Coast of Southwest England" *Remote Sensing* 11, no. 24: 2998.
https://doi.org/10.3390/rs11242998