Spatiotemporal variation of surface albedo and its influencing factors in northern Xinjiang, China

Surface albedo is a quantitative indicator for land surface processes and climate modeling,and plays an important role in surface radiation balance and climate change.In this study,by means of the MCD43A3 surface albedo product developed on the basis of Moderate Resolution Imaging Spectroradiometer(MODIS),we analyzed the spatiotemporal variation,persistence status,land cover type differences,and annual and seasonal differences of surface albedo,as well as the relationship between surface albedo and various influencing factors(including Normalized Difference Snow Index(NDSI),precipitation,Normalized Difference Vegetation Index(NDVI),land surface temperature,soil moisture,air temperature,and digital elevation model(DEM))in the north of Xinjiang Uygur Autonomous Region(northern Xinjiang)of Northwest China from 2010 to 2020 based on the unary linear regression,Hurst index,and Pearson's correlation coefficient analyses.Combined with the random forest(RF)model and geographical detector(Geodetector),the importance of the above-mentioned influencing factors as well as their interactions on surface albedo were quantitatively evaluated.The results showed that the seasonal average surface albedo in northern Xinjiang was the highest in winter and the lowest in summer.The annual average surface albedo from 2010 to 2020 was high in the west and north and low in the east and south,showing a weak decreasing trend and a small and stable overall variation.Land cover types had a significant impact on the variation of surface albedo.The annual average surface albedo in most regions of northern Xinjiang was positively correlated with NDSI and precipitation,and negatively correlated with NDVI,land surface temperature,soil moisture,and air temperature.In addition,the correlations between surface albedo and various influencing factors showed significant differences for different land cover types and in different seasons.To be specific,NDSI had the largest influence on surface albedo,followed by precipitation,land surface temperature,and soil moisture;whereas NDVI,air temperature,and DEM showed relatively weak influences.However,the interactions of any two influencing factors on surface albedo were enhanced,especially the interaction of air temperature and DEM.NDVI showed a nonlinear enhancement of influence on surface albedo when interacted with land surface temperature or precipitation,with an explanatory power greater than 92.00%.This study has a guiding significance in correctly understanding the land-atmosphere interactions in northern Xinjiang and improving the regional land-surface process simulation and climate prediction.

Variability of Soil Moisture and Its Relationship with Surface Albedo and Soil Thermal Parameters over the Loess Plateau

Data from July 2006 to June 2008 observed at SACOL （Semi-Arid Climate and Environment Observatory of Lanzhou University, 35.946°N, 104.137°E, elev. 1961 m）, a semi-arid site in Northwest China, are used to study seasonal variability of soil moisture, along with surface albedo and other soil thermal parameters, such as heat capacity, thermal conductivity and thermal diffusivity, and their relationships to soil moisture content. The results indicate that surface albedo decreases with increases in soil moisture content, showing a typical exponential relation between the surface albedo and the soil moisture. The heat capacity, the soil thermal diffusivity, and soil thermal conductivity show large variations between Julian day 90-212 and 450-578. The soil thermal conductivity is found to increase as a power function of soil moisture. Soil heat capacity and soil thermal diffusivity increase with increases in soil moisture. The SACOL observed soil moisture are also used to validate the AMSR-E/AQUA retrieved soil moisture and there is good agreement between them. The analysis of the relationship between satellite retrieved soil moisture and precipitation suggests that the variability of soil moisture depends on the variation of precipitation over the Loess Plateau.

Relationships Between Surface Albedo,Soil Thermal Parameters and Soil Moisture in the Semi-arid Area of Tongyu,Northeastern China

Continuous observation data collected over the whole year of 2004 on a cropland surtace m Tongyu, a senti-arid area of northeastern China （44°25＇N, 122°52＇E）, have been used to investigate the variations of surface albedo and soil thermal parameters, including heat capacity, thermal conductivity and thermal diffusivity, and their relationships to soil moisture. The diurnal variation of surface albedo appears as a U shape curve on sunny days. Surface albedo decreases with the increase of solar elevation angle, and it tends to be a constant when solar elevation angle is larger than 40°. So the daily average surface albedo was computed using the data when solar elevation angle is larger than 40° Mean daily surface albedo is found to decrease with the increase of soil moisture, showing an exponential dependence on soil moisture. The variations of soil heat capacity are small during Julian days 90 300. Compared with the heat capacity, soil thermal conductivity has very gentle variations during this period, but the soil thermal diffusivity has wide variations during the same period. The soil thermal conductivity is found to increase as a power function of soil moisture. The soil thermal diffusivity increases firstly and then decreases with the increase of soil moisture.

A Study on Parameterization of Surface Albedo over Grassland Surface in the Northern Tibetan Plateau

The relationship of surface albedo with the solar altitude angle and soil moisture is analyzed based on two-year （January 2002 to December 2003） observational data from the AWS （Automatic Weather Station） at MS3478 in the northern Tibetan Plateau during the experimental period of CEOP/CAMP-Tibet （Coordinated Enhanced Observing Period Asia-Australia Monsoon Project on the Tibetan Plateau）. As a double-variable （solar altitude angle and soil moisture） function, surface albedo varies inconspicuously with any single factor. By using the method of approximately separating the double-variable function into two, one-factor functions （product and addition）, the relationship of albedo with these two factors presents much better. The product and additional empirical formulae of albedo are then preliminarily fitted based on long-term experimental data. By comparison with observed values, it is found that the parameterization formulae fitted by using observational data are mostly reliable and their correlation coefficients are both over 0.6. The empirical formulae of albedo though, for the northern Tibetan Plateau, need to be tested by much more representative observational data with the help of numerical models and the retrieval of remote sensing data. It is practical until it is changed into effective parameterization formulae representing a grid scale in models.

Improvement of Surface Albedo Simulations over Arid Regions

To improve the simulation of the surface radiation budget and related thermal processes in arid regions, three sophisticated surface albedo schemes designed for such regions were incorporated into the Biosphere- Atmosphere Transfer Scheme （BATS）. Two of these schemes are functions of the solar zenith angle （SZA）, where the first one has one adjustable parameter defined as SZA1 scheme, and the second one has two empirical parameters defined as SZA2 scheme. The third albedo scheme is a function of solar angle and soil water that were developed based on arid-region observations from the Dunhuang field experiment （DHEX） （defined as DH scheme）. We evaluated the performance of the original and newly-incorporated albedo schemes within BATS using the in-situ data from the Oasis System Energy and Water Cycle Field Experiment that was carried out in JinTa, Gansu arid area （JTEX）. The results indicate that a control run by the original version of the BATS generates a constant albedo, while the SZA1 and SZA2 schemes basically can reproduce the observed diurnal cycle of surface albedo, although these two schemes still underestimate the albedo when SZA is high in the early morning and late afternoon, and overestimate it when SZA is low during noontime. The SZA2 scheme has a better overall performance than the SZA1 scheme. In addition, BATS with the DH scheme slightly improves the albedo simulation in magnitude as compared to that from the control run, but a diurnal cycle of albedo is not produced by this scheme. The SZA1 and SZA2 schemes significantly increase the surface absorbed solar radiation by nearly 70 W m^-2, which further raises the ground temperature by 6 K and the sensible heat flux by 35 W m^-2. The increased solar radiation, heat flux, and temperature are more consistent with the observations that those from the control run. However, a significant improvement in these three variables is not found in BATS with the DH scheme due to the neglect of the diurnal cycle of albedo. Further analysis indicates that during cloudy days the solar radiation simulations of BATS with these three schemes are not in a good agreement with the observations, which implies that a more realistic partitioning of diffuse and direct radiation is needed in future land surface process simulations.

Land Surface Albedo Variations in Sanjiang Plain from 1982 to 2015: Assessing with GLASS Data

As a key parameter for indicating the fraction of surface-reflected solar incident radiation, land surface albedo plays an important role in the Earth’s surface energy budget(SEB). Since the Sanjiang Plain has been severely affected by human activities(e.g., reclamation and shrinking of wetlands), it is important to assess the spatiotemporal variations of surface albedo in this region using a long-term remote sensing dataset. In order to investigate the surface albedo climatology, trends, and mechanisms of change, we evaluated the surface albedo variations in the Sanjiang Plain, China from 1982 to 2015 using the Global LAnd Surface Satellite(GLASS) broadband surface albedo product. The results showed that: 1) an increasing annual trend(+0.000 58/yr) of surface albedo was discovered in the Sanjiang Plain based on the GLASS albedo dataset, with a much stronger increasing trend(+0.001 26/yr) occurring during the winter. Most of the increasing trends occurred over the cultivated land, unused land, and land use conversion types located in the northeastern Sanjiang Plain. 2) The increasing trend of land surface albedo in Sanjiang Plain can be largely explained by the changes of both snow cover extent and land use. The surface albedo in winter is highly correlated with the snow cover extent in the Sanjiang Plain, and the increasing trend of surface albedo can be further enhanced by the land use changes.

Significant Variations of Surface Albedo during a Snowy Period at Xianghe Observatory,China

Surface albedo over typical types of surfaces in the North China Plain was observed using a Multi-field Albedo Observation System before and after several snowfalls from 13 to 27 February 2005. Dramatic variations of the surface albedos of bare land, a frozen pond, and withered grassland during that period were analyzed. Under cloudy sky, the mean surface albedo of bare land was about 0.23, but it immediately rose to 0.85 when the surface was covered by fresh snow. The albedo decreased gradually to normal levels afterwards. The melting processes were different depending on the characteristics of the underlying surfaces. For example, over grassland the surface albedo was relatively lower after snowfall, and as a result, more solar energy could be absorbed and consequently the snow melting process was accelerated. Significant variations of surface albedo cannot be easily captured by satellite observations; therefore, detailed measurements of surface albedo and related parameters are essential for determining the impact of snow on the energy budget of the Earth.

ON THE ESTIMATION OF SURFACE ALBEDO USING SATELLITE DATE

The method to determine the surface albedo using satellite date hasbeen developed and discussed. The relatons betwam the planetary albedo andsurfoce albedo in clear skies are derived with the help of the radiative transfermodelling. To improve the accuracy of the denved surfoce albedo, theparameter X0, i.e., the ratio of the reflected to the total extinction radiation isintroduced into this model. The influences of the model paramenters on thesurfoce albedo have been analyzed. It is found that the error in the derivedsurfoce albedo due to the paramaters together is less than that caused by oulyone parameter because of the complementarity of the parameters. Test resultsin 22 stations over China have been given using the Geostationary SatelliteGMS. The results are generally agreeable except for the case of snow/ice cover.

ON THE RELATIONSHIP BETWEEN PLANETARY AND SURFACE ALBEDO:MODEL'S COMPARISON AND VALIDATION

The simple linear relationship between clear-sky planetary and surface albedo can be adopted for certain accuracy.There are different parameterization schemes of atmospheric correction for different retrieval models.In this paper,several representative retrieval models are compared and tested with observational data from HEIhe basin Field Experiment (HEIFE) in western China.Some evaluations and suggestions on improvement are proposed for models which would be more applicable to plateau and arid areas.

Parameterization of land surface albedo

Remote measurements of Earth’s surface from ground, airborne, and spaceborne instruments show that its albedo is highly variable and is sensitive to solar zenith angle（SZA） and atmospheric opacity. Using a validated radiative transfer calculating toolbox, DISORT and a bidirectional reflectance distribution function library, AMBRALS, a land surface albedo（LSA） lookup table（LUT） is produced with respect to SZA and aerosol optical depth. With the LUT, spectral and broadband LSA can be obtained at any given illumination geometries and atmospheric conditions. It provides a fast and accurate way to simulate surface reflectance over large temporal and spatial scales for climate study.

Re-understanding of land surface albedo and related terms in satellite-based retrievals

Land surface albedo is a critical variable in determining surface energy balance,and regulating climate and ecosystem processes through feedback mechanisms.Therefore,climatic modelers and radiative monitoring require accurate estimates of land surface albedo.With the instrument development,algorithm upgrade,spectral-band-adjustment in wavelength center or band width,and the increasing distinct requirement from diversified communities,various albedo terms have been generated in related satellite-based products.The lack of understanding on the divergence of these terminologies can introduce potential considerable errors in the subsequent applications,or an elevated probability to invert the deduced conclusion.We surveyed the basic concepts of reflectance quantities,retrieval strategies,and models developed since the 1970s,and discuss both strength and opportunity for improvements on land surface albedo extraction,and product generation.In addition,we exemplified the difference of albedo terms using the daily MODIS product(MCD43A)to emphasize the potential risk of the ambiguous usage,over typical IGBP land covers in Northern Kazakhstan.Our investigation shows that relative differences among various albedo terms can reach up to 181%and 50%,while 0.266 and 0.118 of absolute variance respectively in the narrow and broad-band surface albedo,which illuminated cautions against the ambiguous understanding of albedo terminologies or erroneous usage of albedo products.

CHARACTERISTICS OF VARIATION IN SURFACE ALBEDO AND SNOW FORCING OVER THE TIBETAN PLATEAU

The combination of field experiments and satellite observations is the fundamental way to understand the characteristics of spatial-temporal variation in surface albedo over the Tibetan (Qinghai-Xizang) Plateau. Under the condition without snow cover, the relatively regular annual variation cycle of the surface albedo can be expressed by an empirical formula. The effect of snow cover on the surface albedo in winter can be expressed by introducing two variables of snow forcing and sensitivity parameter. The existing satellite retrieved results of surface albedo may provide the digital grid data for describing the geographical distribution. However, some satellite retrieved surface albedos available over the Tibetan Plateau are obviously too low in winter. Taking the satellite derived results in summer as the background field representative of geographical distribution and combining the empirical formula of annual cycle based on the surface observations, a dynamic model of surface albedo is developed for the need of modeling the climatic influence of the underlying surface forcing of the Tibetan Plateau.

NUMERICAL EXPERIMENTS ON THE INFLUENCES OF GENERAL CIRCULATION ANOMALY OVER THE TIBETAN PLATEAU AND SURFACE ALBEDO CHANGE IN NORTHWEST CHINA ON SUMMER PRECIPITATION

An advanced three-level global atmospheric general circulation model has been used to study the summer precipitation anomaly in Northwest China.based on the synoptic fact and the statistical analysis of the precipitation,the surface albedo in Northwest China,and the synoptic systems over the Tibetan(Qinghai-Xizang)Plateau.The results show that either the anticyclone intensified over the plateau or the surface alhedo enhanced in Northwest China results in summer precipitation reduction east of Northwest China.Especially.when both of them appear simultaneously,summer precipitation was obviously reduced and severe drought occurred in most areas of Northwest China.Moreover.the simulated difference of precipitation rate of Northwest China is similar to the actural precipitation distribution in Northwest China in 1995,which is the most severe drought year in Northwest China in the past fifty years.So the tendency in drought severity intensified,drought frequency accelerated,drought persistence period extended,and drought areas expanded in Northwest China in recent years is maybe a result of the influences of human activities(e.g.vegetation was reduced,and desertification worsened)on drought circulation pattens over the Tibetan Plateau.

Evaluation of satellite land surface albedo products over China using ground-measurements

Land surface albedo(LSA)is an important parameter in surface energy balance and global climate change.It has been used in the fields of energy budgets,climate dynamics,and land surface processes.To apply satellite LSA products more widely,the product accuracy needs to be evaluated at different scales and under atmospheric and surface conditions.This study validates and analyzes the errors of the LSA datasets from the Global LAnd Surface Satellites(GLASS)product,the European Space Agency’s Earth Observation Envelope Programme(GlobAlbedo),the Quality Assurance for Essential Climate Variables(QA4ECV)project,the Gap-filled Snow-free Bidirectional Reflectance Distribution Function(BRDF)parameters product(MCD43GF),and the Satellite Application Facility on Climate Monitoring(CM SAF)Albedo dataset from the AVHRR data(CLARA-SAL)against the Chinese Ecosystem Research Network(CERN)measurements at different spatiotemporal scales over China from 2005 to 2015.The results show that LSA estimated by GLASS agrees well with the CERN measurements on a continental scale.The GLASS product is characterized by a correlation coefficient of 0.80,a root-mean-square error of 0.09,and a mean absolute error of 0.06.The consistency between GLASS,GlobAlbedo,and CLARA-SAL is slightly lower over the regions with high aerosol optical depth(AOD)(e.g.Sichuan Basin,northern China)and high cloud cover compared with that in regions with lower AOD and low cloud cover.The estimation errors are related to varying atmospheric and surface conditions and increase with increasing AOD and cloud cover and decreasing enhanced vegetation index.Therefore,algorithms under complex atmospheric and surface conditions(e.g.high AOD,sparse vegetation)should be optimized to improve the accuracy of LSA products.

Estimating sea ice albedo feedback in a regional climate modeling sensitivity experiment

Surface albedo feedback （SAF）, or sea ice albedo feedback over the Arctic Ocean, has an important effect on the Arctic climate, even though it is not the leading contributor to polar amplification. Previous model-based studies on SAF have primarily used global climate models to exploit their global coverage and favorable configurations. This study verified the capability of using regional climate models （RCMs） to investigate SAF by designing a sensitivity experinaent in terms of sea ice coverage. This study modeled two control cases of the years 1980 and 2012, as well as two sensitivity cases performed by switching the sea ice coverages in the control cases. The results proved the Weather Research and Forecast model capable of separating and quantifying the respective contributions of the atmosphere and the surface albedo to the surface radiation budget. Supported by the ALL/CLR model, the balanced surface shortwave radiation absorption was used to calculate SAF. The experiments overestimated SAF, largely because of the canceled cloud effect during model initialization. This study highlights a new possibility of designing experiments for studying climatic sensitivity and feedback using RCMs.

Assessment of precipitation type discrimination methods on glacier of Qilian Mountains

Solid precipitation is not only the main supply for glacier mass,but also exerts an important influence on surface albedo and intensifies glacier melting.However,precipitation type observation is very scarce in the high alpine glaciers,which limits the precise simulation of glacier mass balance.This study assessed three discrimination methods of precipitation types including Ding method,Dai method and Froidurot method based on surface albedo observation data on the Laohugou Glacier No.12(LHG Glacier)in western Qilian Mountains.The results showed that Ding method had a best applicability on the LHG Glacier,the other two need to calibrate parameters when they are used in the high elevation glacier region.Then we fitted the relationship between snowfall probability and fresh snow albedo,and put forward a revised formula to simulate fresh snow albedo based on Ding method,which is expected to reduce the uncertainty in glacier mass and energy balance model.Finally,we found a best air temperature threshold of 4℃for discriminating monthly precipitation types.In order to accurately simulate the glacier melt,it is necessary to obtain the threshold temperature appropriately in different glacier region with different elevation and humidity.

Simulation of East Asian Summer Monsoon by Using an Improved AGCM

The IAP 2-L AGCM is modified by introducing a set of climatological surface albedo data into the model for substituting the model′s original surface albedo parameterization. The comparison between the observations and the simulation results by the modified model shows that the general features of the East Asian summer monsoon can be well reproduced by the modified IAP 2-L AGCM. Especially for the simulation of monsoon precipitation, the modified model can well reproduce not only the monthly mean features of the summer monsoon rainfall over East Asia, but also the stepwise advance and retreat of the East Asian summer monsoon rainbelt. Analysis results demonstrate that the good simulation of the monsoon rainfall is closely related to the reasonable simulation of the large scale general circulation over East Asian region, such as the western Pacific subtropical high, Asian monsoon low and the low level flows. The good performance of the modified model in the rainfall simulation shows its great potential to serve as a useful tool for the prediction of summer drought/flood events over East Asia.

Improved sea-ice radiative processes in a global coupled climate model

The NASA Goddard Institute for Space Studies (GISS) coupled global climatemodel was used to investigate the sensitivity of sea ice to improved representations of sea-iceradiative processes: (1) a more sophisticated surface albedo scheme and (2) the penetration of solarradiation in sea ice. The results show that the large-scale sea-ice conditions are very sensitiveto the aforementioned parameteriza-tions. Although the more sophisticated surface albedo schemeproduces a more realistic seasonal cycle of the surface albedo as compared with the baselinesimulation, the resulting higher albedo relative to the baseline simulation generates much more andthicker ice in the arctic. The penetration of solar radiation in sea-ice itself tends to reduce theice cover and thickness in the entire arctic and the western antarctic, and increase the ice coverand thickness in the eastern antarctic. The combination of (1) and (2) significantly improves thesimulations of the average ice thickness and its spatial distribution in the arctic. The atmosphericresponses associated with sea-ice changes were also discussed. While improvements are seen,particularly of the ice thickness distribution, there are still some unrealistic aspects that willrequire further improvements to the sea-ice component.

A two-dimensional energy balance climate model on Mars

A two-dimensional energy balance climate model has been built to investigate the climate on Mars.The model takes into account the balance among solar radiation,longwave radiation,and energy transmission and can be solved analytically by Legendre polynomials.With the parameters for thermal diffusion and radiation processes being properly specified,the model can simulate a reasonable surface atmospheric temperature distribution but not a very perfect vertical atmospheric temperature distribution compared with numerical results,such as those from the Mars Climate Database.With varying solar radiation in a Martian year,the model can simulate the seasonal variation of the air temperature on Mars.With increasing dust content,the Martian atmosphere gradually warms.However,the warming is insignificant in the cold and warm scenarios,in which the dust mixing ratio varies moderately,whereas the warming is significant in the storm scenario,in which the dust mixing ratio increases dramatically.With an increasing albedo value of either the polar cap or the non-ice region,Mars gradually cools.The mean surface atmospheric temperature decreases moderately with an increasing polar ice albedo,whereas it increases dramatically with an increasing non-ice albedo.This increase occurs because the planetary albedo of the ice regions is smaller than that of the non-ice region.

Radiative forcing over China due to albedo change caused by land cover change during 1990-2010

Land cover change affects surface radiation budget and energy balance by chang- ing surface albedo and further impacts the regional and global climate. In this article, high spatial and temporal resolution satellite products were used to analyze the driving mechanism for surface albedo change caused by land cover change during 1990-2010. In addition, the annual-scale radiative forcing caused by surface albedo changes in China＇s 50 ecological regions were calculated to reveal the biophysical mechanisms of land cover change affecting climate change at regional scale. Our results showed that the national land cover changes were mainly caused by land reclamation, grassland desertification and urbanization in past 20 years, which were almost induced by anthropogenic activities. Grassland and forest area decreased by 0.60% and 0.11%, respectively. The area of urban and farmland increased by 0.60% and 0.19%, respectively. The mean radiative forcing caused by land cover changes during 1990-2010 was 0.062 W/m2 in China, indicating a warming climate effect. However, spatial heterogeneity of radiative forcing was huge among different ecological regions. Farmland conversing to urban construction land, the main type of land cover change for the urban and suburban agricultural ecological region in Beijing-Tianjin-Tangshan region, caused an albedo reduction by 0.00456 and a maximum positive radiative forcing of 0.863 WIm2, which was presented as warming climate effects. Grassland and forest conversing to farmland, the main type of land cover change for the temperate humid agricultural and wetland ecological region in Sanjiang Plain, caused an albedo increase by 0.00152 and a maximum negative radiative forcing of 0.184 W/m2, implying cooling climate effects.