青藏高原冬季积雪异常对东,南亚夏季风影响的初步数值模拟研究

利用一个耦合了简化的简单生物圈模式的大气环流谱模式（ＳＳｉＢ－ＧＣＭ），初步探讨了青藏高原冬季积雪异常对东、南亚夏季季风环流和降水的影响及其机理。结果表明，高原地区积雪增加将使随后地夏季东、南来季风明显减弱，主要表现为东、南亚季风区降水减少，索马里急流、印度季风的印度西南气流弱弱。另外，还提出欧亚大陆雪盖与整个高原雪盖和高原东部雪盖对东、南亚夏季风影响的敏感问题。与欧亚大陆雪盖相比，高原雪盖是影响     

Simulation of Indian summer monsoon circulation and rainfall using RegCM3

Summary The Regional Climate Model RegCM3 has been used to examine its suitability in simulating the Indian summer monsoon circulation features and associated rainfall. The model is integrated at 55 km horizontal resolution over a South Asia domain for the period April–September of the years 1993 to 1996. The characteristics of wind at 850 hPa and 200 hPa, temperature at 500 hPa, surface pressure and rainfall simulated by the model over the Indian region are examined for two convective schemes (a Kuo-type and a mass flux scheme). The monsoon circulation features simulated by RegCM3 are compared with those of the NCEP/NCAR reanalysis and the simulated rainfall is validated against observations from the Global Precipitation Climatology Centre (GPCC) and the India Meteorological Department (IMD). Validation of the wind and temperature fields shows that the use of the Grell convection scheme yields results close to the NCEP/NCAR reanalysis. Similarly, the Indian Summer Monsoon Rainfall (ISMR) simulated by the model with the Grell convection scheme is close to the corresponding observed values. In order to test the model response to land surface changes such as the Tibetan snow depth, a sensitivity study has also been conducted. For such sensitivity experiment, NIMBUS-7 SMMR snow depth data in spring are used as initial conditions in the RegCM3. Preliminary results indicate that RegCM3 is very much sensitive to Tibetan snow. The model simulated Indian summer monsoon circulation becomes weaker and the associated rainfall is reduced by about 30% with the introduction of 10 cm of snow over the Tibetan region in the month of April.     

On the causes of glacial inception at 116 kaBP

To explore processes involved in glacial inception at 116 kaBP, the response of an atmospheric general circulation model (AGCM) to changes in lower boundary conditions is investigated. Two 116 kaBP experiments are conducted to examine the importance of sea surface conditions (sea surface temperature and sea ice distribution): one with the present-day sea surface conditions, and the other with 116 kaBP sea surface conditions. These two different sea surface conditions are obtained from simulations using an earth system climate model of intermediate complexity. Perennial snow cover occurred over the Canadian Archipelago under 116 kaBP orbital and CO₂ forcing with present-day "warm" sea surface conditions, and further expanded over northeastern Canada when 116 kaBP "cool" sea surface conditions were applied. The net positive accumulation in northeastern Canada, with little in Alaska, is in good agreement with geological records. Two additional 116 kaBP experiments are conducted to examine the combined importance of sea surface conditions and land surface conditions (vegetation): one with the present-day sea surface and modified land surface conditions, and the other with 116 kaBP sea surface and modified land surface conditions. Modifying vegetation, based on cooling during summer induced by 116 kaBP sea surface conditions, leads to much larger areas of perennial snow cover. Only when 116 kaBP sea surface conditions are applied, is a realistic global net snow accumulation rate obtained. Contrary to the earlier ice age hypothesis, our results suggest that the capturing of glacial inception at 116 kaBP requires the use of "cooler" sea surface conditions than those of the present climate. Also, the large impact of vegetation change on climate suggests that the inclusion of the vegetation feedback is important for model validation, at least, in this particular period of Earth history.     

陆面模式CLSM的设计及性能检验II.模式检验

利用BOREAS,HEIFE,ARME,GAME-TIPEX等大量的陆面外场观测资料,针对不同类型的陆面过程,对所发展的陆面模式CLSM的性能进行检验.模拟结果与观测资料的对比分析表明:一方面,CLSM能够对积雪变化、干旱/半干旱地区的水热交换等特殊的陆面过程进行合理的描述;另一方面,CLSM对热带雨林地区的植被-大气相互作用、高原地气交换过程同样具有很强的模拟能力.CLSM解决了陆面模式对上述特殊下垫面描述能力有限的实际问题,保证了对特殊下垫面进行合理描述的同时,又保证了对其他不同陆面状况的模拟能力.CLSM改善了陆面模式对全球范围内不同下垫面条件下的陆面过程及地-气交换过程的模拟能力.     

青藏高原MODIS地表反照率和GLASS地表反照率的对比分析

对比分析了青藏高原MODIS地表反照率产品和GLASS地表反照率产品的空间分布连续性、高质量反演结果的比例,应用青藏高原CAMP/Tibet试验期间的高精度观测数据评估了两种产品的精度,通过人工目视解译MODIS地表反射率图像并结合MODIS积雪产品分析了影响两种产品精度的原因,结果表明：1）GLASS地表反照率产品具有比MODIS地表反照率产品更好的空间分布连续性和更高的反演质量;2）绝大多数时段内两种产品都能与地面观测结果保持较好的一致性,能准确地反映地表反照率的异常变化过程;3）局地积雪是影响两种产品精度的重要因素之一;4）积雪条件下,GLASS地表反照率反演算法比MODIS地表反照率反演算法更具优势。研究结果有助于促进人们对地表反照率卫星遥感反演产品的认识,改进青藏高原地表反照率卫星遥感反演算法,提高青藏高原地表反照率卫星遥感反演结果的精度、反演质量和空间分布连续性。     

藏北高原地面加热场的变化及其对气候的影响

利用 1994— 1996年在藏北高原五道梁所观测得到的地面能量收支资料 ,结合同期的大气环流进行了分析研究。结果表明 :高原北部地面加热场强度的变化与高原西部相似 ,而与高原主体东半部的变化相反 ;冬季前期 11月的地面积雪过程对决定整个冬季地面加热场的性质具有重要的意义 ;高原冬季地面热状况的异常 ,引起夏季加热场的异常 ,这可能是造成大气环流异常的原因之一 ,从而影响我国的气候环境 ,因此对高原地面加热场的监测可以为短期气候预测提供依据。     

陆面过程对气候影响的数值模拟:SSiB与IAP/LASG L9R15 AGCM耦合及其模式性能

利用陆面过程模式 SSi B与 IAP/LASG发展的 L9R1 5AGCM的耦合 1 0 a积分试验 ,研究了全球尺度大气与地表的水分和能量交换以及陆地与大气环流和气候的相互作用。模拟表明 :SSi B模式可模拟出陆地上较为真实的表面通量及其日变化 ,较好地定量描述土壤 -植被 -大气连续体系 ( SPAC)中能量和水分的传输过程。因此 ,将其引入气候模式中能够模拟出比 CTL- AGCM更合理的气候平均状态、水汽分布以及水汽输送的气候特征 ,特别是亚洲夏季风水汽输送独特的地域性 ,再现了大气环流 ,尤其是陆面气候的基本特征。并指出 ,陆面过程参数化的引进及其陆面状况的变化显著地改善了全球陆地上的水分平衡状况。利用改进的再循环降水模式 ,进一步研究了陆面过程参数化明显改进降水模拟的物理机制。指出全球陆地 ,特别是盛夏北半球干旱、半干旱地区的再循环降水率明显减小 ,与陆面上表面潜热通量的显著减小区一致 ,从而克服了许多未耦合陆面过程的 AGCMs因对地表水过程非常简单地参数化导致的普遍存在着整个陆地降水偏高 ,改善了全球陆地上的水分平衡状况。因此 ,在充分耦合的陆气环流模式中模拟的降水分布与实况接近。     

Land–sea heating contrast in an idealized Asian summer monsoon

Mechanisms determining the tropospheric temperature gradient that is related to the intensity of the Asian summer monsoon are examined in an intermediate atmospheric model coupled with a mixed-layer ocean and a simple land surface model with an idealized Afro–Eurasian continent and no physical topography. These include processes involving in the influence of the Eurasian continent, thermal effects of the Tibetan Plateau and effects of sea surface temperature. The mechanical effect on the large-scale flow induced by the Plateau is not included in this study. The idealized land–sea geometry without topography induces a positive meridional tropospheric temperature gradient thus a weak Asian summer monsoon circulation. Higher prescribed heating and weaker surface albedo over Eurasia and the Tibetan Plateau, which mimic effects of different land surface processes and the thermal effect of the uplift of the Tibetan Plateau, strengthens the meridional temperature gradient, and so as cold tropical SST anomalies. The strengthened meridional temperature gradient enhances the Asian summer monsoon circulation and favors the strong convection. The corresponding monsoon rainbelt extends northward and northeastward and creates variations of the monsoon rainfall anomalies in different subregions. The surface albedo over the Tibetan Plateau has a relatively weak inverse relation with the intensity of the Asian summer monsoon. The longitudinal gradient of ENSO-like SST anomalies induces a more complicated pattern of the tropospheric temperature anomalies. First, the positive (negative) longitudinal gradient induced by the El Niño (La Niña)-like SST anomalies weakens (strengthens) the Walker circulation and the circulation between South Asia and northern Africa and therefore the intensity of the Asian summer monsoon, while the corresponding monsoon rainbelt extends northward (southward). The El Niño (La Niña)-like SST anomalies also induces colder (warmer) tropospheric temperature over Eurasia and warmer (colder) tropospheric temperature over the Indian Ocean. The associated negative (positive) meridional gradient of the tropospheric temperature anomalies is consistent with the existence of the weak (strong) Asian summer monsoon.     

青藏高原冬春季积雪异常对中国春夏季降水的影响

利用1956年12月～1998年12月共42a，青藏高原及其附近地区78个积雪观测站的雪深和我国160站月降水的距平资料，分析了其气候特征，并用SVD方法分析了冬春季积雪异常与春夏季我国降水异常的关系。用区域气候模式RegCM2模拟了青藏高原积雪异常的气候效应并检验了诊断分析的结果。分析表明，雪深异常，尤其是冬季雪深异常是影响中国降水的一个因子。研究证明，高原冬季雪深异常对后期中国区域降水的影响比春季雪深异常的影响更为重要。数值模拟的结果表明，高原雪深和雪盖的正异常推迟了东亚夏季风的爆发日期，减弱了季风强度，造成华南和华北降水减少，而长江和淮河流域降水增加。冬季雪深异常比冬季雪盖异常和春季雪深异常对降水的影响更为显著。机理分析指出，高原及其邻近地区的积雪异常首先通过融雪改变土壤湿度和地表温度，从而改变了地面到大气的热量、水汽和辐射通量。由此所引起的大气环流变化又反过来影响下垫面的特征和通量输送。在湿土壤和大气之间，这样一种长时间的相互作用是造成后期气候变化的关键过程。与干土壤和大气的相互作用过程有本质差别。     

Interannual variability in the onset of the summer monsoon over the Eastern Bay of Bengal

Summary Climatological characteristics associated with summer monsoon onset over the eastern Bay of Bengal (BOB) are examined in terms of the westerly-easterly boundary surface (WEB). The vertical tilt of the WEB depends on the horizontal meridional temperature gradient (MTG) near the WEB, under the constraint of the thermal wind balance. The switch in the WEB tilt firstly occurs between 90 and 100°E during the first pentad of May. At this time the 850 hPa ridgeline splits over the BOB and heavy rainfall commences over the eastern BOB, indicating the onset of the BOB summer monsoon (BOBSM). The area-averaged MTG (200–500 hPa) is proposed as an index to define the BOBSM onset. A comparison of the onset determined by the MTG, 850 hPa zonal wind, and outgoing longwave radiation (OLR) shows that the MTG index is the most effective in characterizing the interannual variability of the BOBSM onset. Strong precursor signals are found prior to an anomalous BOBSM onset. Composite results show that early (late) BOBSM onset follows excessive (deficient) rainfall over the western Pacific and anomalous lower tropospheric cyclonic circulation which extends zonally from the northern Indian Ocean into the western Pacific, and strong (weak) equatorial westerly anomalies in the preceding winter and spring. Prior to an early (late) BOBSM onset, significant positive (negative) thickness anomalies exist around the Tibetan Plateau, accompanied by anomalous upper tropospheric anticyclonic (cyclonic) circulation. The interannual variations of the BOBSM onset are significantly correlated with anomalous sea surface temperature related to ENSO. These occurs through changes in the Walker circulation and local Hadley circulation, leading to middle and upper tropospheric temperature anomalies over the Asian sector. The strong precursor signals around the Tibetan Plateau may be partly caused by local snow cover anomalies, and an early (late) BOBSM onset is preceded by less (more) snow accumulation over the Tibetan Plateau during the preceding winter.     

青藏高原冬春季雪盖对东亚夏季大气环流影响的研究

通过分析青藏高原积雪的基本特征，指出高原冬春季雪盖在东亚夏季气候形成与异常中的重要作用，同时分别总结了高原冬春季积雪对东亚夏季大气环流影响的诊断研究和数值试验进展，提出了高原冬春季雪盖对气候影响的可能机制。     

青藏高原积雪对亚洲夏季风影响的诊断及数值研究

通过对青藏高原多、少雪年的合成分析及数值试验，研究了青藏高原积雪对亚洲 夏季风和我国东部气候异常的影响。结果表明:青藏高原积雪造成亚洲大气环流较大的年际变化。高原积雪改变了高原陆面春、夏季的热状况，使亚洲夏季风爆发推迟20天左右。高原积雪通过以下物理过程影响亚洲夏季风和我国东部气候:高原积雪多(少)→高原春、夏季的感热弱(强)→感热加热引起的上升运动弱(强)，高原强(弱)环境风场→不利(有利)于高原感热通量向上输送→高原上空对流层加热弱(强)→高原对流层温度低(高)→高原南侧温度对比弱(强)→造成亚洲夏季风弱(强)→我国长江流域易涝(旱)。     

亚洲季风模拟试验中青藏高原积雪强迫问题的讨论

青藏高原积雪对亚洲季风和东亚，南亚时旱涝灾害的影响，百余年来一直为中外气候学家所瞩目。近几年来用大气环流模式进行数值试验已成为该研究领域的主要手段，但是由于假设的积雪强迫不同，模拟结果很不一致。作者概据美国宇航局ＳＭＭＲ微波逐候积雪深度１０年的观测结果，提出了一个真实的青藏高原积雪强迫试验方案。     

Impact of absorbing aerosol deposition on snow albedo reduction over the southern Tibetan plateau based on satellite observations

We investigate the snow albedo variation in spring over the southern Tibetan Plateau induced by the deposition of light-absorbing aerosols using remote sensing data from moderate resolution imaging spectroradiometer (MODIS) aboard Terra satellite during 2001–2012. We have selected pixels with 100 % snow cover for the entire period in March and April to avoid albedo contamination by other types of land surfaces. A model simulation using GEOS-Chem shows that aerosol optical depth (AOD) is a good indicator for black carbon and dust deposition on snow over the southern Tibetan Plateau. The monthly means of satellite-retrieved land surface temperature (LST) and AOD over 100 % snow-covered pixels during the 12 years are used in multiple linear regression analysis to derive the empirical relationship between snow albedo and these variables. Along with the LST effect, AOD is shown to be an important factor contributing to snow albedo reduction. We illustrate through statistical analysis that a 1-K increase in LST and a 0.1 increase in AOD indicate decreases in snow albedo by 0.75 and 2.1 % in the southern Tibetan Plateau, corresponding to local shortwave radiative forcing of 1.5 and 4.2 W m⁻², respectively.

     

青藏高原地面拖曳效应对春季大气环流影响的数值研究

用一个在ＮＣＡＲ的气候模式ＣＣＭ１（Ｒ１５Ｌ１２）基础上改进和发展起来的ＣＣＭ１（Ｒ１５Ｌ７）－ＬＮＷＰ长期数值预报模式，以国家气象中心１９９２年４月１日客观分析资料和ＮＣＡＲ的固定个例（１９７５年１月１６日）为初始场，进行长期数值预报试验，研究了青藏高原地面拖曳效应对春季大气环流的影响．结果表明，当月平均的青藏高原地面拖曳系数与全球其它陆面月平均拖曳系数之差由０．０００８增大至０．００４时，预报效果得到了明显改进．青藏高原至巴尔喀什湖之间的（３００ｈＰａ和５００ｈＰａ）温度差可增大３℃，从孟加拉湾一带至青藏高原的温度差可减小１．５℃，使得在青藏高原及其北侧高空西风增强（增强中心在４０°Ｎ的２５０ｈＰａ层）．在青藏高原南侧的２５°Ｎ的２５０ｈＰａ出现西风的减弱中心，有利于晚春季节东亚南支西风急流的北移．青藏高原的地面拖曳效应有利于在春季加大其迎风坡和背风坡效应，加强北半球经向三圈环流及全球经向环流．     

NUMERICAL EXPERIMENTS ON THE EFFECT OF INDIAN SUBCONTINENT VEGETATION COVER ON SUMMER MONSOON

In the present paper,a global two-dimensional climate model developed by Chinese Academy of Meteorological Sciences (CAMS) with inclusion of a simple parameterization of land surface processes developed recently by Zhang and Li is used to investigate the effect of vegetation cover of the Indian subcontinent on summer monsoon circulation and its rainfall.First of all,we carried out a control experiment in which the distribution of vegetation species was based on obser-vations.Then,with removal of the vegetation cover on the subcontinent,a sensitivity experiment was performed.For both of the experiments,the integrations were made from the beginning of April to the end of July.The results show that,after the vegetation was removed from the subcontinent,the monthly mean updraft and the monsoon circulation to the south of the Plateau was weakened in June and July.Especially,the change was more manifest in June.Also,in the south side of the Plateau,the easterly component was strengthened in the lower troposphere,the monsoon rainfall was reduced over the central India.Moreover,the rainfall was obviously reduced over the southern India from the end of June to the beginning of July.     

Interannual Variability of Snow Depth over the Tibetan Plateau and Its Associated Atmospheric Circulation Anomalies

The interannual variability of wintertime snow depth over the Tibetan Plateau(TP) and related atmospheric circulation anomalies were investigated based on observed snow depth measurements and NCEP/NCAR reanalysis data.Empirical orthogonal function(EOF) analysis was applied to identify the spatio-temporal variability of wintertime TP snow depth.Snow depth anomalies were dominated by a monopole pattern over the TP and a dipole structure with opposite anomalies over the southeastern and northwestern TP.The atmospheric circulation conditions responsible for the interannual variability of TP snow depth were examined via regression analyses against the principal component of the most dominant EOF mode.In the upper troposphere,negative zonal wind anomalies over the TP with extensively positive anomalies to the south indicated that the southwestward shift of the westerly jet may favor the development of surface cyclones over the TP.An anomalous cyclone centered over the southeastern TP was associated with the anomalous westerly jet,which is conducive to heavier snowfall and results in positive snow depth anomalies.An anomalous cyclone was observed at 500 hPa over the TP,with an anomalous anticyclone immediately to the north,suggesting that the TP is frequently affected by surface cyclones.Regression analyses revealed that significant negative thickness anomalies exist around the TP from March to May,with a meridional dipole anomaly in March.The persistent negative anomalies due to more winter TP snow are not conducive to earlier reversal of the meridional temperature gradient,leading to a possible delay in the onset of the Asian summer monsoon.     

Assessing the Impacts of Eurasian Snow Conditions on Climate Predictability with a Global Climate Model

On the basis of two ensemble experiments conducted by a general atmospheric circulation model (Institute of Atmospheric Physics nine-level atmospheric general circulation model coupled with land surface model, hereinafter referred to as IAP9L_CoLM), the impacts of realistic Eurasian snow conditions on summer climate predictability were investigated. The predictive skill of sea level pressures (SLP) and middle and upper tropospheric geopotential heights at mid-high latitudes of Eurasia was enhanced when improved Eurasian snow conditions were introduced into the model. Furthermore, the model skill in reproducing the interannual variation and spatial distribution of the surface air temperature (SAT) anomalies over China was improved by applying realistic (prescribed) Eurasian snow conditions. The predictive skill of the summer precipitation in China was low; however, when realistic snow conditions were employed, the predictability increased, illustrating the effectiveness of the application of realistic Eurasian snow conditions. Overall, the results of the present study suggested that Eurasian snow conditions have a significant effect on dynamical seasonal prediction in China. When Eurasian snow conditions in the global climate model (GCM) can be more realistically represented, the predictability of summer climate over China increases.     

青藏高原积雪与亚洲季风环流年代际变化的关系

利用高原测站的月平均雪深资料和NCEP／NCAR再分析资料，分析了20世纪70年代末以来，青藏高原积雪的显著增多与亚洲季风环流转变的联系。研究表明，高原南侧冬春季西风的增强及西风扰动的活跃是造成青藏高原冬春积雪显著增多的主要原因，高原积雪的增多与亚洲夏季风的减弱均是亚洲季风环流转变的结果；20世纪70年代末以来，夏季华东降水的增多、华南降水的减少及华北的干旱化与青藏高原冬春积雪增多及东亚夏季风的减弱是基本同步的，高原冬春积雪与华东夏季降水的正相关、与华北及华南夏季降水的负相关主要是建立在年代际时间尺度上，因此，高原积雪与我国夏季降水关系的研究应以亚洲季风环流的年代际变化为背景。     

青藏高原冬季雪盖增大对夏季东亚大气环流影响的数值试验

利用二维非线性能量（Ｅ－ε）闭合的边界层数值模式,研究了中纬度地区海岸日间局地海风环流的特征,并对不同海风特性条件下的ＴＩＢＬ高度廓线用幂指数关系进行了数学拟合。对大量的数值试验结果分析表明,ＴＩＢＬ廓线幂指数（α）的取值与海风特性有关,其大小在０．４－１．１之间,α＝０．５只在海风充分发展时的陆上适用,文中还给出了不同海风发展程度（阶段）下的α取值。