| High Mountain Asia（HMA）,including the Tibetan Plateau and the adjacent mountains,is the region with the most alpine glaciers in the world.Known as the"Water Tower of Asia",these glaciers feed many of Asia’s rivers and are an important freshwater resource,affecting the regional water cycle and the ecological environment.Since the 1990s,the characteristics and causes of glacier changes in the context of global warming have attracted much attention from scholars,especially in the regions that remain stable or show no clear retreat（Karakoram anomaly）.Most modern glaciers in the High Mountain Asia are located in inaccessible areas,and remote sensing is often used to monitor the changes of glaciers in large regions.Firstly,the impact of snow cover on glacier area was studied.The analysis was focused on the high mountain ranges surrounding the Tarim Basin,where the spatial distribution of snow cover is quite variable.The study was based on multi-temporal remote sensing data to monitor glacier and snow cover area in the Tarim Basin high mountain area.Then,a long time series of glacier surface velocity in the Parlung Zangbo Basin in southeast Tibetan Plateau was extracted,where the seasonal precipitation is abundant,and the observed seasonal glacier velocity was evaluated.These observations documented a large temporal and spatial variability in glacier velocity,which was analyzed in detail.In this paper,three aspects of glacier movement in the high mountain area of Asia are studied:glacier area,glacier surface velocity and fluctuations of the glacier surface.The seasonality of surface velocity and the oscillations of the glacier surface were analyzed in detail.This study can contribute to understand glacier movement in response to weather and climate variability.The main research contents and results are as follows:（1）The method developed in this study relies on the frequency of usable image pixels to minimize the impact of clouds and to delineate glaciers.The major innovation is to composite several images pixel by pixel to determine the median surface reflectance of each pixel.This does not require that cloud-free observations of all pixels are available on the same day,just that a sufficient number of cloud-free samples is acquired during a longer period of time for each pixel.The composite image improves the reliability of estimated change in glacier area,because the median is a robust statistic.The NDSI method combined with threshold setting provided systematic estimates of the annual glacier area and seasonal snow cover.The algorithm was successfully applied using the Google Earth Engine platform for efficient processing of data on the large study area.Using this method,cloud-free and snow-free glacial areas can be synthesized pixel by pixel from a large number of consecutive MODIS summer images.（2）MODIS data was used to calculate the Normalized Difference Snow Index（NDSI）and a threshold was applied to extract the Tarim Basin glacier area and seasonal snow cover.Between 2000 and 2020,the total area of the Tarim Basin Glacier declined at a rate of 0.94%per year.Because of differences in atmospheric circulation and environmental conditions,changes in the glacier area of the Tarim Basin show differences across five sub-regions.The rate of glacier area loss was fastest（2.98%per year）in the East Tian Shan,while Pamir Mountains and East Kunlun had the slowest rates,i.e.0.50%per year and 0.81%per year,respectively.（3）The post-processing algorithm of extracting glacier flow velocity from Landsat-8 OLI data was improved.The Parlung Zangbo Basin（PZB）area in southeast Tibetan Plateau is affected by the monsoon,and the frequency of usable optical images is seriously reduced by clouds.The glaciers in southeast Tibetan Plateau have the characteristics of high accumulation,high ablation and fast movement.When extracting the glacier velocity,null value appears in the glacier velocity results due to cloud cover.The accuracy of the retrievals can be improved by using the time redundancy in the retrievals of glacier flow velocity.In this paper,the glacier velocity is calculated using overlapping images at different times,and then the glacier velocity is estimated by averaging available maps at a given moment in time.Stacking images then sampling at a given date improves the signal-to-noise ratio with respect to a single glacier velocity map.This method solves the problem of the impact on retrieved glacier velocity of cloud-affected observations and improves the accuracy of the retrieved glacier surface velocity.（4）The variability of seasonal and annual flow velocities in the Parlung Zangbo region was analyzed in detail.Overall the glacier flow rate in the Parlung Zangbo region shows an increasing trend from 2013 to 2020,but the annual variability changed.From2013 to 2017,there was no significant trend in the annual glacier flow velocity,while from 2017 to 2020,there was an increasing trend.The accumulated ice mass could have caused seasonal velocity changes in response to changes in the mass balance during2017–2020.（5）By analyzing the results of glacier surface velocity,the occurrences of seasonal and periodic variations of time in glacier surface velocity was observed.To further explore the causes of wave-like features in glacier surface flow,the harmonic analysis method was used to analyze the longitudinal profiles of glacier surface elevation and surface reflectance.The observed waves in the Yanong Glacier for the first time and estimated the wavenumber is 0.003 m-1.The similar results were obtained by digital elevation data and multispectral images.These results suggest the occurrence of waves in glacier surface flow.A solid body of theoretical knowledge exits on wave-like motion in glaciers.In the near future the further research plan was to combine remote sensing,observations with the theory of glacier waves. |
