Variation Characteristics Of Land Surface Energy Balance In Tibet From Multi-Satellite Observations

The land surface energy balance（SEB）is the principal driving factor of the Earth’s climate change and drives the hydrological and heat cycle in a certain location.The SEB includes four primary fluxes:Net radiation,Ground heat flux,sensible heat flux and Latent heat flux.However,the variation characteristics of SEB are very complex as well as its mechanism,particularly in the Tibetan Plateau a region with high solar radiation activity.The Tibetan Plateau is Earth’s highest and largest landmass and strongly impacts the Asian climate,especially on the Asian monsoon,which is a climate-sensitive region and the main water source for many rivers in South and East Asia.It is important to estimate SEB variation and understand the mechanism of the energy fluxes to the local climate in the Tibetan Plateau.A detailed SEB variation over the Tibetan Plateau is required to assess the dynamics of climate change in the region.In this thesis,the variation characteristics of SEB in the Tibetan Plateau are estimated and investigated from multi-satellite observations as well as its variation mechanism.The main works and results are summarized as follows:（1）A novel algorithm is proposed to estimate SEB using remote sensing data.The Simplified Estimation of Surface Energy Balance（SE2B）is a semi-empirical method to estimate SEB using remote sensing data.Estimated surface energy fluxes are validated by in situ ground measurements of two micrometeorological tower networks,Global Flux Network（FLUXNET）and Coordinated and Enhanced Observation Period（CEOP）Asia-Australia Monsoon Project（CAMP）over Tibet.The method is simultaneously validated for daily and monthly data.Pearson correlation coefficient was used to establish the correlation between the estimated and observed surface energy fluxes while standard error,mean bias error,root mean square error and mean absolute error are used to estimate the precision of the algorithm.The results suggested that SE2B is an accurate method to estimate surface energy fluxes with an average of 1.87 Wm^-2 mean bias error and 0.93 of Pearson’s correlation coefficient.（2）Two decadal variations of SEB are analyzed using linear regression slope,Sen’s slope and Mann-Kendall test on the mean annual images of corresponding surface energy flux.It is found that net radiation and latent heat flux are increased by 0.37 and 1.18 Wm-2,respectively.In comparison,ground heat flux and sensible heat flux are decreased by 0.49 and 0.33 Wm^-2 per decade over Tibet during the last two decades.It is also observed that net radiation and latent heat flux show a remarkable increase over east and southeast Tibet where forests and shrubs are abundant.In contrast,SH and G show inverse behavior over the same region.SH increases over southeast Tibet while G increases over the northern barren lands of Tibet.（3）Sensitivity analysis is performed to understand the mechanism of SEB variations.The basic assumption is how much percentage change will occur in the corresponding surface energy flux if the influencing parameter changes by±20%.In the current study,the air temperature is taken as a meteorological parameter while land surface temperature,albedo and normalized difference vegetation index are taken as biophysical parameters.It is found that meteorological and biophysical parameters significantly impact surface energy fluxes.Sensitivity is further successively analyzed for these changes.It is computed that±20%changes in air temperature and land surface temperature vary surface energy fluxes up to 22.54%and 30.57%,respectively.This study provides comprehensive and insightful details about SEB variation characteristics in Tibet.The proposed simplified estimation scheme enables the vast community of remote sensing scientists to estimate regional SEB with minimal remote sensing data.The long-term variation analysis enlightens the pattern of climate change in Tibet and eventually in the whole region as Tibet largely controls regional climate and Asian monsoon.In the final phase of the present study,the established relations between atmospheric,meteorological and biophysical parameters with surface energy fluxes explain the variation of SEB in the region.It is expected that these relations will be better understood by analyzing various regions of the world in the future.Policymakers and concerned authorities can use this study’s results to make better and more sustainable strategies for future projects.