| The Eastern Tien Shan is very sensitive to climate change in the western part of the country and its retreat and response to climate plays an important indicative role.Due to the perennial low temperatures in the region,moisture landing and preservation is mostly in a solid state.The snow layer gradually transforms into ice under the influence of its own pressure and sunlight exposure over the years,and as long as the terrain has a certain slope,the ice moves extremely slowly from higher to lower places,driven by the upper pressure and gravity,and glaciers are formed.Elevation monitoring of glaciers allows their changing characteristics and response to climate change to be determined.In addition,glacier melt and accumulation has important implications for rivers as well as ecosystems and economic production.In SAR technology is commonly used to observe the elevation of target areas because of its advantages of all-day,all-weather,high resolution and high accuracy.Due to the complex topography of mountain glaciers,there is a high degree of uncertainty in remote sensing monitoring of glacier surface elevation,and the spatial and temporal patterns of glacier surface elevation change over a large area are not clear.Question 1: What is the accuracy of remote sensing monitoring in high mountain areas caused by topographic relief?Question 2: How can the uncertainty of remote sensing monitoring be constrained according to the morphological characteristics of the glacier surface and the pattern of glacier surface elevation change to improve the accuracy of remote sensing monitoring? Question 3: What are the driving factors causing the change of glacier surface elevation and what are the spatial and temporal variation patterns? To address the above three questions,25 glaciers on Bogda Mountain in the Eastern Tien Shan were selected as the object of this paper because they are typical glaciers in arid or semi-arid regions and have important water supply functions.Based on Sentinel 1A data(2017-2021),meteorological data(2000-2020),glacier cataloguing data(CGI2),digital elevation data and landsat data,the elevation changes of Bogda Mountain over 20 years were obtained by using repeated orbital interferometry.Based on the characteristics of glacier surface elevation patterns,an In SAR mountain glacier surface change coherence and accuracy enhancement algorithm is proposed,and an In SAR surface elevation change monitoring model is established to improve the accuracy of glacier change monitoring by In SAR technology.The surface elevation change pattern of Bogda Mountain glacier is also analysed in conjunction with climatic and topographic factors.The main research elements and conclusions are as follows.(1)To address the influence of systematic errors on remotely sensed surface elevation monitoring,the surface elevation change of relatively gentle water bodies and bare land areas are used as references for regional glacier surface elevation change optimization,which improves the accuracy of glacier surface elevation change monitoring.In view of the influence of topographic factors on remotely sensed surface elevation monitoring,a method for optimizing surface elevation change results based on topographic features is proposed.The elevation results obtained from the interference were optimised for topographic factors such as slope,slope direction and terrain relief,respectively,and the areas with standard deviation of topographic factors greater than 10 were removed and then interpolated.The results showed that the standard deviation of glacier surface elevation in different periods was reduced by about42% after optimization using topographic factors,while the standard deviation of surface elevation in non-glacier areas was reduced by about 14%,indicating that the uncertainty of surface elevation in glacier areas monitored by In SAR is stronger than that in non-glacier areas,and the In SAR monitoring based on topographic factors can effectively reduce the uncertainty of glacier surface elevation data.(2)The In SAR glacier surface elevation change optimization method was studied according to the surface elevation change characteristics of the glacier mainstream line,and an In SAR glacier surface elevation change monitoring model was established.Using the glacier mainstream surface elevation change model,the overall surface elevation change distribution of the glacier was obtained according to the law of glacier surface elevation change with height.According to the elevation change results after the model optimization,Hegou Glacier No.8was selected as the validation object and its elevation change value was calculated as-0.42m/a.The obtained results were compared with other literature actual measurement results and found to be in good agreement with each other.(3)The results of glacier elevation changes over 20 years were obtained from the glacier mainstem surface elevation model.The results show that the overall trend of glacier surface elevation in the Bogda Mountains has been decreasing over these 20 years.The driving factors for the change in glacier surface elevation in the Bogda were investigated.Decreasing precipitation and increasing temperature are the main factors for the decrease of glacier surface elevation in the study area.The elevation distribution of glaciers in the study area is lower than that of other mountainous areas in the Tianshan Mountains,and the pattern of glacier changes from low to high altitude areas declines slowly and then rises.In the elevation range of 4270-4580 m,glacier elevation changes from retreat to accumulation.In relation to slope,most of the glacier slopes in the study area are below 42°,and the overall change in glacier elevation tends to decrease as the slope rises.In relation to slope orientation,more glaciers cover the north slope than the south slope,and the retreat rate of north-facing glaciers is higher than that of other facing glaciers,with a decreasing trend in the range of change from the north slope clockwise to the south slope,and a gradual expansion from the south to the north slope.This is closely related to factors such as the low elevation of north-facing glaciers and the greater decrease in precipitation on the northern slopes of the mountain than on the southern slopes. |
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