Spatiotemporal Changes Of Global Land Surface Albedo From Remote Sensing Observations

Surface albedo is one of the critical parameters in surface radiation and energy budget(SREB),which characterized the surface reflectivity for shortwave solar radiation,and is a controlling variable for the ground-air energy exchange.In recent decades,surface albedo has significantly changed due to the global climate change and the impacts of human activities,which has great impacts on regional and global climate.Therefore,it is very important for analyzing the spatiotemporal changes of surface albedo from satellite observations.In this study,the global surface albedo trends and their drivers were evaluated with the long-term and spatiotemporal continuous Global Land Surface Satellite(GLASS)albedo product.The highlights of this study includs(1)The estimation method for snow albedo was improved;(2)The estimation accuracy and spatiotemporal continuous of GLASS albedo datasets were evaluated;(2)The global surface albedo trends from 1982 to 2015 were investigated;(3)The relationships between the oscillation of surface albedo anomaly and atmospheric circulations have been found;(4)The global drivers of surface albedo changes as well as the relationships between surface albedo changes and vegetation dynamics were explored.The main conclusions and achievements are summarized as follows:(1)A direct-estimation algorithm(DEA)for mapping snow albedo was developed.The validation results show that the overall accuracy(The validation results with the GC-Net sites are:R~2:0.657,RMSE:0.0556,Bias:-0.0325)and temporal resolution was improved.This algorithm is a constituent part of the deriving algorithms for GLASS albedo product,and has been applied in generating the GLASS phase-2 surface broadband albedo product.The GLASS phase-2 surface broadband albedo product was validated with the observations of Fluxnet sites with an R~2 of 0.6348,and RMSE of 0.0693.The analyzing results show that the GLASS phase-2 surface albedo product is a long-term and spatiotemporal continuous dataset,which is suitable for analyzing the global trends and patterns of surface albedo.(2)The global surface albedo trends map from 1982 to 2015(34 years)were derived from GLASS phase-2 surface albedo product.The decreasing trends were mostly observed in Greenland,Central Siberia,southern Alaska,southern Africa arid grassland,western Australia,southern edge of Sahara Desert,and Loess Plateau in China;The increasing trends were mostly observed in northern North America,Central Europe,Central Asia,Antarctic,tropical forest regions in Southeast Asia,and Northeast China.The surface albedo annual oscillation phenomenon was discovered,and has been preliminary explained by the Arctic Oscillation(AO)and Southern Oscillation(SO)indies in this study.(3)The spatial distribution map of surface albedo drivers has been derived based on the GLASS datasets and Modern-Era Retrospective analysis for Research and Application version 2(Merra-2)reanalysis data.The results show that northern high-latitude is mainly affected by snow cover extent;Southeast United States,Southeast China,tropical grassland in Central Africa,northwestern Europe,eastern South America,and parts of Australia are mainly affected by vegetation dynamics;Syria and Iraq of Middle-East,southern Indian,and southern Australia are mainly affected by soil surface wetness.(4)The mechanism of surface albedo changes induced by vegetation growth variations under the background of global climate change was investigated.The global vegetation trends maps,lag-effect of vegetation growth to climate factors,and the surface albedo changes map induced by vegetation were obtained.The results show that the decrease of surface albedo in Southern edge of Sahara Desert,southern Africa arid grassland,and central Australia can be explained by the increase trends of vegetation growth.(5)The Greenland island,southern Africa arid grassland,and Northeast China were selected as hotspot area for analyzing.The surface albedo of Greenland in July show a-0.0113/decade decreasing trend.The most significant decreasing trend can be found in the sea shore of Greenland.Moreover,there is a negative relationship between the decreasing trend and altitude.The surface albedo of southern Africa arid grassland show a-0.006/decade decreasing trend,which has a strong relationship with the vegetation dynamics.The surface albedo changes and their drivers in Northeast China were also explored with the GLASS surface albedo product.The results show that there is a+0.0037/decade increasing trend in Northeast China,and the surface albedo anomaly is closely connected with AO index;The increasing trends mostly occur in Hulunbuir grassland,Sanjiang Plain,and Songnen Plain,and the decreasing trends mostly occur in Greater Khingan mountain,eastern Lesser Khingan,and hilly land in the western Liaoning Province.The trends and drivers for spatiotemporal changes of global surface albedo have been derived by this study.These results update our knowledge about the patterns of global surface albedo changes,and provide a global distribution map for the drivers of surface albedo change,which lays a solid foundation for studying the responses and feedbacks of surface albedo to global climate changes and human activities.