Ice cover determination of the lakes of Baltic and White sea basins on the base of Jason-2 satellite observations

Thy work aims to development of a simple method for distinction between open water and ice cover on the base of geophysical data record (GDR) of altimetry satellite Jason-2 for large and middle-sized freshwater inland water bodies. The method is applied to the lakes of Baltic and White Seas basins: Ladoga, Onega, Vygozero and Segosero. The method for ice-water discrimination based on data of altimetry satellites was initially developed for Topex/Poseidon satellite and was successfully applied to the Caspian, Aral and other Seas and Baikal Lake. This method was based on the joint analysis of the two parameters: the backscatter coefficient at 13.6 GHz and the average value of the brightness temperature at 18 and 37 GHz and the conclusion about the ice cover can be made when these parameters exceed a certain threshold. It is shown that the method works well for large water bodies such as Ladoga and Onega Lakes and experiences significant difficulties for the middle-sized water bodies, such as Vygozero, and Segosero. This is largely due to the small amount of valid data on the backscatter coefficient for not very large water basins, which is caused by the influence of the land on the shape of the reflected altimetry pulses. For the medium water basins more productive method is based on an analysis of the difference of brightness temperatures of land and water. Along track variation of the average brightness temperature at 18.7 and 34 GHz TB/2=(TB18.7+TB34)/2 for cycles 1–179 of Jason-2 observations, constructed for winter and summer periods showed that for all lakes the difference of brightness temperatures of land and water strongly reduced when the water freezes. This allows us to determine the freezing of water based on a decrease in difference of brightness temperatures of land and water areas. Based on the proposed method we constructed time series of brightness temperature differences and demonstrated that these dependences have a pronounced seasonal variation for all lakes considered. The transition from summer values (open water) to winter values (ice) is sharp enough, which allowed us to determine sufficiently accurately the date of freezing of the lakes.