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.

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.

Calculation of Solar Albedo and Radiation Equilibrium over the Qinghai-Xizang Plateau and Analysisof Their Climatic Features

Using radiation data from the Automatic Weather Stations (AWSs) for thermal balance obser-vations, which were set up at Lhasa, Nagqu, Xigaze and Nyingchi by the Sino-Japanese Asian Monsoon Mechanism Co-operative Project in 1993–1996, and 1985–1989 Earth Radiation Balance Experiment (ERBE) measurements of Langley Research Center/NASA of US, and 1961–1996 monthly mean data from 148 surface stations over the Qinghai-Xizang Plateau (QXP) and its neighborhood, study is performed on empirical calculation methods of surface albedo, surface total radiation, planetary albedo and outgoing longwave radiation with the climatic features of radiation balance at the surface and the atmospheric top examined. Evidences suggest that the empirical formulae for surface albedo, planetary albedo, surface to-tal radiation and outgoing longwave radiation from the atmospheric top are capable of describing their seasonal and interannual variations over the QXP. The surface albedo is marked by noticeable seasonal variation and yearly mean of 0.22 with the maximum of 0.29 in January and minimum of 0.17 in July and August; in winter the albedo has great horizontal difference, bigger in the moun-tains than in the river valleys, and small in summer. The planetary albedo shows a smaller range of its annual variation with the yearly mean of 0.37, the maximum (minimum) occurring in February and March (autumn). In winter its high-value regions are mainly at Gar (Shiquanhe) in the western QXP and from the southwestern Qinghai to the northeastern Tibet and the low-value area at the northern slope of the central Himalayas; in summer, however, the albedo distribution displays clear-ly a progressive decrease from southeast to northwest. As for the surface total radiation, its values and annual varying range are smaller in the east than in the southwest. Its high-value center is at the southern slope of the Himalayas in winter and makes a conspicuous westward migration in spr-ing, remaining there for a long time, and it begins to retreat eastward in autumn. Monthly mean values of the surface net radiation are all positive and larger in summer than in winter. The net ra-diation is significantly intensified under the combined effect of surface total radiation and surface albedo from spring to early summer, resulting in the strongest sector in the mid plateau with its center staying nearly motionless from March to September, and is reduced in autumn dominantly by surface effective radiation. The earth-atmosphere system loses heat outward from October to next February and gains in other months. On an average, the plateau gains heat of 15 W m-2 on an annual basis. Key words The Qinghai-Xizang Plateau - Albedo - Radiation balance - Climatic feature (1)This work was supported under the auspices of the National (G1998040800) and CAS’s Key Project for Basic Research on Tibetan Plateau (KZ951-A1-204; KZ95T-06).

Radiation balance and the response of albedo to environmental factors above two alpine ecosystems in the eastern Tibetan Plateau

Understanding the energy balance on the Tibetan Plateau is important for better prediction of global climate change. To characterize the energy balance on the Plateau, we examined the radiation balance and the response of albedo to environmental factors above an alpine meadow and an alpine wetland surfaces in the eastern Tibetan Plateau, using 2014 data. Although our two sites belong to the same climatic background, and are close geographically, the annual incident solar radiation at the alpine meadow site(6,447 MJ/(m2·a)) was about 1.1 times that at the alpine wetland site(6,012 MJ/(m2·a)),due to differences in the cloudiness between our two sites. The alpine meadow and the alpine wetland emitted about 38%and 42%, respectively, of annual incident solar radiation back into atmosphere in the form of net longwave radiation; and they reflected about 22% and 18%, respectively, of the annual incident solar radiation back into atmosphere in the form of shortwave radiation. The annual net radiation was 2,648 and 2,544 MJ/(m2·a) for the alpine meadow site and the alpine wetland site, respectively, accounting for only about 40% of the annual incident solar radiation, significantly lower than the global mean. At 30-min scales, surface albedo exponentially decreases with the increase of the solar elevation angle; and it linearly decreases with the increase of soil-water content for our two sites. But those relationships are significantly influenced by cloudiness and are site-specific.

Improved Parameterization of Snow Albedo in WRF+Noah:Methodology Based on a Severe Snow Event on the Tibetan Plateau

Snowfall and the subsequent evolution of the snowpack have a large effect on the surface energy balance and water cycle of the Tibetan Plateau(TP).The effects of snow cover can be represented by the WRF coupled with a land surface scheme.The widely used Noah scheme is computationally efficient,but its poor representation of albedo needs considerable improvement.In this study,an improved albedo scheme is developed using a satellite-retrieved albedo that takes snow depth and age into account.Numerical experiments were then conducted to simulate a severe snow event in March 2017.The performance of the coupled WRF/Noah model,which implemented the improved albedo scheme,is compared against the model’s performance using the default Noah albedo scheme and against the coupled WRF/CLM that applied CLM albedo scheme.When the improved albedo scheme is implemented,the albedo overestimation in the southeastern TP is reduced,reducing the RMSE of the air temperature by 0.7°C.The improved albedo scheme also attains the highest correlation between the satellite-derived and the model-estimated albedo,which provides for a realistic representation of both the snow water equivalent(SWE)spatial distribution in the heavy snowbelt(SWE>6 mm)and the maximum SWE in the eastern TP.The underestimated albedo in the coupled WRF/CLM leads to underestimating the regional maximum SWE and a consequent failure to estimate SWE in the heavy snowbelt accurately.Our study demonstrates the feasibility of improving the Noah albedo scheme and provides a theoretical reference for researchers aiming to improve albedo schemes further.

Vertical response of snow albedo to seasonal climate change in the Tibetan Plateau

The snow cover in the Tibetan Plateau(TP)responds keenly to global climate and hydrological shifts,with snow albedo variation serving as a pivotal indicator of these changes.In this study,we explored snow albedo changes over the period(2001-2022)in the TP combined with the high-resolution near-surface meteorological forcing datasets(2001-2022).The study utilized Ding’s method to separate precipitation patterns,and then employed path analysis to evaluate the vertical response of snow albedo to air temperature,rainfall,and snowfall across four periods.The findings are as follows:(1)Snow albedo in area above 4000 m ranged from 0.4 to 0.7,while below 4000 m,snow albedo was primarily below 0.4.Snow albedo was generally higher in the northern TP.(2)During the snow accumulation period(October to December),snow albedo showed a decreasing trend in most areas of the TP.Conversely,snow albedo exhibited overall increasing trends during the snow stable period(January to February),snowmelt period(March to May),and snowless period(June to September).Especially in the central TP,snow albedo showed significant decrease during the snow accumulation period,and it increased significantly in the other periods.(3)Air temperature,rainfall,and snowfall influenced directly and predominantly snow albedo changes in the TP.Especially,air temperature and snowfall were the primary driving factors in most areas.(4)During different periods,air temperature was the main factor driving changes in snow albedo below 5000 m,but snowfall had a stronger influence above 5000 m.Except during the snow accumulation period,the impact of rainfall on snow albedo decreased with increasing altitude.During the snowless period,rainfall affected snow albedo obviously,but snowfall remained the dominant factor in areas above 6500 m.These results provide new insights on climate-driven changes in the snow albedo over the TP.

藏北高原GlobAlbedo地表反照率的精度分析

为了分析欧洲航天局多星观测数据联合反演的全球地表反照率产品Glob Albedo在青藏高原的反演精度,促进其在青藏高原地—气相互作用研究中的应用,利用藏北高原BJ站和西大滩站观测的上行和下行太阳短波辐射资料,对比分析了Glob Albedo的精度,并与MODIS地表反照率产品MCD43B3进行了比较。结果表明：空间分辨率1 km的Glob Albedo短波波段（0.3～5.0μm）的地表反照率与地面观测结果总体上具有较好的一致性,但是精度受积雪覆盖比例的影响较大。积雪覆盖比例〈0.1时,Glob Albedo短波波段的地表反照率与高质量地面观测结果的均方根误差介于0.0100～0.0218,Glob Albedo的精度完全能够满足气候和陆面模式的精度要求。反之,它们的均方根误差介于0.0252～0.1461,存在较大的不确定性。对比Glob Albedo和MCD43B3,前者的精度略高于后者：Glob Albedo短波波段地表反照率与高质量地面观测结果的均方根误差介于0.0195～0.0959,MCD43B3短波波段地表反照率与高质量地面观测结果的均方根误差介于0.0273～0.1269。

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.

ON THE RELATIONSHIP BETWEEN THE CLEAR-SKY PLANETARY AND THE SURFACE ALBEDO OVER THE QINGHAI-XIZANG PLATEAU

Satellite observations offer the potential of mapping and monitoring of the distribution and variation of surface albedo. Numerous studies have been conducted aiming at derivation of surface albedo from satellite sensor data. In regard to the transform of planetary albedo to surface albedo, it is the most important to establish the relationship between the clearsky and the surface albedo.

Drivers of elevation-dependent warming over the Tibetan Plateau

青藏高原(海拔≥3000 m地区)对全球气候变化的变暖响应是空间不均匀的,其增温幅度会随着海拔升高而增大,被称为海拔依赖性增温.青藏高原海拔依赖性增温具有季节依赖性,在冬季最为显著,达0.21℃km-1.在以往的研究中,众多因素被认为是青藏高原海拔依赖性增温的可能驱动因素,但关于这些因素相对重要性的研究较少.基于多个数据集,本文应用辐射核(radiative kernel)技术方法定量诊断了近几十年(1979-2018年)冬季不同物理过程对青藏高原海拔依赖性增温的贡献.结果表明,与积雪变化相关的地表反照率反馈在其中起主导作用.观测数据分析显示,在过去40年的冬季,高海拔地区的积雪覆盖率显著减少,导致地表反射的短波辐射减少,从而促进了海拔依赖性增温.

Impacts of land surface darkening on frozen ground and ecosystems over the Tibetan Plateau

Tibetan Plateau(TP) is known as the “Third Pole” of the Earth. Any changes in land surface processes on the TP can have an unneglectable impact on regional and global climate. With the warming and wetting climate, the land surface of the TP saw a darkening trend featured by decreasing surface albedo over the past decades, primarily due to the melting of glaciers, snow,and greening vegetation. Recent studies have investigated the effects of the TP land surface darkening on the field of climate, but these assessments only address one aspect of the feedback loop. How do these darkening-induced climate changes affect the frozen ground and ecosystems on the TP? In this study, we investigated the impact of TP land surface darkening on regional frozen ground and ecosystems using the state-of-the-art land surface model ORCHIDEE-MICT. Our model results show that darkening-induced climate changes on the TP will lead to a reduction in the area of regional frozen ground by 1.1×10~4±0.019×10~4km~2, a deepening of the regional permafrost active layer by 0.06±0.0004 m, and a decrease in the maximum freezing depth of regional seasonal frozen ground by 0.06±0.0016 m compared to the scenario without TP land surface darkening.Furthermore, the darkening-induced climate change on the TP will result in an increase in the regional leaf area index and an enhancement in the regional gross primary productivity, ultimately leading to an increase in regional terrestrial carbon stock by0.81±0.001 PgC. This study addresses the remaining piece of the puzzle in the feedback loop of TP land surface darkening, and improves our understanding of interactions across multiple spheres on the TP. The exacerbated regional permafrost degradation and increasing regional terrestrial carbon stock induced by TP land surface darkening should be considered in the development of national ecological security barrier.

Disturbance of light-absorbing aerosols on the albedo in a winter snowpack of Central Tibet

A field observation on the albedo of the snowpack in Central Tibet was conducted in the Nam Co region in the winter of 2011. Snow properties, including grain size and density, were measured in the field, and surface-layer snow samples （down to 5 cm） were collected. The average concentrations of black carbon and dust were 72 ppbm （close to that in the glaciers of Mt. Nyainqentanglha） and 120 ppmm, respectively. Inverse trends were found to exist between the albedo of the snowpack and light-absorbing aerosols （LAAs） as well as grain size growth. Modeling showed that black carbon, dust, and grain growth in the winter snowpack can reduce the broadband albedo by 11%, 28%, and 61%, respectively.

Aerosol optical properties and its direct radiative forcing over Tibetan Plateau from 2006 to 2017

Studies on optical properties of aerosols can reduce the uncertainty for modelling direct radiative forcing(DRF)and improve the accuracy for discussing aerosols effects on the Tibetan Plateau(TP)climate.This study investigated the spatiotemporal variation of aerosol optical and microphysical properties over TP based on OMI and MERRA2,and assessed the influence of aerosol optical properties on DRF at NamCo station(30°46.44′N,90°59.31′E,4730 m)in the central TP from 2006 to 2017 based on a long measurement of AERONET and the modelling of SBDART model.The results show that aerosol optical depth(AOD)exhibits obvious seasonal variation over TP,with higher AOD500nm(>0.75)during spring and summer,and lower value(<0.25)in autumn and winter.The aerosol concentrations show a fluctuated rising from 1980 to 2000,significant increasing from 2000 to 2010 and slight declining trend after 2013.Based on sensitivity experiments,it is found that AOD and single scattering albedo(SSA)have more important impact on the DRF compared withαvalues and ASY.When AOD440nm increases by 60%,DRF at the TOA and ATM is increased by 57.2%and 60.2%,respectively.When SSA440nm increases by 20%,DRF at the TOA and ATM decreases by 121%and 96.7%,respectively.

近地表水汽密度对多年冻土区地表辐射的影响研究——以北麓河地区为例

气候暖湿化背景下,青藏高原地区大气水汽含量的增大通过影响地表辐射进而影响多年冻土地表能量分配及其热稳定性。以青藏高原中部北麓河地区的气象数据与活动层水热数据为基础,分析了2 m处空气相对湿度和夏季典型降雨事件对地表反照率及辐射四分量的影响。结果表明:大气水汽通过削弱太阳短波辐射,吸收地面长波辐射,增加向下长波辐射,进而降低地表反照率。大气水汽对地表辐射的影响具有明显的季节性特征,夏季大气水汽对太阳短波辐射的削弱作用最明显,并且发射的向下长波辐射较多;冬季大气水汽对太阳短波辐射的削弱作用相对较弱,发射向下长波辐射较少。在研究时段内北麓河地区夏季和冬季空气相对湿度每增加10%,太阳短波辐射日均值分别减少54.9和9.8 W/m^(2),向下长波辐射日均值分别增加14.8和3.9 W/m^(2)。秋季空气中水汽含量的变化对地表反照率的影响最大,秋季空气相对湿度每减小10%,地表反照率增加0.15;春季最小,春季空气相对湿度每增加10%,地表反照率仅降低0.01。夏季不同典型降雨事件导致近地表水汽密度和浅层土壤含水量增大,使地表反照率降低。夏季不同类型降雨事件对地表反照率的影响程度表现为:大雨>中雨>小雨。研究结果为暖湿化气候背景下青藏高原中部多年冻土区地表能量平衡分析提供参考。

2000—2016年黄土高原地区荒漠化遥感分析

鉴于利用NDVI-albedo特征空间的方法进行黄土高原荒漠化的研究较少,利用MODIS归一化差异植被指数(normalized difference vegetation index,NDVI)和地表反照率(albedo)数据,构建NDVI-albedo特征空间,计算荒漠化差值指数(desertification difference index,DDI)和植被条件反照率干旱指数(vegetation condition albedo drought index,VCADI),并利用实测土壤湿度数据进行验证,分析黄土高原2000—2016年土地荒漠化和旱情的时空分布规律和影响因素。结果表明,DDI、VCADI与土壤湿度分别呈正相关和负相关,均通过了显著性检验,均可作为土地荒漠化和旱情监测指标;17年间,黄土高原重度荒漠化面积逐渐缩小,土地荒漠化改善面积占84.55%,荒漠化加重区域仅占5.61%;旱情改善面积占67.07%,加重区域占19.44%;荒漠化与降水量呈负相关关系,与气温呈正相关关系,而旱情与降雨量、年均温度均呈正相关关系。

青藏高原晴空行星反照率与地面反照率关系的研究

本文利用了大气-地球系统的物理模型推导了晴空行星反照率与地面反照率之间的相互关系,指出当仅考虑一次地面反射时,行星反照率是地面反照率的一次线性函数,当考虑到大气与地面的多次反射时,两者的关系设为二次抛物线函数,并利用1986年中美青藏高原联合考察期间的地面辐射资料以及同期NOAA-9的GAC资料对上述关系进行了试验,结果表明,两种关系都能较好地反映出青藏高原行星反照率与地面反照率之间的关系,但二次函数表现的物理意义更为明确。 青藏高原作为一个特殊的地理类型,其行星反照率与地面反照率的相互关系与全球纬向平均相比较也有明显的差异,本文讨论了这些差异的物理意义,同时分别讨论了该地区可见光和近红外这二个通道的滤过反照率与地面分光谱反照率之间的关系。 青藏高原晴空反照率与地面反照率关系的研究为利用AVHRR资料反演青藏高原地面反照率的数值分布图提供了理论依据和具体方法。

1998年夏季青藏高原辐射平衡分量特征

利用1998年夏季第二次青藏高原气象科学试验（TIPEX）获得的改则、当雄和昌都三个热源观测站的数据和相关资料，统计和分析了高原夏季辐射平衡分量和热源强度的变化特征。结果表明高原地面总辐射平均强度以西部最强，中部次之，东部最小。6月中旬后随着雨季到来，地表反射率均有所降低，中部和东部的辐射强度明显减弱，西部雨季降水和云量都比较少，辐射强度变化不明显。高原中部和东部的净辐射在6月中旬有明显的突变现象，西部突变期出现在7月上旬．以中部的辐射加热强度最大，东部次之，西部最小。湿期随着地面长波辐射的减少，热源强度明显增大。

青藏高原纳木错站地表反照率观测与MODIS资料的对比分析

利用中国科学院纳木错多圈层相互作用综合观测研究站内太阳光度计观测的大气气溶胶光学厚度和整层气柱水汽总量,作为大气辐射传输模式的输入参数,模拟计算了2007年5月—2008年8月无雪期晴空条件下,正午时段该站的太阳总辐射和散射辐射,得到两者的比例S。基于MODIS发布的MCD43B3产品中的短波段黑空和白空反照率以及比例S,计算得到实际大气条件下地表反照率的卫星反演值,进而与地面观测值进行对比分析。结果显示,两者没有显著差别,可以满足气候模式对地表反照率绝对偏差为0.02的精度要求,且均方根偏差约为0.0156,最大偏差为0.046。雨季纳木错站的土壤含水量增加,使得该站晴空时观测的5 min平均地表反照率呈线性下降。

青藏高原地区MODIS反照率的精度分析

应用2002—2004年青藏高原CAMP/Tibet试验期间4个地面站点的反照率观测结果定量分析Terra MODIS 1km分辨率短波SW波段(0.3～5.0μm)反照率全反演结果和当量反演结果的精度。对于全反演结果,黑空反照率、白空反照率与地面观测结果的均方根差分别为0.0187和0.0168;对于当量反演结果,黑空反照率、白空反照率与地面观测结果的均方根差分别为0.0766和0.0761。综合全反演结果和当量反演结果,则黑空反照率、白空反照率与地面观测结果的均方根差分别为0.0679和0.0675。当地面观测结果与MODIS反照率当量反演结果均为"无雪"状态时,黑空反照率、白空反照率与地面观测结果的均方根差分别为0.0352和0.0364;当地面观测结果为"积雪"状态,MODIS反照率当量反演结果为"无雪"状态时,黑空反照率、白空反照率与地面观测结果的均方根差分别高达0.1556和0.1541。