## 5 Discussion
The temperature started to rise gradually after 3 January 2021, but remained below 0 \({}^{\circ}\)C. From 18 April 2021, the temperature reached 0 \({}^{\circ}\)C and started to gradually rise. The lake ice movement then entered the third stage, where the snow melted during the day due to rising temperatures and then froze again at night when the temperature dropped. The freeze-thaw cycle continued during this stage. Similar freeze-thaw cycles have been observed in Lake Baikal in Russia and Nam Co Lake on the Tibetan Plateau [3; 19]. The results of our time series monitoring of lake ice indicated that each stage of lake ice--freezing, accumulation, and melting--showed a strong correlation with the local temperature. In a study of river ice, [PERSON] et al. [2] indicated that river iceaccumulation periods are longer than melting periods, but periods of rising temperatures are longer than periods of falling temperatures. Our lake ice monitoring results were similar; we found that Lake Khovsgol's ice accumulation period lasted for about 4 months from January to late April and its ice melting period lasted for about 2 months from late April to early May. However, local temperatures always increased, except for a drop from December 12 to early January. Wind speed is another factor that affects the movement of ice in Lake Khovsgol [3]. According to the wind speed data that we collected (Figure 2b), the wind speed increased from 8 January to 2 February 2021, and the lake ice was actively moving during this period. In contrast, from 22 February to 4 March, the wind speed variation was smaller and the lake ice movement was milder. In addition, according to the research from [PERSON] et al. [3] on the ice of Nam Co Lake, higher wind speeds in December correspond to earlier freeze-up and larger wind speed values, which lead to thicker lake ice. The freezing and melting of lake ice is related to many factors, such as lake depth and runoff [17; 72]. [PERSON] et al. [79] underlined that the mean depth determines the amount of heat accumulated in a lake and is thus responsible for the delayed appearance of ice [81]. Our results showed that Lake Khovsgol started to freeze from the shallowest water on the southern shore (the area located in the northern part of the lake is the deepest and has the most heat storage). After the date at which the temperature dropped below freezing, we observed a time delay in ice-cover freezing that was roughly linear with the lake depth, with each additional meter of lake depth corresponding to an additional day of delay [17; 72]. Thus, the icing trend of Lake Khovsgol is from south to north. The opposite is true for melting ice, and although POT results for the third stage are not available, the visible band of Sentinel-2 optical images showed that the lake ice breaks up from the north (Figure 11e), which is exactly where groundwater flows into Lake Khovsgol. The study shows that many permanent rivers flow into Lake Khovsgol [82], and the runoff direction of the lake is from north to south. In conclusion, the mechanism of ice formation and the factors influencing its motion are too complicated for us to provide more than quantitative analysis and reasonable hypotheses based on our own observations and the conclusions of others' studies. Figure 11: True color images of Lake Khovsgol at different times (B4, B3, B2). (**a**) shows that the lake started to freeze from the south, while the north remained water. (**b**) shows that the ice cover has completely covered the whole lake and the ice ridges/mounds are clearly visible. The red area is the selected stable area. (**c**) shows the intense solar radiation on the lake surface in April, which formed bubbles and air channels on the ice surface, giving the ice a white, snow-like appearance. (**d**) shows an example of the third phase obscured by clouds. (**e**) shows the lake ice breaking up from the north.