# Ice Production in Ross Ice Shelf Polynyas during 2017-2018 from Sentinel-1 SAR Images
## 3 Method
### Detection of Polynya Area/Exent  
Polynyas in front of the Ross Ice Shelf and McMurdo Sound occur frequently due to the katabatic winds. Newly formed frazil and grease ice are self-organized into rows/strips approximately parallel to the wind direction (northward) and appear as white streaks that are early and visible in SAR imagery [41, 42] as indicated in Figure 2 (green polygons as the mapped polynyas). The area characterized by white streaks (i.e., inside the green polygons) was called an \"open polynya area\" in some studies [41, 50]. The open polynya area freezes quickly when the wind slows down, and the ice continues to grow (and thicken) until the next wind event that blows this newly formed ice away, ready for the subsequent new ice formation. So, the polynya ice production caused by such wind events is the ice volume blown away by the next wind event. The yearly polynya ice production is the total ice volume produced in the polynya by all wind events in the year. Noting that each ice production, although in the same general polynya area, would differ in production area/extent, shape, and thickness that are totally determined by the intensity, speed, and direction of each individual katabatic wind event and the length of the ice growth period before the next wind event. Figure 2 presents the evolution of such a large wind-driven ice production event, from no polynya (all covered by ice) on April 3 (Figure 2a), to a well-defined polynya with new ice formation on April 5 (Figure 2b), and to when the polynya ice completely merged with other ice on April 7 (Figure 2c). Polynyas in Figure 2b,e, although in the same general areas (MSP in the west and RSIP in the east), have different areas/extents, shapes, and thicknesses, due to different katabatic wind events (one on 5 April and the other on 19 August). Although Sentinel-1 can scan the polar region daily, the study area is not always covered or fully covered, especially in 2017 when the study area was only scanned every other day; therefore, not all wind-driven polynya ice production events were imaged. Sometimes, the image only covers a portion of the study area. In these two cases, we try to determine the situation from the next available images with larger coverage, on which the outline/trace of the last polynya would be kept and mapped. For example, Figure 2e shows the same polynya not completely mapped/covered on 19 August (Figure 2d). In the period of 17-21 September 2018 (Figure 3), three continuous ice formation/polynya events could be seen, marked with different color lines (red, green, and blue). The event 1 (red) was the first and biggest one and it is seen in all images (Figure 3a-d); the event 2 (green) started on 19 September (Figure 3b), and the event 3 (blue) started on 20 September (Figure 3c). On 21 September, the traces of all three ice formation events became obscure, except the event 1. Thus, even if one event was not captured due to no image collection at that time, it could be captured in a later image, based on the trace of the event. For example, in the wind-driven polynya event of 3-7 April 2017 (Figure 2a-c), the images were only available every other day. From the images on 5 April, only one wind-driven polynya can be seen. If any event occurred on 4 or 6 April, the event trace/outline could be seen on 5 or 7 April, just as seen from subsequent images in Figure 3. However, Figure 2 shows no additional event trace on the image of 5 April and no event trace on the image 7 April, meaning no event occurred on either 4 April or 6 April. Figure 2: Examples of Sentinel-1 SAR images on 3 April 2017 (**a**) (no polynya), 5 April 2017 (**b**) (clear polynya), and 7 April 2017 (**c**) (disappearance of polynya); 19 August 2017 (**d**) and 20 August 2017 (**e**) together explain the incomplete scan in (**d**); 28 October 2018 (**f**) indicating the thin ice (green dots) from passive microwave data overlaying the Sentinel-1 SAR image in the McMurdo Sound.