Research On Identification Of The Mountain Glacier Surface Cover Types By Combining Optical And SAR Remote Sensing Data

Glaciers are sensitive indicators of climate change and one of the key components of the cryosphere.They have significant impacts on the energy balance between the earth surface and the atmosphere,and atmospheric and oceanic circulations.However,in recent decades,climate warming has led to a series of problems,such as the accelerating glacier melting,declining solid freshwater reserves and rising sea level.Therefore,the accurate and rapid identification and monitoring for glaciers are increasingly more important for the prevention and mitigation of ice-snow disaster/hazards and for studying climate change at local to regional scales.Due to the rugged terrains,remote locations and difficult observations by ground-based meteorological stations,satellite remote sensing have undoubtedly become an important means of glaciology,such as for the identification and studies on changes in mountain glaciers.Optical sensing provides a good method for extracting the extent of snow and/or ice cover of glaciers,but effectiveness and accuracy of this method can be easily affected by atmospheric conditions,such as cloudiness and others;active microwave sensing has the advantage in better penetration of cloud,fog and snow cover for all-weather and all-time earth observation,but for mountain glaciers,the backscattering signal of active microwave sensing can be affected by the topographical fluctuations,resulting in distorted or missing earth surface information.In this article,the Glacier No.12 of Laohugou Valley in the Qilian Mountains,located in the northwest of Qilian Mountains,was chosen as research area.For making full use of the complementary advantages of optical sensing and SAR remote sensing and for improving the recognition accuracy of mountain glacier surface cover types,the data collected by optical satellite Sentinel-2 and SAR satellite Sentinel-1 during the period from the mid-April 2019 to mid-June 2020 was chosen as research data in our work.Additionally,based on exisiting research results and update literature,the types of modern glacier surface cover were classified into five categories: dry snow,wet snow,bare glacial ice surface,glacial surface lake and surface moraine(superficial till).Firstly,using the data of Sentinel-1 and Sentinel-2,respectively,the supervised classification method was selected for classifying the surface cover types of Glacier No.12 of the Laohugou Valley.The result shows that the classification accuracy of the Sentinel-2 data for four types of surface covers(snow cover,surface moraines,exposed glacial ice and glacier surface lake)is higher than that of the Sentinel-1 data,and the convolution neural network(CNN)algorithm has the highest classification accuracy,but it is unable to effectively distinguish the dry and wet snow cover on the glacier surface;the accuracy of the Sentinel-1 data in identifying the glacier surface cover types is generally low;Sentinel-1 data has high recognition accuracy for dry and wet snow covers on glacier surface.Thus,polarimetric SAR wet snow model is used for extracting the extents of dry and wet snow cover.High-precision DEM data are helpful in improving the recognition accuracy of dry and wet snow cover,and the recognition accuracy of VH polarization is higher than that of VV polarization.By combining the data of the Sentinel-1 and Sentine-2,our research results showed that during the period from mid-April 2019 to mid-June 2020,on Glacier No.12 of Laohugou Valley,glacial lakes were mainly distributed in the low-lying areas of the firn basin(accumulation area),the top of the west branch and the intersection zone of the east and west branches;the area of glacier surface lakes would enlarge during the glacier melting period,and the glacial lake occupies a small area of the glacier,so it will not affect the downstream area;moraines were mainly distributed in the glacier tongue/terminal and the in areas near the mountains on both sides of the glacier(lateral moraines);dry snow,with limited area of distribution,was mainly distributed on the of the glacier on the mountain ridge and the term of the east and west branches of the glacier;the wet snow,in the areas near the glacier top in the east,west branches and above;the naked ice mainly distributed in the areas near the glacier tongue in the east,west branches and below.The area of the ice lake is smaller than others and has no effect on the downstream area.The glacier melting increased gradually from April and slowed down gradually from October,with the most intense melting in July and August.