## 4 Observation of Floating-Oil-Slick Evolution Off the Shore of Qatar  
### Case #Q1: 27-28 March 2021  
Figure 7 presents the oil slicks detected for Case #Q1. On the Sentinel-2 image (Figure 7a-left), we applied HSBA and object contour determination twice, since the oil slick includes dark and bright pixels. In Figure 7a-right, the dark-oil pixels detected consisted of a white oil object and green oil contour, and the bright-oil pixels consisted of only a red oil contour. The HSBA could not classify the bright-oil object since its oil index values were very close to the background. The bright and background-value distribution is not bimodal [33]. The dark-oil part detected lacked some object pixels due to the inhomogeneity of the oil slick. One look-alike (on the left) with low oil index values was also detected by HSBA (in white); however, we can classify it as a look-alike due to the lack of oil contour. Figure 7b,c illustrate the oil slicks detected, including oil object and contour, from Sentinel-3/1 images, respectively. We realized that, as was the case with the oil slicks detected by Sentinel-2, the contours of the oil objects identified by Sentinel-1/3 are fragmentary due to inhomogeneity. Figure 8 collocates Sentinel-2/3/1 images to observe the changes (in shape and position) of the oil slicks detected over about one day (Figure 8a), approximately 8 h (Figure 8b), and about 32 h (Figure 8c). To facilitate the oil-drift observation, we merged the oil objects and contours, as well as the dark and bright-oil pixels detected by Sentinel-2. In Figure 8a, the collocation of Sentinel-2/3 images indicates that the oil slicks detected moved about 18.5 km southeastward. Compared to the initial oil shape on the Sentinel-2 image, that identified on the Sentinel-3 image evolved significantly after about one day. It split into two parts, and many oil pixels disappeared, presumably due to strong winds and/or evaporation. In Figure 8a, the oil had moved slightly westward, by about 5.2 km, after 8 h. It is hard to accurately estimate the oil movement distance in this case because many oil parts did not move in exactly the same direction and with a similar velocity. The lack of many oil pixels also changed the oil shape significantly, probably due to strong winds and/or evaporation. Figure 8c indicates that the oil detected had moved southward after 32 h; however, it is hard to estimate the distance accurately due to the significant evolution in the oil shape. Figure 7: Floating-oil-slick detection for Case #Q1, 27–28 March 2021. (**a–c**) Extracted scenes from Sentinel-2 (27 March, 06:56:21), Sentinel-3 (28 March, 06:34:43), and Sentinel-1 (28 March, 14:33:06) images, respectively. (**Left**) Sentinel-2 RGB, Sentinel-3 OLCI tristimulus, and Sentinel-1 NRCS images, respectively. (**Right**) Oil slicks detected from Sentinel-2/3/1 images (**left**), respectively. Surface wind and current fields corresponding to the Sentinel-2/3/1 scenes (Figure 7a-c) are shown in Figure 9. Wind and current directions are displayed with the up-north reference (0\({}^{\circ}\)), meaning that wind or current blows or moves from south to north. From 06:00 to 21:00 on 27 March, wind intensity was relatively low (1-5 m/s), and the wind direction changed significantly between the northwest and northeast. Then, from 21:00 to 06:00 the following day, the wind speed quickly increased from 1 m/s to 10.5 m/s, blowing mainly southeastward. Meanwhile, the current also moved in different directions, from the southeast at 06:00 on 27 March, then southwest and northwest, at 18:00. From 21:00 on 27 March, to 06:00 on 28 March, it moved between the northeast and north. The wind and current observations indicate that the oil detected by Sentinel-2 moved in the same direction as the current (between the southeast and southwest) in the first phase (06:00-18:00, 27 March) since the wind was blowing between the northwest and northeast at that time with a low wind speed (about 2 m/s).