Atmospheric hydrological budget with its effects over Tibetan Plateau

Based on 1961-2000 NCEP/NCAR monthly mean reanalysis datasets, vapor transfer and hydrological budget over the Tibetan Plateau are investigated. The Plateau is a vapor sink all the year round. In summer, vapor is convergent in lower levels (from surface to 500 hPa) and divergent in upper levels (from 400 to 300 hPa), with 450 hPa referred to as level of non-divergence. Two levels have different hydrologic budget signatures: the budget is negative at the upper levels from February to November, i.e., vapor transfers from the upper levels over the plateau; as to the lower, the negative (positive) budget occurs during the winter (summer) half year. Evidence also indicates that Tibetan Plateau is a "vapor transition belt", vapor from the south and the west is transferred from lower to upper levels there in summer, which will affect surrounding regions, including eastern China, especially, the middle and lower reaches of the Yangtze. Vapor transfer exerts significant influence on precipitation in summertime months. Vapor transferred from the upper layers helps humidify eastern China, with coefficient -0.3 of the upper budget to the precipitation over the middle and lower reaches of the Yangtze (MLRY); also, vapor transferred from east side (27.5o-32.5oN) of the upper level has remarkable relationship with precipitation, the coefficient being 0.41. The convergence of the lower level vapor has great effects on the local precipitation over the plateau, with coefficient reaching 0.44, and the vapor passage affects the advance and retreat of the rainbelt. In general, atmospheric hydrologic budget and vapor transfer over the plateau have noticeable effects on precipitation of the target region as well as the ambient areas.     

青藏高原大气水汽收支特征及影响

Based on 1961-2000 NCEP/NCAR monthly mean reanalysis datasets, vapor transfer and hydrological budget over the Tibetan Plateau are investigated. The Plateau is a vapor sink all the year round. In summer, vapor is convergent in lower levels (from surface to 500 hPa) and divergent in upper levels (from 400 to 300 hPa), with 450 hPa referred to as level of non-divergence. Two levels have different hydrologic budget signatures: the budget is negative at the upper levels from February to November, i.e., vapor transfers from the upper levels over the plateau; as to the lower, the negative (positive) budget occurs during the winter (summer) half year. Evidence also indicates that Tibetan Plateau is a "vapor transition belt", vapor from the south and the west is transferred from lower to upper levels there in summer, which will affect surrounding regions, including eastern China, especially, the middle and lower reaches of the Yangtze. Vapor transfer exerts significant influence on precipitation in summertime months. Vapor transferred from the upper layers helps humidify eastern China, with coefficient -0.3 of the upper budget to the precipitation over the middle and lower reaches of the Yangtze (MLRY); also, vapor transferred from east side (27.5°-32.5°N) of the upper level has remarkable relationship with precipitation, the coefficient being 0.41. The convergence of the lower level vapor has great effects on the local precipitation over the plateau, with coefficient reaching 0.44, and the vapor passage affects the advance and retreat of the rainbelt. In general, atmospheric hydrologic budget and vapor transfer over the plateau have noticeable effects on precipitation of the target region as well as the ambient areas.     

印度洋偶极子中的西极子对西藏高原盛夏降水的影响

利用西藏高原地区1987-2016年的逐月夏季降水资料和印度洋偶极子指数资料分析了两者的关系,结果表明：高原地区盛夏降水与表征西印度洋异常海温的西极子指数表现出良好的相关关系,在西极子指数正异常年时高原降水偏多10%~30%,其中高原中部偏多最为显著,而在负异常年时与之相反。分析其机理研究发现,在正西极子异常年,南海和西太暖池区域的深对流加强、西太副高偏西偏南和印度热低压的减弱使得来自热带的水汽更容易深入高原腹地,其次,南亚高压东体异常增强,配合低空异常辐合,都使得高原降水偏多。同时,高原上空局地纬圈环流在高原中部（90 °E附近）上空（400 hPa以上）有异常辐合上升区,使得高原中部更容易发展暖湿切变线、高原低涡等中尺度涡旋低值系统,造成更多的降水。本研究从高原气候变化响应海洋年际变化的角度分析了区域降水的季节差异,可以为高原气候预测提供新的思路。     

Estimation of Daily Vapor Pressure Deficit Using MODIS Potential Evapotranspiration on the Tibetan Plateau

Vapor pressure deficit (VPD) is an important parameter in modelling hydrologic cycles and vegetation productivity. Meteorological stations are scarce in remote areas, which often results in imprecise estimations of VPD on the Tibetan Plateau. Moderate Resolution Imaging Spectroradiometer (MODIS) provides evapotranspiration data, which may offer the possibility of scaling up VPD estimations on the Tibetan Plateau. However, no studies thus far have estimated VPD using MODIS evapotranspiration data on the Tibetan Plateau. Therefore, this study used MODIS potential evapotranspiration (PET) to estimate VPD in alpine meadows, alpine steppes, croplands, forests and shrublands for the year, spring, summer, autumn and winter in 2000-2012. A series of root-mean- squared-error (RMSE) and mean-absolute-error (MAE) values were obtained for correlating measured VPD and estimated VPD using MODIS PET data for each listed time period and vegetation type: whole year (0.98-2.15 hPa and 0.68-1.44 hPa), spring (0.95-2.34 hPa and 0.72-1.54 hPa), summer (1.39-2.60 hPa and 0.89-1.96 hPa), autumn (0.78-1.93 hPa and 0.56-1.36 hPa), winter (0.48-1.40 hPa and 0.36-0.98 hPa), alpine steppes (0.48- 1.39 hPa and 0.36-1.00 hPa), alpine meadows (0.58-1.39 hPa and 0.44-0.90 hPa), croplands (1.10-2.55 hPa and 0.82-1.74 hPa), shrublands (0.98-1.90 hPa and 0.78-1.37 hPa), and forests (1.40-2.60 hPa and 0.98-1.96 hPa), respectively. Therefore, MODIS PET may be used to estimate VPD, and better results may be obtained if future studies incorporate vegetation types and seasons when the VPD data are estimated using MODIS PET on the Tibetan Plateau.     

青藏高原降水季节分配的空间变化特征

青藏高原是全球气候变化影响的敏感区域。在全球气候变暖的背景下,其水文气候过程发生了显著的变化,直接影响到区域水资源演化。然而,目前对该区域水文气候过程的时空演变规律仍认识不足。本文以青藏高原气象站点降水观测数据为基准,结合水汽通量资料,对13种不同源降水数据集质量进行对比分析;并选用质量较好的IGSNRR数据集识别了青藏高原降水季节分配特征的空间分布格局。结果表明,青藏高原东南、西南以及西北边缘地区降水集中度和集中期较小,夏季降水占全年降水比例不足50%;随着逐渐向高原腹地推进,降水集中度和集中期逐渐增大,雨季逐渐缩短且推迟,雨季降水占全年降水比例逐渐增大。降水季节分配的空间分布格局与水汽运移方向保持一致,即主要是由西风和印度洋季风的影响所致。基于此,识别出西风的影响区域主要位于高原35°N以北,印度洋季风的影响区域主要位于高原约30°N以南,而高原中部(30°N~35°N)降水受到西风和印度洋季风的共同影响。该结果有助于进一步理解和认识青藏高原水文气候过程空间差异性。     

夏半年青藏高原“湿池”的水汽分布及水汽输送特征

采用1948-2007年共计60年的NCEP/NCAR再分析资料.计算了夏半年(4-9月)青藏高原大气中的可降水量、水汽输送通量和水汽输送通量散度,分析了夏半年青藏高原可降水量的分布和变化特征,青藏高原及其附近的水汽输送.结果表明:在对流层中层的青藏高原上空,夏季是一个明显的大气水汽含量高中心,"湿池"特征非常显著,湿池主要有三个大的可降水量中心,即高原的西南部、东南部和高原南侧.4-9月,高原上的可降水量变化很大,高原的增湿的速度小于减湿的速度.水汽进人高原主要通过三条水汽通道,即西风带水汽输送通道、印度洋-孟别拉湾水汽通道和南海-孟加托湾水汽通道.水汽主要在高原西南侧、喜马拉雅山中段和高原东南侧进入高原.     

亚洲降水中δ18O沿不同水汽输送路径的变化

利用IAEA/WMO全球监测网和青藏高原的监测站,建立了由赤道地区经我国西南水汽通道至长江中下游的南方水汽输送路径、沿西风带自我国西部经华北至日本的北方水汽输送路径以及自南亚穿喜马拉雅山到我国青藏高原的水汽输送路径的取样剖面,比较了三条水汽路径在不同季节降水中啄18O的变化及其与温度、降水量的关系。沿南方水汽路径,低纬度地区取样站降水中平均啄18O的季节差异较小。沿北方水汽路径,郑州以西取样站平均啄18O的季节差均大于郑州以东的取样站。随着经度的增加,降水中平均啄18O的季节差减小。沿高原水汽路径,印度次大陆南部降水中的啄18O相对较高,随着纬度的增加,降水中啄18O逐渐减小。在翻越喜马拉雅山后,由于强烈的洗涤作用,降水中啄18O急剧下降。     

亚洲夏季风北部边缘带变化及中高纬度行星波对其影响

本文使用1961—2016年NCEP1再分析资料和GPCC全球降水分析资料,确定了亚洲夏季风北部边缘带的空间范围,分析了季风边缘带的南北边界位置、降水、面积的相互关系和年代/际变化特征,讨论了造成季风边缘带夏季降水异常的影响因子。主要结论如下：亚洲夏季风北部边缘带平均位置位于青藏高原中部经黄土高原和中国东北地区向亚洲东岸延伸的带状区域上,根据下垫面性质、区域生态环境和气候特征,将季风北边缘带划分为青藏高原区（85°E~105°E）、黄土高原区（105°E~115°E）和中国东北区（115°E~135°E）3段,季风边缘带降水的年际变化与其南边界位置有显著的正相关,青藏高原季风边缘带面积变化与其南界位置显著负相关,黄土高原季风边缘带和东北季风边缘带面积与北边界位置显著正相关,且3段季风边缘带的位置、面积、降水均有明显的年际、年代际变化特征。季风边缘带夏季降水偏少与欧亚中高纬对流层上层自西向东传播的欧亚（EU）遥相关波列密切相关,季风边缘带夏季降水偏少时期,亚洲低纬度地区对流活动偏弱、非洲东岸近赤道地区200 hPa异常辐合可能造成索马里急流和亚洲夏季风强度整体偏弱,200 hPa亚洲急流强度弱且位置偏北,500 hPa中国北方受西风带异常高压控制,东亚夏季风降水主要集中在中国南方地区,季风边缘带夏季降水异常偏少。季风边缘带夏季降水偏多与欧亚中高纬对流层上层沿亚洲急流向东传播的丝绸之路（SRP）波列密切相关,200 hPa、500 hPa环流形势与季风边缘带夏季降水偏少时期基本相反,东亚夏季风降水空间分布呈北多南少特征,季风边缘带夏季降水异常偏多。     

青藏高原夏季上空水汽含量演变特征及其与降水的关系

利用青藏高原(以下简称高原) 近30 年(1979-2008 年) 14 个探空站的温度和湿度观测资料以及83 个地面台站的月平均降水资料,分析了高原夏季上空水汽含量与地面降水的联系以及高原地区的降水转化率问题。结果表明：1) 高原夏季水汽含量在空间分布上表现出随海拔高度增高而减少的特征,其中东北部为最大值,东南部为次大值,而西北部为最小值。夏季降水整体上由东南向西北递减;2) EOF分解表明,高原夏季水汽含量存在两种主要的空间分布型：即全区一致变化型和南北反向变化型,其中以唐古拉山脉北侧为界呈现出的水汽含量南北反向型与降水的第一特征向量场表现出的南北反向型在空间分布上十分相似;3) 在年际变化上,高原夏季水汽含量的南北反向型与降水的南北反向型之间存在较一致的对应关系：即水汽含量出现南多北少时,高原南部降水普遍偏多而北部降水普遍偏少,反之亦然;4) 高原夏季平均降水转化率在3%~38%之间,其空间差异非常明显,高原南部降水转化率明显大于北部地区。     

Distribution of winter-spring snow over the Tibetan Plateau and its relationship with summer precipitation in Yangtze River

The distribution of winter-spring snow cover over the Tibetan Plateau(TP) and its relationship with summer precipitation in the middle and lower reaches of Yangtze River Valley(MLYRV) during 2003–2013 have been investigated with the moderate-resolution imaging spectrometer(MODIS) Terra data(MOD10A2) and precipitation observations. Results show that snow cover percentage(SCP) remains approximately 20% in winter and spring then tails off to below 5% with warmer temperature and snow melt in summer. The lower and highest percentages present a declining tendency while the middle SCP exhibits an opposite variation. The maximum value appears from the middle of October to March and the minimum emerges from July to August. The annual and winter-spring SCPs present a decreasing tendency. Snow cover is mainly situated in the periphery of the plateau and mountainous regions, and less snow in the interior of the plateau, basin and valley areas in view of snow cover frequency(SCF) over the TP. Whatever annual or winter-spring snow cover, they all have remarkable declining tendency during 2003–2013, and annual snow cover presents a decreasing trend in the interior of the TP and increasing trend in the periphery of the TP. The multi-year averaged eight-day SCP is negatively related to mean precipitation in the MLYRV. Spring SCP is negatively related to summer precipitation while winter SCP is positively related to summer precipitation in most parts of the MLYRV. Hence, the influence of winter snow cover on precipitation is much more significant than that in spring on the basis of correlation analysis. The oscillation of SCF from southeast to northwest over the TP corresponds well to the beginning, development and cessation of the rain belt in eastern China.     

Characteristics of abrupt changes of snow cover and seasonal freeze-thaw layer in the Tibetan Plateau and their impacts on summer precipitation in China

In this paper, a variation series of snow cover and seasonal freeze-thaw layer from 1965 to 2004 on the Tibetan Plateau has been established by using the observation data from meteorological stations. The sliding T-test, M-K test and B-G algorithm are used to verify abrupt changes of snow cover and seasonal freeze-thaw layer in the Tibetan plateau. The results show that the snow cover has not undergone an abrupt change, but the seasonal freeze-thaw layer obviously witnessed a rapid degradation in 1987, with the frozen soil depth being reduced by about 15 cm. It is also found that when there is less snow in the plateau region, precipitation in South China and Southwest China increases. But when the frozen soil is deep, precipitation in most of China apparently decreases. Both snow cover and seasonal freeze-thaw layer on the plateau can be used to predict the summer precipitation in China. However, if the impacts of snow cover and seasonal freeze-thaw layer are used at the same time, the predictability of summer precipitation can be significantly improved. The significant correlation zone of snow is located in middle reaches of the Yangtze River covering the Hexi Corridor and northeastern Inner Mongolia, and the seasonal freeze-thaw layer exists in Mt. Nanling, northern Shannxi and northwestern part of North China. The significant correlation zone of simultaneous impacts of snow cover and seasonal freeze-thaw layer is larger than that of either snow cover or seasonal freeze-thaw layer. There are three significant correlation zones extending from north to south: the north zone spreads from Mt. Daxinganling to the Hexi Corridor, crossing northern Mt. Taihang and northern Shannxi; the central zone covers middle and lower reaches of the Yangtze River; and the south zone extends from Mt. Wuyi to Yunnan and Guizhou Plateau through Mt. Nanling.     

Impacts of snow cover and frozen soil in the Tibetan Plateau on summer precipitation in China

This paper presents an analysis of the mechanisms and impacts of snow cover and frozen soil in the Tibetan Plateau on the summer precipitation in China, using RegCM3 version 3.1 model simulations. Comparisons of simulations vs. observations show that RegCM3 well captures these impacts. Results indicate that in a more-snow year with deep frozen soil there will be more precipitation in the Yangtze River Basin and central Northwest China, western Inner Mongolia, and Xinjiang, but less precipitation in Northeast China, North China, South China, and most of Southwest China. In a less-snow year with deep frozen soil, however, there will be more precipitation in Northeast China, North China, and southern South China, but less precipitation in the Yangtze River Basin and in northern South China. Such differences may be attributed to different combination patterns of melting snow and thawing frozen soil on the Plateau, which may change soil moisture as well as cause differences in energy absorption in the phase change processes of snow cover and frozen soil. These factors may produce more surface sensible heat in more-snow years when the frozen soil is deep than when the frozen soil is shallow. The higher surface sensible heat may lead to a stronger updraft over the Plateau, eventually contributing to a stronger South Asia High and West Pacific Subtropical High. Due to different values of the wind fields at 850 hPa, a convergence zone will form over the Yangtze River Basin, which may produce more summer precipitation in the basin area but less precipitation in North China and South China. However, because soil moisture depends on ice content, in less-snow years with deep frozen soil, the soil moisture will be higher. The combination of higher frozen soil moisture with latent heat absorption in the phase change process may generate less surface sensible heat and consequently a weaker updraft motion over the Plateau. As a result, both the South Asia High and the West Pacific Subtropical High will be weaker, hence causing more summer precipitation in northern China but less in southern China.     

我国高空风速的气候学特征

利用全国119个探空站1980~2006年13个等压面和地面附近的月平均风速资料,分析了不同高度年、季节平均风速的气候学特征。结果表明,全国平均200hPa以下风速随高度增加而增加,200hPa~30hPa之间风速随高度增加而降低,30hPa以上风速随高度增加而再呈增加趋势;春、秋季平均风速随高度变化与年平均相似,但冬、夏季的垂直分布差异明显;700hPa及其以上最大风速出现在1月,最小风速在7月或8月;850hPa和地面最大风速发生在4月;对流层风速年较差从下向上增加,在200hPa附近风速年较差最大。我国地面风速在东、西部地区较大,中部地区较小;500hPa年平均风速分布呈西低东高态势,最大中心出现在辽东半岛东部;200hPa年平均风速在江淮地区出现高值中心;500hPa冬季最大风速区在大陆南部,夏季北移并向西扩展;200hPa各季强风速区基本呈东西走向的带状分布,其中春季在江淮地区,夏季移至西北,秋季位于黄淮地区,冬季位置最南。     

近48年青藏高原强降水量的时空分布特征

基于青海和西藏地区48个气象台站近48 a(1961~2008年)的逐日降水资料,分析青藏高原冬、夏半年强降水量的时空演变特征。结果表明:青藏高原强降水量与总降水量的空间分布相似,夏半年为由东南向西北递减,冬半年则由唐古拉山脉东段的高原腹地向四周递减;夏(冬)半年强降水量存在准3、准6 a(7~8 a)的年际振荡以及准9~10 a(15 a)的年代际振荡;夏半年高原北(南)部强降水量以增加(减少)趋势为主,强降水量呈现出微弱的减少趋势,而冬半年高原大多数地区均呈现出明显的增加趋势,在1976年发生突变现象。     

青藏高原强降水日数的时空分布特征

 根据青海和西藏48个气象台站近48 a（1961-2008年）的逐日降水和气温资料,分别以日降水量超过5 mm和25 mm作为冬半年（11月～翌年3月）和夏半年（5～9月）强降水的临界值,分析了青藏高原冬、夏半年强降水日数的时空分布特征。结果表明：（1）高原强降水日数与总降水量的空间分布型非常相似,夏半年均表现为由东南向西北递减,而冬半年则为由高原腹地向四周递减。（2）夏（冬）半年强降水主要集中在7月上旬～8月中旬（11月上旬和3月中下旬）。（3）夏（冬）半年强降水存在准6 a（5～6 a）的年际振荡以及准10～11 a（15 a）的年代际振荡。（4）强降水日数变化趋势的空间差异较大,夏半年高原北（南）部强降水日数普遍以增加（减少）趋势为主,而冬半年除雅鲁藏布江流域呈减少趋势外,高原大多数地区均表现出显著增加趋势。     

近42 年来青藏高原年内降水时空不均匀性特征分析

根据青藏高原1967-2008 年逐日站点降水资料,定义了高原降水集中度(PCD) 和集中期(PCP)。并运用EOF、相关分析等方法分析高原PCD和PCP的时空分布特征、PCD与高原强降水的关系以及PCP前期强影响信号。结果表明：高原大部分地区PCD处于0.4~0.8 之间,PCP则处于36~41 候之间。高原PCD以全区一致型的空间分布为主;而PCP 则以南北反向型分布为主,全区一致型分布次之。整个高原PCD均呈减弱趋势,而PCP均表现为提前特征。除高原南侧个别地区,高原PCD 无论与高原强降水日数还是强降水量均呈显著正相关。同时,高原南北部PCP对应的水汽输送存在显著差异, 高原南部PCP主要受孟加拉湾季风爆发的影响。     

近50a青藏高原东部夏半年强降水事件的气候特征

基于青藏高原东部47个站点1963-2012年的5~9月逐日降水资料,分析了近50 a该区夏半年强降水的时空分布特征和相对强度。结果表明:青藏高原东部夏半年强降水事件在7月出现的频次最多,以持续1 d的单站暴雨为主;强降水量和频次在近50 a呈弱增长趋势,其存在准12 a的年代际震荡,且在1978年之后,强降水量同时存在大致准3 a的演变周期,在各自然分带强降水量和频次的变化趋势存在差异;夏半年强降水量和频次呈现出自东南向西北阶梯性递减的分布特征;青藏高原东部夏半年强降水的相对强度与强降水量呈反向特征,其中以柴达木地区相对强度为最大,藏东川西区为最少;各自然分带的强降水量和频次与夏半年降水量有很好的相关关系,而强降水的相对强度与夏半年降水量表现出不同的正负相关性。     

1979-2012年夏季黄土高原空中云水资源时空分布

利用欧洲数值预报中心(ECMWF)发布的新一代全球全分辨率ERA-Interim再分析数据,采用经验正交函数(EOF)、小波分析、回归分析等方法,分析1979-2012年夏季黄土高原空中云水资源分布特征。结果表明:(1)夏季黄土高原空中云水资源远大于该地区实际年降水量,具有较大空中云水资源开发利用潜势;(2)空间上云水资源表现为两种模态--西北部、东南部反位相振荡(EOF1)以及中部云水资源偏多西北、东南两端偏少(EOF2),且具有显著年际变化周期;(3)黄土高原的空中云水资源主要来自东海,当水汽输送反气旋环流中心偏南(北)时,影响EOF1(EOF2)空间模态;(4)云水、云冰量峰值分别出现在700 hPa、400 hPa左右,当700 hPa存在水汽辐合及上升运动时有利于黄土高原空中云水资源开发。     

西北地区东部夏季水汽输送特征及其与降水的关系

采用ERA Interim 再分析资料和160 站逐月站点降水资料,运用经验正交函数（EOF）分析、合成分析等方法揭示了西北东部3 个分区的水汽输送的区域气候特征、与降水EOF气候模态相对应的整层水汽输送特征以及降水偏多（少）年的水汽输送异常特征。结果表明：西北地区东部夏季经向水汽输送的大值区处于900 hPa~800 hPa 高度上;纬向水汽输送大值位于700 hPa~500 hPa 高度上。对西北东部降水做EOF 分析,第一模态为全区一致型,与降水相对应的西风影响区主要盛行西风水汽输送,季风边缘区的南部盛行西南风水汽输送;第二模态为东南-西北型,东风、东北风水汽输送流入西北东部地区;第三模态为东北－西南型,西风和西北风水汽输送将水汽带到西风影响区内。     

伊朗高原和北非感热异常对夏季塔里木盆地降水的影响

基于1971—2019年日本气象厅提供的JRA-55地表感热、大气环流再分析数据和国家气象信息中心提供的我国陆面逐月格点降水数据,研究了夏季伊朗高原和北非感热异常对同期塔里木盆地降水的可能影响。结果表明：（1） 夏季伊朗高原感热和北非感热均与塔里木盆地夏季降水密切联系,将2个区域加热共同考虑,其与塔里木盆地降水的关系要比仅考虑单一区域加热更为紧密。（2） 当伊朗高原感热整体偏强,且北非感热呈北弱南强异常分布时,对应中亚副热带西风急流相对偏南,中亚和蒙古高原上空分别为异常气旋和反气旋控制,塔里木盆地上空南风加强;热带印度洋水汽在阿拉伯海与中亚的异常环流配合下北上进入塔里木盆地;以上条件共同导致了同期塔里木盆地降水的增多,反之亦然。（3） 北非和伊朗高原加热均可单独影响塔里木盆地夏季降水,其中伊朗高原感热对大尺度环流和水汽输送的影响均显著,因此其与降水的关系也更加密切。北非感热加热的影响主要体现在大尺度环流方面,对水汽输送的影响和伊朗高原存在差异,主要体现在印度半岛上空的异常反气旋位置偏南,导致阿拉伯海水汽无法进入塔里木盆地。