Spatiotemporal Variations Of Land Surface Albedo And Its Feedback To Surface Temperature On The Tibetan Plateau

The Tibetan Plateau is an important ecological security barrier in China and even Asia,and has become an early warning and sensitive area for global climate change.Global warming has caused widespread and rapid changes in all spheres of the climate system,becoming increasingly prominent in the Tibetan Plateau region.Land surface albedo regulates the radiation energy balance between the surface and the atmosphere,and is a key parameter that affects the surface energy budget and the interaction between the surface and the atmosphere.The change of underlying surface can significantly affect the land surface albedo,and the subtle change of land surface albedo will also be feedback to the climate system,which has an important impact on regional and global climate.Therefore,it is of great significance to study the temporal and spatial variation of land surface albedo and its feedback to temperature.Based on the long time series MODIS（Modern Resolution Imaging Spectroradiometer,MODIS）land surface albedo data,vegetation data,snow data,combined with GLASS（Global Land Surface Satellite,GLASS）and NCEP（National Centers for Environmental Prediction,NCEP）radiation data sets,this paper uses Theil-Sen Medium,Mann Kendall significance test,empirical orthogonal function analysis,correlation analysis and singular value decomposition analysis are used to systematically analyze the spatial and temporal variation pattern of Tibetan Plateau and its main influencing factors（snow cover and vegetation）,and quantify the contribution of albedo feedback to surface temperature.The main conclusions are as follows:（1）The multi-year average land surface albedo of the Tibetan Plateau is about 0.22±0.7%.There are obvious differences in spatial distribution,which is high in the northwest and low in the southeast.With the increase of altitude,the land surface albedo increases.The Kunlun Mountains,Karakoram Mountains,Tanggula Mountains,Nyainqentanglha Mountains,the Himalayas and Qilian Mountains are high value areas,and also sensitive areas for land surface albedo changes in the past 20 years.The spatial distribution characteristics of land surface albedo in the four seasons are high in the northwest and low in the southeast,but different seasons have different distribution characteristics.The spatial distribution of land surface albedo in spring and autumn is similar,and the land surface albedo in summer is significantly lower than that in other seasons,the area with high land surface albedo in winter has the widest coverage.The average land surface albedo of the four seasons is in the order of winter>spring>autumn>summer.（2）The annual average land surface albedo of the Tibetan Plateau showed a slight growth trend from 2001 to 2020,with a change rate of5.77×10^-4decade^-1,and the volatility of the land surface albedo from 2011 to 2020 was significantly greater than that from 2001 to2010.The land surface albedo increased significantly in the central Qilian Mountains,the eastern Kunlun Mountains and the Himalayas,accounting for 8.26%of the total area,while the land surface albedo decreased significantly in the margin of Qilian Mountains,the western Kunlun Mountains,Qaidam Basin,Hengduan Mountains and Nyainqentanglha Mountains,accounting for 12.72%of the total area.（3）The variation of land surface albedo is closely related to surface cover.High snow cover areas correspond to high value areas of land surface albedo,and high vegetation cover areas correspond to low value areas of land surface albedo.There is a significant positive correlation between land surface albedo and snow cover.The extremely significant positive correlation regions are mainly distributed in Qilian Mountains,the Himalayas,Gangdise Mountains,Tanggula Mountains,Nyainqentanglha Mountains,Bayan Har Mountains and other regions.The change of snow cover in these regions is the main reason for the formation of land surface albedo sensitive areas in recent years.There is a significant negative correlation between land surface albedo and normalized difference vegetation index,and the land surface albedo gradually increases with the decrease of tree crown and forest cover.Land surface albedo is more sensitive in snow covered areas than in areas where snow and vegetation coexist.Improvement and degradation of vegetation in low vegetation covered areas are more likely to cause changes in land surface albedo.The snow cover rate on the Tibetan Plateau has generally shown an increasing trend,with a significant growth trend in the Qilian Mountains,Kunlun Mountains,Bayan Har Mountains,and other places,while a significant decrease trend has been observed in the Hengduan Mountains and Nyainqentanglha Mountains,with overall characteristics similar to the trend of land surface albedo.However,the vegetation on the Tibetan Plateau has generally shown a green trend,with only partial degradation occurring in the Hengduan Mountains.The change of land surface albedo is not significant in areas with significant improvement/degradation of vegetation,while the change of land surface albedo is more severe in areas with significant increase/decrease of snow cover.The spatial characteristics of the land surface albedo and the snow cover rate are distributed in a consistent abnormal manner,which is opposite to the normalized difference vegetation index.At the same time,the phase change of the land surface albedo and the snow cover rate and the time coefficient of the normalized difference vegetation index is in good consistency.However,the interannual variation of land surface albedo is largely affected by the change of snow cover.（4）The downward shortwave radiation from the Tibetan Plateau showed an overall decreasing trend from 2001 to 2020,exhibiting a spatial distribution pattern of high in the west and low in the east.The reduced area is mainly distributed in Kunlun Mountains,Qilian Mountains,Himalayas,Bayan Har Mountains,Nyainqentanglha Mountains and Tanggula Mountains,and the increased area is mainly distributed in Hengduan Mountains,Altun Mountains and Qaidam Basin.Compared with downward shortwave radiation,albedo changes（Δα）relatively little.The albedo increase areas are mainly distributed in the Kunlun Mountains,Qilian Mountains,Himalayas,Northern Tibetan Plateau,Tanggula Mountains and the Bayan Har Mountains,while the decrease areas are mainly distributed in the Altun Mountains,Hengduan Mountains,Nyainqentanglha Mountains and Qaidam Basin.Contribution of all year,spring and winter albedo feedback to surface temperature change(ΔT_SAF).TheΔT_SAF values are-0.01K,-0.26K,and-0.01K respectively,all of which have a cooling effect on the surface of the Tibetan Plateau.In spring and winter,the albedo feedback cools significantly in Kunlun Mountains,Northern Tibetan Plateau,Bayan Har Mountains,Himalayas and Gangdise Mountains,about 3.5K;In the Hengduan Mountains,the Altun Mountains,and the Qaidam Basin,the surface of the Tibetan Plateau has warmed by approximately 4K.The spatial distribution ofΔαandΔT_SAF has a good corresponding relationship.The area of albedo increase corresponds to the area of surface cooling,and the area of albedo decrease corresponds to the area of surface warming.Downward shortwave radiation largely regulates the albedo feedback intensity.The contribution of albedo feedback to the change of surface temperature depends on the change of surface albedo and downward shortwave radiation,but the change of surface albedo largely affects the change of albedo feedback to surface temperature.This study obtained the spatial-temporal variation characteristics of the surface albedo of the Tibetan Plateau,revealed the relationship between the surface albedo of the Tibetan Plateau and vegetation and snow cover since the 21st century,quantified the contribution of the surface albedo feedback to the surface temperature,and helped to further explore the response of the surface albedo to climate change and the study of the feedback mechanism.