基于SAR的触底湖冰遥感研究进展

A review of remote sensing of bedfast lake ice using SAR

湖冰的变化是区域乃至全球气候变化的敏感指示器。已有研究大多关注于湖冰的物候,或表层冻结的类型,而对其纵向的类型(浮冰和触底冰)关注相对较少。相对于浮冰,触底冰能够在整个封冻期对湖底基质提供0℃以下的保护环境,延滞湖底冻土融区的发育及碳排放等,因此,准确识别和分类浮冰和触底冰对于冰冻圈多个领域的研究均有重要意义。星载合成孔径雷达(Synthetic Aperture Radar, SAR)为大范围识别和周期性监测湖冰类型及其变化提供了先进的手段。近年来,随着遥感技术特别是合成孔径雷达技术的进一步发展,遥感数据的种类日益多样化,时空分辨率显著提高,给湖冰研究带来了新的机遇和挑战。目前,对湖冰的遥感研究大多集中在湖冰的物候或厚度上,而对湖冰的纵向发育(能否触底)关注较少。到目前为止,还没有关于利用合成孔径雷达遥感识别触底湖冰的文献综述。因此,有必要全面系统地回顾该领域的研究进展,总结取得的成就,找出存在的问题和挑战,进一步推动SAR数据在快速湖冰监测中的应用,为相关的气候变化和湖泊生态研究提供支持。本文对SAR遥感识别触底冰和浮冰的相关研究进行了全面综述,阐明其识别的基本原理和常用的方法,并梳理了湖冰类型识别对相关研究的支撑作用和指示意义。研究表明:(1)浮冰的后向散射增强主要由冰水界面的高粗糙度和气泡导致,但在不同的波长下,两者分别能够产生的后向散射增量仍有待进一步研究;(2)识别方法从阈值法逐步发展到面向对象分割、时间序列聚类,以及机器学习等,然而,识别精度仍受冰面融水、积雪覆盖、湖泊边界精度等因素影响;(3)触底湖冰的产生能够体现封冻期湖冰的纵向发育过程,因此可以反映冰厚对气候变化的响应,以及触底湖冰对不同深度湖泊底部冻土层的影响,这对气候、生态,以及淡水资源研究等具有指示意义。

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°C 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.