## 4 Observation of Floating-Oil-Slick Evolution Off the Shore of Qatar
### Case #Q1: 27-28 March 2021
Then, in the second phase, from 21:00 on 27 March, to 06:00 on 28 March, the oil moved in the same direction as the wind (southeast) since the current was moving between the northeast and north. In particular, the significant increase in wind speed (from 2 m/s to 10.5 m/s) in the second phase is assumed to be the main contribution to the oil drift toward the southeast. Likewise, strong wind is assumed to have split the oil detected in the Sentinel-2 into two parts, which were observed on the Sentinel-3 image. Figure 8: Collocation of Sentinel-2/3/1 images (Case #Q1, 27–28 March 2021) for observations of oil-slick evolution in periods of about (**a**) 24 h (between Sentinel-2/3), (**b**) 8 h (between Sentinel-3/1), and (**c**) 32 h (between Sentinel-2/1). From 06:00 to 15:00 on Mar. 28, corresponding to the oil slicks detected by Sentinel-3/1, the wind continued blowing southeastward, with an increase in intensity from 10.5 m/s to 12.5 m/s, while the current moved toward the northwest. Note that the oil shape detected on the Sentinel-3/1 images (quite) significantly evolved due to the strong wind. However, the oil-drift distance and direction between Sentinel-3/1 images could not be accurately determined since the two superimposed oil slicks were very close. We argue that the difference between wind and current directions, i.e., southeast vs. northwest, respectively,  
Figure 9: Surface wind and current speed and direction (indicated by the arrows), corresponding to the extracted Sentinel-2/3/1 scenes (Figure 7a–c), respectively. **(i)** ERA-5 wind vectors for **(a)** 06:00, 27 March 2021; **(b)** 06:00, 28 March; and **(c)** 14:00, 28 March. **(ii)** CMES current vectors for **(d)** 06:30, 27 March; **(e)** 06:30, 28 March; and **(f)** 14:30, 28 March. **(iii)** Mean values of wind and current fields from 06:00, 27 March, to 14:00 28 March. is the main reason for the oil-drift stagnation. While the oil tended to move in the same direction as the current (northwest), it was also impacted by the wind blowing southeast at a velocity of 12 m/s. As a result, one oil part moved in a northwesterly direction, while the other one moved in a direction between southwestward and westward. Case #Q1 showed that it was not easy to accurately conclusively determine the wind and current impact on the oil drift for both short-term (i.e., several hours) and long-term (about one day) observations if the wind and current fields significantly changed, and/or the two did not move in the same direction. In addition, the large scale (and perhaps the oil thickness) of the oil detected was probably another factor affecting its movement. The results also showed that the oil-drift velocity and direction were mostly influenced by the current circulation, as the corresponding wind speed was low. As the wind blew with high intensity, e.g., increasing from 2 m/s to 10.5 m/s (Figure 9), it contributed more significantly to the movement of the oil slick (Figure 7a). Likewise, strong winds could impact the evolution of the oil shapes, i.e., splitting oil slicks into many parts, a consideration which also affected the detection of the oil objects and contours (lack of some oil pixels), as shown in Figure 7a-c. Additionally, as wind and current vectors were the main contributors to the oil drift, the difference between the wind and current directions might decelerate the oil movement.