Time-lagged Effects of the Spring Atmospheric Heat Source over the Tibetan Plateau on Summer Precipitation in Northeast China during 1961–2020:Role of Soil Moisture

The spring atmospheric heat source(AHS)over the Tibetan Plateau(TP)has been suggested to affect the Asian summer monsoon and summer precipitation over South China.However,its influence on the summer precipitation in Northeast China(NEC)remains unknown.The connection between spring TP AHS and subsequent summer precipitation over NEC from 1961 to 2020 is analyzed in this study.Results illustrate that stronger spring TP AHS can enhance subsequent summer NEC precipitation,and higher soil moisture in the Yellow River Valley-North China region(YRVNC)acts as a bridge.During spring,the strong TP AHS could strengthen the transportation of water vapor to East China and lead to excessive rainfall in the YRVNC.Thus,soil moisture increases,which regulates local thermal conditions by decreasing local surface skin temperature and sensible heat.Owing to the memory of soil moisture,the lower spring sensible heat over the YRVNC can last until mid-summer,decrease the land–sea thermal contrast,and weaken the southerly winds over the East Asia–western Pacific region and convective activities over the South China Sea and tropical western Pacific.This modulates the East Asia–Pacific teleconnection pattern,which leads to a cyclonic anomaly and excessive summer precipitation over NEC.

Utilizing a new soil effective temperature scheme and archived satellite microwave brightness temperature data to estimate surface soil moisture in the Nagqu region, Tibetan Plateau of China

Since the early 2000s, many satellite passive microwave brightness temperature （BT） archives, such as the Advanced Microwave Scanning Radiometer for the Earth Observing System （AMSR-E） BTs, have become the useful resources for assessing the changes in the surface and deep soil moistures over both arid and semi-arid regions. In this study, we used a new soil effective temperature （T scheme and the archived AMSR-E BTs to estimate surface soil moisture （SM） over the Nagqu region in the central Tibetan Plateau, China. The surface and deep soil temperatures required for the calculation of regional-scale T were obtained from outputs of the Community Land Model version 4.5 （CLM4.5）. In situ SM measurements at the CEOP-CAMP/Tibet （Coordinated Enhanced Observing Period Asia-Australia Monsoon Project on the Tibetan Plateau） experimental sites were used to validate the AMSR-E-based SM estimations at regional and single-site scales. Furthermore, the spatial distribution of monthly mean surface SM over the Nagqu region was obtained from 16 daytime AMSR-E BT observations in July 2004 over the Nagqu region. Results revealed that the AMSR-E-based surface SM estimations agreed well with the in situ-based surface SM measurements, with the root mean square error （RMSE） ranging from 0.042 to 0.066 m3/m3 and the coefficient of determination （R2） ranging from 0.71 to 0.92 during the nighttime and daytime. The regional surface soil water state map showed a clear spatial pattern related to the terrain. It indicated that the lower surface SM values occurred in the mountainous areas of the northern, mid-western and southeastern parts of Nagqu region, while the higher surface SM values appeared in the low elevation areas such as the Tongtian River Basin, Namco Lake and bog meadows in the central part of Nagqu region. Our analysis also showed that the new T^scheme does not require special fitting parameters or additional assumptions, which simplifies the data requirements for regional-scale applications. This scheme combined with the archived satellite passive microwave BT observations can be used to estimate the historical surface SM for hydrological process studies over the Tibetan Plateau regions.

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.

Analysis of spatial distribution and multi-year trend of the remotely sensed soil moisture on the Tibetan Plateau

Long-term highly accurate surface soil moisture data of TP(Tibetan Plateau)are important to the research of Asian monsoon and global atmospheric circulation.However,due to the sparse in-situ networks,the lack of soil moisture observations has seriously hindered the progress of climate change researches of TP.Based on the Dual-Channel soil moisture retrieval algorithm and the satellite observation data of AMSR-E(Advanced Microwave Scanning Radiometer for EOS),we have produced the surface soil moisture data of TP from 2003 to 2010 and analyzed the seasonal characteristic of the soil moisture spatial distribution and its multi-year changing trend in area of TP.Compared to the in-situ observations,the accuracy of the soil moisture retrieved by the proposed algorithm is evaluated.The evaluation result shows that the new soil moisture product has a better accuracy in the TP region than the official product of AMSR-E.The spatial distribution of the annual mean values of soil moisture and the seasonal variations of the monthly-averaged soil moisture are analyzed.The results show that the soil moisture variations in space and time are consistent with the precipitation distribution and the water vapor transmission path in TP.Based on the new soil moisture product,we also analyzed the spatial distribution of the changing trend of multi-year soil moisture in TP.From the comparisons with the precipitation changing trend obtained from the meteorological observation sites in TP,we found that the spatial pattern of the changing trend of soil moisture coincides with the precipitation as a whole.

Can the tropical storms originated from the Bay of Bengal impact the precipitation and soil moisture over the Tibetan Plateau?

This study investigates the impacts of tropical storms originated from the Bay of Bengal(BOBTSs) on the precipitation and soil moisture over the Tibetan Plateau(TP) in April–June(AMJ) and September–December(SOND) during 1981–2011 based on the best track dataset provided by Joint Typhoon Warning Centre(JTWC). Results indicate that there are about 1.35 BOBTSs influence the TP in each year and most of them occurred in May and October, and the BOBTSs in AMJ influence the TP with larger extension and higher latitudes than those in SOND. The maximum regional precipitation induced by the BOBTSs accounts for more than 50% for the total precipitation in the corresponding month and about 20% for the season. Further analysis reveals that the surface soil moisture anomalies induced by the BOBTSs can persist only 20–25 days in AMJ, and the case is also true for the snow depth in SOND. Numerical simulations by using the regional climate model of Weather Research and Forecasting(WRF) suggest that the soil moisture anomalies in the sub-surface can last 2 months whereas for the surface it can persist only about 20 days, which agrees well with the observation analysis. Overall, the effect of the preceding BOBTSs on the snow depth and soil moisture anomalies over the TP cannot maintain to summer, and there is no robust connection between the BOBTSs and summer precipitation anomalies in East China. Moreover, since the mid-1990 s, the spring rainfall induced by the BOBTSs over the TP seems to be enhanced to a certain degree because of the intensified BOBTSs.

5月中国土壤湿度异常对7月华南-青藏高原东部偶极子型降水年际变化的影响

利用1979—2019年ERA5再分析资料和站点降水资料,研究了5月中国土壤湿度异常对7月华南和青藏高原东部偶极子型降水年际变化的影响及其可能的物理过程。结果表明,当5月青藏高原土壤湿度偏湿,华中地区土壤湿度偏干时,对应7月华南(高原东部)降水偏多(偏少),两地降水呈偶极子型分布。通过进一步的诊断分析发现,青藏高原(华中地区)土壤湿度正(负)异常可从5月持续至7月,使得7月中国北方地区地表湍流热通量正异常,进而使得对流层中低层大气增暖,中国北方与贝加尔湖之间经向温度梯度和大气斜压性增强,天气尺度的瞬变波活动增强。通过瞬变的涡度强迫有利于中国北方及蒙古地区准正压异常高压和Rossby波波源的形成,相关的Rossby波向东南方向传播至我国南方,使得华南地区出现准正压结构的异常低压,有利于西北太平洋副热带高压东移,南亚高压西移。对应中国北方及蒙古-华南地区对流层中低层为反气旋-气旋式环流异常,进而导致华南地区(高原东部)降水增多(减少)。此外,中国北方-蒙古地区的异常高压与局地偏干的土壤湿度之间的正反馈过程,有利于上述物理过程的维持和增强,进而有利于7月偶极子降水的异常,反之亦然。

Development of a daily soil moisture product for the period of2002–2011 in Chinese mainland

Soil moisture is an essential climate variable(ECV) concerned widely. Due to its high spatial variability, it is costly to measure soil moisture at tens of kilometers scale. In this study, a ten-year(2002–2011) daily soil moisture dataset at 0.25° spatial resolution for Chinese mainland was produced through assimilating the Advanced Microwave Scanning Radiometer for Earth Observing System(AMSR-E) brightness temperature(TB) data into a land surface model(LSM). The obtained soil moisture data was evaluated against soil moisture-measuring networks deployed in two wet areas and one dry area of the Tibetan Plateau.The results show that for the wet areas the accuracy of the soil moisture product obtained from the assimilation is considerably higher than that of both AMSR-E official soil moisture products and land surface simulation results, and for the dry area their accuracy is comparable to each other. The spatial pattern of the soil moisture from the new product is consistent with that of soil porosity from an independent survey-based dataset, further confirming the credibility of the new product. According to this product, the transition regions in China show stronger seasonal variation of soil moisture than dry and wet regions, and drier regions have stronger inter-annual variability of soil moisture than wetter regions, particularly during transitional seasons(spring and autumn). The soil moisture product is accessible at the National Tibetan Plateau Data Center.

基于高分遥感影像的亚高寒草甸土壤水分含量反演

通过高分卫星遥感影像计算植被供水指数来反演亚高寒草甸土壤水分含量,结合高分辨率遥感影像(GF-2)和中分辨率的遥感影像(Landsat-7)进行土壤水分反演模型建模验证,揭示高分遥感影像结合植被供水指数法在青藏高原东北缘亚高寒草甸草原上的适用性,同时分析研究区土壤水分分布及其影响因素。基于高分二号(GF-2)、Landsat-7影像数据,以甘南藏族自治州当周草原为研究区,利用植被供水指数(VSWI,vegetation supply water index)构建土壤水分反演模型得到研究区土壤水分含量反演图,通过半方差函数及主成分分析法探索研究区土壤水分空间分布及影响因素。结果表明:研究区土壤水分含量分布状态呈现出一定程度的空间变异,体现在整个研究区内以及各个地块之间,土壤水分含量主要介于0.11%~60.44%之间;土壤水分含量与坡度、海拔、坡向、NDVI、地表温度均呈正相关关系,分布主要受NDVI、坡向、坡度、海拔的影响。综上,利用植被供水指数法结合高分遥感影像监测土壤水分含量是可行的,基于GF-2遥感影像所建立的模型拟合度最优,较Landsat-7遥感影像更具优势。

青藏高原南部4种土地利用方式土壤质量评价

为了阐明青藏高原南部土壤质量的基本特征,本研究采集了该地区4种典型土地利用类型(草地、耕地、乔木林地和灌木林地)的土壤样品,测定土壤相关理化指标,利用主成分分析法筛选最小数据集指标,并运用土壤质量指数法(SQI)定量评估土壤质量。结果表明:土地利用方式对土壤理化性质具有显著影响,饱和导水率属于极敏感指标,电导率、全氮、有机质、砂粒属于高度敏感指标,pH、容重、粉粒、黏粒属于中度敏感指标。青藏高原南部土壤质量评价最小数据集由全氮、粉粒、黏粒、电导率构成,最小数据集能够较为准确地体现青藏高原南部土壤质量信息(P<0.01)。根据最小数据集计算的SQI表现为乔木林地>草地>耕地>灌木林地,且乔木林地土壤质量显著高于其他3种土地利用类型(P<0.05)。评价结果对于青藏高原南部地区土地的合理利用和生态保护具有重要的参考意义。

青藏高原春季土壤湿度与我国长江流域夏季降水的联系及其可能机理

土壤湿度作为陆面过程的重要因子,对局地及邻近地区的大气环流和天气气候有重要影响.青藏高原的土壤湿度观测站点稀少,时间较短,鉴于此,本文使用经过部分观测站点检验的卫星反演数据,研究了春季高原土壤湿度的年际变化与后期夏季我国东部降水的联系和可能机理.结果表明:在全球变暖的背景下,高原土壤湿度总体呈现出显著增加的趋势,去除该线性趋势后,我们定义了一个高原土壤湿度指数TPSMI来定量表征高原土壤湿度的年际变化特征,发现表层、中层、深层的土壤湿度年际变率趋于一致,且春季土壤湿度与夏季土壤湿度显著相关(相关系数可达0.56).当TPSMI偏大时,即高原东部土壤湿度偏大,而西部偏小时,夏季在高原东部(西部)存在一个潜热(感热)热源,二者共同作用下,在对流层中高层从高原西部经我国大陆直至东北地区激发出一个气旋—反气旋—气旋波列,该波列呈相当正压结构,有利于东北冷涡的加强及冷空气向南爆发;与此同时,南亚高压加强东伸,西太副高西伸加强,低空南方暖湿气流与北方干冷气流在长江流域汇合,伴随着上升运动加强,从而有利于夏季长江流域降水增多;反之,当TPSMI偏小时,夏季长江流域降水减少.

藏北高原土壤温湿变化特征分析

利用"全球协调加强观测计划(CEOP)亚澳季风之青藏高原试验"(CAMP/Tibet,2001—2010)的观测资料,从不同的时间尺度分析了藏北高原不同地点不同深度的土壤温度和土壤湿度变化特征.结果表明:10 cm以上日平均土壤温度呈正弦变化,而10 cm以下土壤温度变化不大;各层土壤温度最高都出现在7～8月;年际气候的差异至少可以反映到40 cm土壤;各层土壤湿度无明显日变化,存在明显月变化,夏季降水量的多少对各层土壤湿度都有明显的影响.

藏北高原土壤的温湿特征

通过藏北高原两个站点 （ Ｄ１１０ 和安多） 土壤温湿特性的分析， 表明浅层土壤温度的变化幅度明显的比深层的要大， 而且浅层土壤温度受地表随机天气过程的影响较大。浅层 （２０ ｃｍ ）土壤在未冻结前湿度的变化幅度不但受形成降水的地表随机天气过程的影响， 而且受其下层土壤湿度状况的影响。下层土壤湿度越小， 浅层 （２０ ｃｍ ） 土壤湿度的变化幅度越大。土壤湿度和土壤温度之间存在着明显的相互关系， 土壤的湿度状况能够影响土壤温度变化的幅度和土壤温度变化的趋势。

不同海拔高度高寒草地土壤理化指标分析

海拔对植物的影响呈现垂直地带性,植物与土壤之间有着密切的关系,为了探明土壤特征与海拔之间的是否存在明显的规律性,探究海拔高度与土壤性质之间的相关关系,进而解释海拔高度影响生态结构和功能的过程。对青藏高原高寒草地不同海拔高度（2800～4100 m）的土壤进行取样,分别采用电位测定法、重铬酸钾容量法-外加热法、扩散吸收法、碳酸氢钠浸提-钼锑抗比色法、Mg（NO3）2氧化-BaSO4比浊法、EDTA容量法、乙酸铵浸提-火焰光度法对pH值、有机质、有效氮、有效磷、全硫、有效钙和镁、有效钾、钠进行测定。结果表明：土壤pH值、有机质、有效氮、有效磷、有效钙和镁、全硫含量与海拔之间具有显著的相关关系,土壤有机质、全硫、有效氮、有效磷和有效镁的含量呈现随着海拔高度的升高先是逐渐降低,含量最低点出现在海拔3200～3400 m,后随着海拔的升高逐渐增加,有效钙含量及pH值随着海拔高度的增加而递减的趋势。然而,有效钾、钠的含量与海拔之间不存在明显的相关关系。总的来说,多种能够以有机形式或其他稳定形式存在的营养元素含量在不同海拔高度呈现一定的规律性变化,而易淋溶的营养元素的含量由于缺乏稳定形式导致不同海拔之间没有明显的规律性。

青藏高原多年冻土活动层土壤水分对高寒草甸覆盖变化的响应

对青藏高原高寒草甸30%、60%和93%三种覆盖度下,多年冻土活动层的土壤水分随季节变化的观测研究,结果表明:多年冻土活动层土壤水分分布对植被覆盖变化响应强烈.年内不同时期,植被覆盖度为65%和30%的土壤表层20cm深度内水分含量及分布相似,每次降水后30%覆盖度土壤水分的变率略大于65%覆盖度的;而93%覆盖度土壤水分在年内解冻开始到冻结前均小于前两种覆盖类型;植被覆盖度越小,土壤冻结和融化响应时间越早,响应历时也越短;浅层土壤冻结和融化对植被覆盖度的响应程度较强,接近深层土壤冻结和融化对植被覆盖度的响应程度降低.覆盖度为30%和65%土壤水分在整个冻结过程的减少幅度比93%覆盖度土壤大10%～26%,而融化期水分增加幅度更大为1.5%～80%;土壤冻融的相变水量对植被覆盖度变化响应明显,植被覆盖度降低,土壤冻结和融化相变水量增大.由于受植被蒸腾与地表蒸散发和土壤温度梯度的影响,融化期土壤剖面的水分重新分配,总体上呈现水分向剖面上部和底部迁移,剖面中部60～80cm深度左右的土壤出现"干层".

青藏高原唐古拉地区暖季土壤水分对地表反照率及其土壤热参数的影响

为了研究青藏高原暖季土壤水分对冻土区地表热状况的影响,选取2010-2012年5-9月在青藏高原唐古拉气象场获取的气象及其活动层数据,分析了表层土壤水分对地表反照率以及土壤热参数的影响.结果表明:唐古拉站暖季表层土壤含水量集中在0.15~0.27之间,地表反照率值集中在0.14~0.24之间,日平均土壤热导率的波动范围在0.9~2.0 W·m^(-1)·K^(-1)之间,土壤热容的波动范围主要集中在0.8×10~6~1.8×10~6J·m^(-3)·K^(-1)之间,而土壤热扩散率则主要集中在0.6×10^(-6)~2.2×10^(-6)m^2·s^(-1)之间.土壤水分对地表反照率影响较大,随着土壤水分的增长,地表反照率呈现出明显的减小趋势.土壤水分对地表反照率的影响还受到植被生长周期的影响,土壤水分和地表反照率之间的关系在植被枯萎期和生长期有明显的差异性.唐古拉地区土壤热参数也明显受到土壤水分变化的影响,随着土壤水分的增加,土壤热导率、热容和热扩散率都为增大趋势,但是土壤水分对土壤热导率的影响较为显著,而对土壤热扩散率的影响则不显著.

利用TRMM/TMI资料反演青藏高原中部土壤湿度

用辐射传输理论提出的地表微波辐射极化指数PI的定义,分别指出了PI对土壤湿度、地面粗糙度、植被层和大气层的影响。用热带降水测量(TRMM—Tropical Rainfall Measuring Mission)卫星上携带的微波辐射仪(TMI—TRMM Microwave Imager)的1B11的6年亮温数据,统计得到青藏高原中部地区PI值月平均分布。并用归一化距平,反演得到了该区域年、季以及干湿季土壤湿度变化的空间图像。结果表明,PI距平分布图可以很好地表征土壤湿度的变化,从而为大尺度评估高原土壤湿度变化提供了理论依据。另外,在同一时间段内,在已知区域平均PI值与平均土壤湿度的条件下,用归一化距平的方法可以定量反演该区域的土壤湿度。

青藏高原春季土壤湿度异常对我国夏季降水影响的模拟研究

利用耦合的全球海气模式(NCAR CCSM3),对青藏高原春季土壤湿度异常影响我国夏季7月降水的机制进行了数值模拟。结果表明,高原6～62 cm深度的中层土壤湿度异常与表层土壤湿度异常有很好的一致性,相对而言,中层土壤湿度异常的持续性较好。若5月高原中层土壤偏湿,则春末至夏初高原地面蒸发、潜热通量增加,而感热通量、地面温度降低,高原表面的加热作用减弱,使得印度高压西撤偏晚,环流系统的季节性转换偏晚,东亚地区形成有利于我国夏季出现第Ⅰ类雨型的环流分布形势,使我国东部雨带偏北,华北地区多雨,江淮地区降水偏少,华南地区降水偏多;反之亦然。

青藏高原高寒草甸土壤无机氮对增温和降水改变的响应

基于海北站野外长期增温和降水改变控制平台,研究高寒草甸生态系统生长季土壤无机氮对增温和降水改变的响应。结果表明,增温使铵态氮降低47.5%(p=0.001),硝态氮降低46.1%(p=0.021)。降水的改变对无机氮的影响存在不对称性,增加降水使铵态氮增加74.7%(p=0.046),硝态氮增加154%(p=0.017);减少降水使铵态氮降低,对硝态氮无显著影响。铵态氮、硝态氮随着土壤湿度的增加而增加,与土壤温度无显著关系。这表明增温和降水改变主要通过改变土壤湿度而不是土壤温度影响生长季土壤无机氮。因此预测,未来气候变化背景下,土壤湿度的增加可能导致青藏高原高寒草甸土壤无机氮的可利用性增加。

基于光学与被动微波遥感的青藏高原地区土壤水分反演

青藏高原地区高精度的土壤水分反演对高原能水循环、全球大气循环研究有着极大的影响.因此,获取青藏高原土壤水分时空布信息是一个迫切需要解决的问题.温度植被干旱指数(TVDI),是基于光学与热红外遥感通道数据反演土壤水分的重要方法,但在研究区域较大、地表覆盖格局差异显著时,TVDI模型反演精度会受到地表温度(Ts)等因素的影响.被动微波AMSR-E数据精确记录了像元内的土壤水分信息,但空间分辨率低.本文利用同时期的MODIS与被动微波数据,发展了针对青藏高原地区高精度土壤水分反演算法.首先,在TVDI模型中,利用修正型土壤调整植被指数(MSAVI)代替归一化植被指数(NDVI),以改进NDVI易饱和的缺点;其次,利用ASTER GDEM数据,对地形高程和纬度差异引起的地表温度变化进行了校正;然后,通过神经网络训练建立基于TVDI、被动微波以及辅助气象数据的土壤水分反演模型,并应用该模型反演了青藏高原地区三个观测网(CAMP/Tibet、玛曲和那曲)的土壤水分;最后,利用实测土壤水分数据对反演结果进行验证,结果表明该模型的精度均方根误差(RMSE)数值可达到0.031~0.041 m^3·m^(-3).本文还应用该算法反演了青藏高原连续的土壤水分的空间分布,并比较了土壤水分的变化趋势与实测降水变化趋势,结果表明二者变化量的正负关系一致.

2009-2010年青藏高原土壤湿度的时空分布特征

利用2009年7月1日至2010年6月30日中国气象局研制的多源土壤温湿度融合分析产品,分析了青藏高原地区不同深度的土壤湿度分布特征.结果表明:青藏高原土壤湿度具有显著的季节变化特征,即春季土壤湿度最大,夏季次之,秋季最小;土壤湿度呈现出浅层和深层低湿、中间层高湿的特点,且土壤湿度由浅到深层变化幅度逐渐减小.随着温度回升,3-8月为土壤湿度增加时段,湿度增加区域从藏东南向西北、塔里木盆地向藏东北扩展,9月以后土壤湿度呈大范围减小.随着季节变化,浅层土壤湿度高湿度区域从南部向北部移动,中间层土壤湿度的变化与浅层相反,深层土壤湿度季节变化差异不大,高湿度区域基本位于高原南部.