A review of remote sensing of bedfast lake ice using SAR
The change of lake ice is a sensitive indicator of regional and global climate change. Most of the exist-ing studies have focused on the phenology of lake ice,or the type of surface freezing,while less on its vertical type (i. e. floating ice and bedfast ice). Compared with floating ice,bedfast ice can provide a protective envi-ronment below 0 ℃ to the lake bed during the entire freezing period,and delay the development and carbon emissions of the talik. Therefore,accurate identification and classification of both ice types are of great signifi-cance to many research fields of the cryosphere. Due to the fact that most lakes that have a large amount of bed-fast ices are located in remote and cold regions,it is difficult to conduct large-scale and long-term field observa-tions,resulting in limited knowledge of the distribution of bedfast lake ices. Satellite-based synthetic aperture ra-dar (SAR) provides an advanced means for large-scale identification of lake ice types and periodic monitoring their transitions. Since the 1990s,SAR remote sensing has been widely used for identifying bedfast ice,achiev-ing a series of methods and datasets,providing important and advanced technical means for observing bedfast ice in large and remote lakes. In recent years,with the further development of remote sensing technology,SAR in particular,the variety of remote sensing data has become increasingly diverse,and the spatiotemporal resolution has significantly improved,bringing new opportunities and challenges to lake ice studies. At present,most re-mote sensing studies on lake ice focus on the phenology or thickness of lake ice,but limited attention has been paid to the vertical development of lake ice (whether it can touch the bottom or not). Up until present,there is no literature review on the use of SAR remote sensing to identify bedfast ice. Therefore,it is necessary to com-prehensively and systematically review the research progress in this field,summarize the achievements,identify the existing problems and challenges,and further promote the application of SAR data in bedfast lake ice moni-toring,and support related climate change and lake ecology studies. In this paper,a comprehensive review of the research on the classification of bedfast ice and floating ice by SAR remote sensing is conducted. The basic principles and common methods are summarized,and the supporting role and indicative significance of lake ice type for the related research are combed. The results show that:(1) The enhanced backscatter of floating ice is mainly caused by the combined effect of bubbles and high roughness of the ice-water interface,but the backscat-ter increment produced by each of them at different wavelengths still needs further study;(2) Classification methods have developed from threshold method to object-oriented segmentation,time series clustering and ma-chine learning. However,the accuracy is still limited by factors such as ice melt,snow cover and lake bound-ary;(3) Lake ice type can reflect the vertical development process of lake ice during the freezing period,there-fore it reflects the response of ice thickness to climate change and its impact on the permafrost layer at the lake bottom with different bathymetries,which has indicative significance for the studies of climate,ecology and freshwater resources. Compared to lake ice phenology that focuses on reflecting climate change through the tim-ing of freezing and thawing,bedfast lake ice reflects more about the vertical development process of lake ice dur-ing the freezing period. Therefore,it focuses on reflecting the response of ice thickness to climate change and its impact on the permafrost layer at the bottom of different depths of lakes,which is of great significance for cli-mate,ecology,and freshwater resource research. This review summarizes the research progress of SAR remote sensing in identifying bedfast ice,which is anticipated to promote the usage of SAR data (e. g. Sentinel-1) in this field.
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