| Affected by global warming,the material balance of most glaciers has been in a state of continuous loss in recent years.China's mountain glacier reserves are abundant,and most of them are concentrated in southeastern Tibet.These glaciers are not only one of the main sources of freshwater resources,but also have an important impact on the climate of southeastern Tibet and even the entire southwestern region of China.Moreover,the glaciers may cause a series of natural disasters.Therefore,studying the motion status of mountain glaciers and related factors that affect their dynamic changes can provide theoretical and data support for glacier degradation,climate analysis,and glacial disaster early warning.To this end,this article mainly did the following work:Firstly,this paper systematically analyzes the current methods commonly used in glacier motion monitoring.Based on the advantages and disadvantages of each method and the characteristics of mountain glaciers,finally this paper selects pixel offset tracking technology(POT)to extract the glacier velocity field.However,the mismatching of the surface of the glacier is very easy to occur.Too low coherence between image pairs will lead to an increase in the number of mismatched pixels,which will lead to a decrease in the coverage of the monitoring space,and the number of mismatched pixels will also increase affect the accuracy and reliability of the POT solution results.In order to overcome this problem,this paper proposes the PO-SBAS model,by limiting the time baseline length of the image pairs to maintain the correlation between the image pairs and reduce the impact of mismatched matching.In addition,more redundant observations are obtained through the combination of the images,which can reduce the interference of mismatching noise during the calculation,thereby improving the accuracy of the calculation.Secondly,considering the relationship between the scattering characteristics of ice surface features and seasonal changes,the ground scattering characteristics are closely related to the mismatch of matching.Therefore,based on the PO-SBAS model,this paper proposes a season-based time-baseline optimal combination method to make the PO-SBAS model more suitable for glacier motion monitoring.Based on the proposed method,the analysis of the Bugyai Kangri Glacier in eastern Tibet was carried out.To obtain the orbital SAR images of 76 C-band Sentinel-1 satellites covering the study area from 2015 to 2019.First,the PO-SBAS model is used to extract the glacial velocity field in the study area,and the number of mismatched pixels in the glacial area is used as the evaluation index to analyze the influence of different time baseline lengths on the mismatched match in different seasons.The results show that the surface scattering characteristics of the glacier surface changed sharply in the early ablation period,and the matching quality decreased with the increase of the baseline length in time.The matching quality of the baseline combination at48 days and more in the late ablation period declined rapidly.The image quality across the freezing period and the ablation period decreases poorly as the length of the time baseline increases.Then,based on the above analysis,a time-baseline optimized combination method based on seasons is proposed.The SAR images of the freezing period,the pre-ablation period and the post-ablation period each constitute a sub-baseline set.Among them,during the freezing period,the baseline length is controlled within 96 days;in the early ablation period and across the freezing period and the ablation period,the shortest time baseline is selected;in the late ablation period,the baseline length is controlled within 48 days.In this baseline combination mode,the PO-SBAS model is optimized to solve the time-series glacial velocity field.Thirdly,based on the PO-SBAS time series solving model after baseline combination optimization,this paper extracts the Bugyai Kangri Glacier motion field,and analyzes the relationship between the spatiotemporal evolution of the motion field and the glacier scale,topography,altitude,climate and other factors.Studies have shown that(1)The average flow velocity of the Bugyai Kangri Glacier can reach 0.94 m/d.From the perspective of the glacier motion field and the spatial distribution of glacier,the speed of the glacier movement is positively correlated with glacier area.The larger the glacier area,the faster the movement speed.The movement speed of the accumulation zone is higher than that of the ablation zone,and the closer to the end of the ice tongue,the slower the movement speed,and the faster the movement speed through the steep slope area.The end of the ice tongue is the most sensitive area for glaciers to respond to climate change.The chronological fluctuation of glacial flow velocity is the most intense,and the chronological difference of glacial flow velocity gradually decreases as the elevation rises.Affected by the climate,the glaciers on the southwestern slope of the area move faster than the northeast slope of the leeward,and the speed fluctuations are more intense,making them more sensitive to climate change.(2)From the perspective of the time series change of the Bugyai Kangri Glacier motion field,the velocity of the glacier fluctuates seasonally,and the movement speed in the spring and summer seasons is greater than in the autumn and winter seasons.The velocity at the end of the ice tongue increases with temperature,and the response to climate change is synchronized.As the elevation rises,the response time of the glacier velocity to the climate change gradually lags,and the seasonal change amplitude also gradually decreases.(3)From the changes in the elevation of the surface of the Bugyai Kangri Glacier,the thickness of the glacier has dropped by 1.07 m from 2000 to 2014,the mass balance of the glacier is in a state of deficit,and the thickness of the ice tongue is thinned to varying degrees.The elevation of the glacier surface gradually stabilizes as the altitude rises.Among them,the Zuxuehui Glacier(S1)of the southern slope has the largest thickness reduction,reaching-76.7 m.The Dongbeijia(N4)glacier of the northern slope has a thinner area and the altitude is higher,the average thickness change on the central axis is-15.70 m.Combined with the analysis of the Bugyai Kangri glacier motion field and elevation changes,when the material balance of the glacier is in a state of deficit for a long time,the thickness of the ice tongue gradually decreases and the flow velocity of the glacier slows down.According to the different distribution of water and heat conditions in the local area,the south slope glacier with richer water vapor sources has a relatively smaller retreat than the north slope glacier,and the glacier flow rate is faster. |
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