增暖背景下东亚夏季风区降水演变的空间模态及其形成机制

基于CMIP5中全新世(Mid-Holocene,6 ka BP)试验及RCP8.5试验的对比,本文研究了不同增暖情景下东亚夏季风区降水演变的空间模态及其成因.结果表明,两种增暖情景下东亚夏季风区降水演变的空间模态存在显著差异.轨道辐射主导的中全新世暖期期间,东亚夏季风区降水演变的空间模态为经向"三极子"结构;而大气C...     

Impact of global SST on decadal shift of East Asian summer climate

East Asia experienced a significant interdecadal climate shift around the late 1970s, with more floods in the valley of the Yangtze River of central-eastern China and more severe drought in North China since then. Whether global SST variations have played a role in this shift is unclear. In the present study, this issue is investigated by ensemble experiments of an atmospheric general circulation model (AGCM), the GFDL AM2, since one validation reveals that the model simulates the observed East Asian Summer Monsoon (EASM) well. The results suggest that decadal global SST variations may have played a substantial role in this climate shift. Further examination of the associated atmospheric circulation shows that these results are physically reasonable.     

东亚上空急流位置经向变化及其与东亚夏季气候异常的联系

利用1979—2010年逐月CMAP降水资料和NCEP/NCAR再分析资料集,通过定义夏季东亚急流位置指数,采用统计和动力诊断方法研究了东亚上空急流位置经向变化及其与东亚夏季气候的联系。所定义的东亚急流位置指数较好地反映了东亚上空急流位置的经向移动。结果表明:东亚上空急流经向位置的移动存在显著的年际变化,其主要周期为2~3 a和8 a。当夏季东亚急流位置偏北(南)时,从低纬到高纬,东亚地区降水异常主要呈现出偏多—偏少—偏多(偏少—偏多—偏少)的经向分布;相应地,气温则在副热带西太平洋地区偏低(高),我国华东、华北及日本地区气温偏高(低),西伯利亚东部较高纬地区气温偏低(高)。东亚上空急流经向位置异常年,异常环流随高度呈略有西倾的准正压结构。东亚上空急流经向位置的偏北(南)与由西太平洋—南海热带地区非绝热加热相关的经圈环流异常有关,亦与中纬度波扰能量东传有关,并由此可部分解释我国长江中下游至日本地区的气温异常偏高(低)。     

WRF模式对中国夏季降水的动力降尺度模拟研究

采用NCEP的FNL再分析资料驱动WRF模式,对中国10 a(2000—2009年)夏季降水进行双重动力降尺度(双重嵌套)模拟,将子、母区域模拟结果和观测进行对比,以检验双重动力降尺度对中国夏季降水模拟的"增值"能力。结果表明:单重动力降尺度(单重嵌套)方法能较好模拟出中国10 a夏季平均降水的空间分布,对季风雨带"北跳"特征模拟较好,但模拟降水具有系统性正偏差。在母区域的强迫下,双重动力降尺度模拟的降水分布与单重动力降尺度相比,没有发生根本性变化。但由于子区域的分辨率要高于母区域,双重动力降尺度比单重动力降尺度能提供更多有价值的降水细节。双重动力降尺度的这种"增值"能力存在地域依赖性,在华南地区和江淮地区,双重动力降尺度模拟出的降水分布、量值和逐日演变都要好于单重动力降尺度。但在华北地区,双重动力降尺度没有表现出明显的"增值"。     

Dynamical downscaling of ERA-40 in complex terrain using the WRF regional climate model

Results from a first-time employment of the WRF regional climate model to climatological simulations in Europe are presented. The ERA-40 reanalysis (resolution 1°) has been downscaled to a horizontal resolution of 30 and 10?km for the period of 1961?C1990. This model setup includes the whole North Atlantic in the 30?km domain and spectral nudging is used to keep the large scales consistent with the driving ERA-40 reanalysis. The model results are compared against an extensive observational network of surface variables in complex terrain in Norway. The comparison shows that the WRF model is able to add significant detail to the representation of precipitation and 2-m temperature of the ERA-40 reanalysis. Especially the geographical distribution, wet day frequency and extreme values of precipitation are highly improved due to the better representation of the orography. Refining the resolution from 30 to 10?km further increases the skill of the model, especially in case of precipitation. Our results indicate that the use of 10-km resolution is advantageous for producing regional future climate projections. Use of a large domain and spectral nudging seems to be useful in reproducing the extreme precipitation events due to the better resolved synoptic scale features over the North Atlantic, and also helps to reduce the large regional temperature biases over Norway. This study presents a high-resolution, high-quality climatological data set useful for reference climate impact studies.     

Impact of the Atlantic multidecadal variability on East Asian summer climate in idealized simulations

本文利用基于地球系统模式CESM1开展的北大西洋多年代际振荡理想化数值试验,研究了北大西洋多年代际振荡对东亚夏季气候的影响.结果显示,北大西洋多年代际振荡可以通过中纬度罗斯贝波以及热带开尔文波的传播两种途■影响东亚夏季气候.当北大西洋多年代际振荡处于正位相时,一方面,偏暖的北大西洋通过激发一条从北大西洋向下游传播的中纬度大气罗斯贝波列导致东亚陆地气压降低而西北太平洋气压升高,使得东亚-西北太平洋之间的海陆气压差增强;另一方面,偏暖的北大西洋激发赤道开尔文波东传,激发西北太平洋对流层低层出现反气旋式环流异常.通过以上两种途■,正位相的北大西洋多年代际振荡最终导致东亚夏季风增强,东亚地区夏季出现北湿南干和偏暖的气候.     

Performance assessment of three convective parameterization schemes in WRF for downscaling summer rainfall over South Africa

Austral summer rainfall over the period 1991/1992 to 2010/2011 was dynamically downscaled by the weather research and forecasting (WRF) model at 9 km resolution for South Africa. Lateral boundary conditions for WRF were provided from the European Centre for medium-range weather (ECMWF) reanalysis (ERA) interim data. The model biases for the rainfall were evaluated over the South Africa as a whole and its nine provinces separately by employing three different convective parameterization schemes, namely the (1) Kain–Fritsch (KF), (2) Betts–Miller–Janjic (BMJ) and (3) Grell–Devenyi ensemble (GDE) schemes. All three schemes have generated positive rainfall biases over South Africa, with the KF scheme producing the largest biases and mean absolute errors. Only the BMJ scheme could reproduce the intensity of rainfall anomalies, and also exhibited the highest correlation with observed interannual summer rainfall variability. In the KF scheme, a significantly high amount of moisture was transported from the tropics into South Africa. The vertical thermodynamic profiles show that the KF scheme has caused low level moisture convergence, due to the highly unstable atmosphere, and hence contributed to the widespread positive biases of rainfall. The negative bias in moisture, along with a stable atmosphere and negative biases of vertical velocity simulated by the GDE scheme resulted in negative rainfall biases, especially over the Limpopo Province. In terms of rain rate, the KF scheme generated the lowest number of low rain rates and the maximum number of moderate to high rain rates associated with more convective unstable environment. KF and GDE schemes overestimated the convective rain and underestimated the stratiform rain. However, the simulated convective and stratiform rain with BMJ scheme is in more agreement with the observations. This study also documents the performance of regional model in downscaling the large scale climate mode such as El Niño Southern Oscillation (ENSO) and subtropical dipole modes. The correlations between the simulated area averaged rainfalls over South Africa and Nino3.4 index were ?0.66, ?0.69 and ?0.49 with KF, BMJ and GDE scheme respectively as compared to the observed correlation of ?0.57. The model could reproduce the observed ENSO-South Africa rainfall relationship and could successfully simulate three wet (dry) years that are associated with La Niña (El Niño) and the BMJ scheme is closest to the observed variability. Also, the model showed good skill in simulating the excess rainfall over South Africa that is associated with positive subtropical Indian Ocean Dipole for the DJF season 2005/2006.     

Equilibrium climate response of the East Asian summer monsoon to forcing of anthropogenic aerosol species

We used an online aerosol–climate model to study the equilibrium climate response of the East Asian summer monsoon (EASM) to increases in anthropogenic emissions of sulfate, organic carbon, and black carbon aerosols from 1850 to 2000. Our results show that each of these aerosol species has a different effect on the EASM as a result of changes in the local sea–land thermal contrast and atmospheric circulation. The increased emission of sulfate aerosol leads to a decrease in the thermal contrast between the land and ocean, a southward shift of the East Asian subtropical jet, and significant northerly wind anomalies at 850 hPa over eastern China and the ambient oceans, markedly dampening the EASM. An increase in organic carbon aerosol results in pronounced surface cooling and the formation of an anomalous anticyclone over the oceans north of 30°N. These effects cause a slight increase in the sea–land thermal contrast and southerly flow anomalies to the west of the anticyclonic center, strengthening the northern EASM. An increase in organic carbon emission decreases the sea–land thermal contrast over southern China, which weakens the southern EASM. The response of the summer 850-hPa winds and rainfall over the East Asian monsoon region to an increase in black carbon emission is generally consistent with the response to an increase in organic carbon. The increase in black carbon emission leads to a strengthening of the northern EASM north of 35°N and a slight weakening of the southern EASM south of 35°N. The simulated response of the EASM to the increase in black carbon emission is unchanged when the emission of black carbon is scaled up by five times its year 2000 levels, although the intensities of the response is enhanced. The increase in sulfate emission primarily weakens the EASM, whereas the increases in black carbon and organic carbon emissions mitigate weakening of the northern EASM.     

Dynamic downscaling of summer precipitation prediction over China in 1998 using WRF and CCSM4

To study the prediction of the anomalous precipitation and general circulation for the summer(June–July–August) of1998, the Community Climate System Model Version 4.0(CCSM4.0) integrations were used to drive version 3.2 of the Weather Research and Forecasting(WRF3.2) regional climate model to produce hindcasts at 60 km resolution. The results showed that the WRF model produced improved summer precipitation simulations. The systematic errors in the east of the Tibetan Plateau were removed, while in North China and Northeast China the systematic errors still existed. The improvements in summer precipitation interannual increment prediction also had regional characteristics. There was a marked improvement over the south of the Yangtze River basin and South China, but no obvious improvement over North China and Northeast China. Further analysis showed that the improvement was present not only for the seasonal mean precipitation, but also on a sub-seasonal timescale. The two occurrences of the Mei-yu rainfall agreed better with the observations in the WRF model,but were not resolved in CCSM. These improvements resulted from both the higher resolution and better topography of the WRF model.     

Sensitivity of the WRF model simulation of the East Asian summer monsoon in 1993 to shortwave radiation schemes and ozone absorption

Sensitivity of the Weather Research and Forecasting (WRF) model simulation of the East Asian summer monsoon (EASM) in 1993 to solar radiation parameterizations and ozone absorption was investigated. Three numerical experiments were conducted using the National Centers for Environmental Prediction/Department of Energy (NCEP/DOE) and the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis data as time-varying surface and lateral boundary forcings, respectively: (a) a control run (“CTL”) with the Dudhia radiation scheme and the model top placed at 50 hPa, (b) the “SWG” experiment which is the same as the CTL except the Goddard radiation scheme, and (3) the “SWT” run which is the same as the SWG but the model top was raised to 5 hPa and the vertical levels increased from 31 to 35. The use of the Goddard scheme results in considerable improvement in reproducing the model’s thermal structures, such as zonal mean air temperature, its latitudinal gradient and vertically integrated temperature. This leads to better agreements in the simulation of the upper tropospheric zonal winds through the thermal wind relationship which, in turn, rectifies the lowlevel circulations through dynamical coupling between the upper and lower troposphere. The Taylor diagram analysis quantitatively indicates that the SWT and the SWG are discernable from each other with slightly improved simulations in the SWT. This suggests a nontrivial role of ozone absorption and accompanied stratospheric heating in EASM simulation.     

IPCC AR4气候模式对东亚夏季风年代际变化的模拟性能评估

文中使用多种观测资料和分类的方法评估了IPCC AR4(政府间气候变化委员会第4次评估报告)气候模式(亦称Coupled Model Intercomparison Program 3, CMIP3)对东亚夏季风降水与环流年代际变化的模拟性能.结果表明,在评估的19个模式中,有9个模式可以较好地再现中国东部地区多年平均降水场,但仅有3个模式(第1类模式)可以较好地对东亚夏季风降水的年代际变化作出模拟,这3个模式是:GFDL-CM2.0、MIROC3.2(hires)和MIROC3.2(medres),其中模式GFDL-CM2.0具有最好的模拟性能.进一步的分析表明,大部分模式对东亚夏季风变化模拟能力的缺乏是因为这些模式没有抓住东亚夏季风降水变化的主要动力和热力学机制,即东亚地区在过去所出现的大范围对流层变冷和变干.而第1类模式由于较好地再现了东亚地区垂直速度场(动力学因子)和水汽场(热力学因子)的变化特征,因此较好地模拟出中国东部南涝北旱的气候变化特征.本文的评估清楚地表明,当选择不同模式进行集合时,模式对某一研究变量的模拟性能好坏极大地影响了集合的结果.当模拟性能较好的模式在一起进行集合时,所得到的结果更加接近于真实的观测结果.就特定的研究变量而言,这种集合更加优于将可得到的所有模式进行集合.这说明,虽然多模式集合一般优于单个模式的结果,但应考虑使参与集合的模式对所研究变量具有一定的模拟能力.     

Northern East Asian low and its impact on the interannual variation of East Asian summer rainfall

The strength of the East Asian summer monsoon and associated rainfall has been linked to the western North Pacific subtropical high (WNPSH) and the lower-tropospheric low pressure system over continental East Asia (EA). In contrast to the large number of studies devoted to the WNPSH, little is known about the variability of the East Asian continental low. The present study delineates the East Asian continental low using 850-hPa geopotential height. Since the low is centered over northern EA (NEA), we refer to it as the NEA low (NEAL). We show that the intensity of the NEAL has large interannual variation, with a dominant period of 2–4 years. An enhanced NEAL exhibits a barotropic structure throughout the whole troposphere, which accelerates the summer-mean upper-tropospheric westerly jet and lower-tropospheric monsoon westerly to its south. We carefully identify the anomalous NEAL-induced rainfall anomalies by removal of the tropical heating effects. An enhanced NEAL not only increases rainfall locally in northern Northeast China, but also shifts the East Asian subtropical front northward, causing above-normal rainfall extending eastward from the Huai River valley across central-northern Japan and below-normal rainfall in South China. The northward shift of the East Asian subtropical front is attributed to the following processes without change in the WNPSH: an enhanced NEAL increases meridional pressure gradients and the monsoon westerly along the East Asian subtropical front, which in turn induces a cyclonic shear vorticity anomaly to its northern side. The associated Ekman pumping induces moisture flux convergence that shifts the East Asian subtropical front northward. In addition, the frequent occurrence of synoptic cut-off lows is found to be associated with an enhanced NEAL. Wave activity analysis indicates that the interannual intensity change of the NEAL is significantly associated with the extratropical Polar Eurasian teleconnection, in addition to the forcing of the tropical WNP heating.     

A comparison of East Asian summer monsoon simulations from CAM3.1 with three dynamic cores

This paper examines the sensitivity of CAM3.1 simulations of East Asian summer monsoon (EASM) to the choice of dynamic cores using three long-term simulations, one with each of the following cores: the Eulerian spectral transform method (EUL), semi-Lagrangian scheme (SLD) and finite volume approach (FV). Our results indicate that the dynamic cores significantly influence the simulated fields not only through dynamics, such as wind, but also through physical processes, such as precipitation. Generally speaking, SLD is superior to EUL and FV in simulating the climatological features of EASM and its interannual variability. The SLD version of the CAM model partially reduces its known deficiency in simulating the climatological features of East Asian summer precipitation. The strength and position of simulated western Pacific subtropical high (WPSH) and its ridge line compare more favourably with observations in SLD and FV than in EUL. They contribute to the intensification of the south-easterly along the south of WPSH and the vertical motion through the troposphere around 30° N, where the subtropical rain belt exists. Additionally, SLD simulates the scope of the westerly jet core over East Asia more realistically than the other two dynamic cores do. Considerable systematic errors of the seasonal migration of monsoon rain belt and water vapour flux exist in all of the three versions of CAM3.1 model, although it captures the broad northward shift of convection, and the simulated results share similarities. The interannual variation of EASM is found to be more accurate in SLD simulation, which reasonably reproduces the leading combined patterns of precipitation and 850-hPa winds in East Asia, as well as the 2.5- and 10-year periods of Li?CZeng EASM index. These results emphasise the importance of dynamic cores for the EASM simulation as distinct from the simulation??s sensitivity to the physical parameterisations.     

WRF模式对江西2006—2015年夏季气候模拟的性能分析

基于NCEP/FNL再分析资料,利用中尺度天气预报模式（WRF）对2006—2015年1月1日—8月31日的天气形势进行模拟,分析探讨了模式对江西省夏季(6—8月)气温和降水的模拟能力。结果表明：WRF模式能准确模拟出江西省气温和降水的空间分布气候特征,模拟结果与中国区域高时空分辨率地面气象要素驱动数据集（CMFD）接近。其中,降水的模拟精度低于气温模拟;模拟的气温在鄱阳湖地区出现低值,与CMFD的偏差最大。WRF模式模拟的地面反照率偏大导致气温模拟结果偏低。     

Potential impacts of East Asian winter monsoon on climate variability and predictability in the Australian summer monsoon region

This study explores potential impacts of the East Asian winter monsoon (EAWM) on summer climate variability and predictability in the Australia–Asian region through Australia–Asia (A-A) monsoon interactions. Observational analysis is conducted for the period of 1959 to 2001 using ERA-40 wind reanalysis and Climate Research Unit rainfall and surface temperature monthly datasets. Statistically significant correlations are established between the Australian summer monsoon and its rainfall variations with cross-equatorial flows penetrating from South China Sea region and northerly flow in the EAWM. The underlying mechanism for such connections is the response of the position and intensity of Hardley circulation to strong/weak EAWM. A strong EAWM is associated with an enhanced cross-equatorial flow crossing the maritime continent and a strengthened Australia summer monsoon westerlies which affect rainfall and temperature variations in northern and eastern part of the Australian continent. Furthermore, partial correlation analysis, which largely excludes El Niño-Southern Oscillation (ENSO) effects, suggests that these connections are the inherent features in the monsoon system. This is further supported by analyzing a global model experiment using persistent sea surface temperatures (SSTs) which, without any SST interannual variations, shows similar patterns as in the observational analysis. Furthermore, such interaction could potentially affect climate predictability in the region, as shown by some statistically significant lag correlations at monthly time scale. Such results are attributed to the impacts of EAWM on regional SST variations and its linkage to surface conditions in the Eurasian continent. Finally, such impacts under global warmed climate are discussed by analyzing ten IPCC AR4 models and results suggest they still exist in the warmed climate even though the EAWM tends to be weaker.     

Intercomparison of different physics schemes in the WRF model over the Asian summer monsoon region

WRF模式对包含西太平洋、东印度洋和中国大陆的大区域气候模拟能力的提高对东亚夏季风气候的预测非常重要。本文的研究目的是找出合理的物理方案组合来模拟中国大陆、东印度洋和西太平洋这一特大区域的降水和温度。通过模拟2008年夏季气候并与观测进行对比分析,主要比较了Mellor–Yamada–Janjic(MYJ)和Yonsei University(YSU)边界层方案、WSM3和WSM5微物理过程方案、Betts–Miller–Janjic(BMJ)和Tiedtke积云参数化方案对这一区域的气候模拟结果。研究表明:各种物理方案的组合对于温度空间分布的模拟与观测类似;边界层方案的选择对降水影响较大,MYJ方案比YSU方案对降水的模拟效果要好;积云方案Tiedtke对雨带分布,特别是赤道辐合带(ITCZ)降水模拟更加合理。     

The East Asian summer monsoon: an overview

Summary The present paper provides an overview of major problems of the East Asian summer monsoon. The summer monsoon system over East Asia (including the South China Sea (SCS)) cannot be just thought of as the eastward and northward extension of the Indian monsoon. Numerous studies have well documented that the huge Asian summer monsoon system can be divided into two subsystems: the Indian and the East Asian monsoon system which are to a greater extent independent of each other and, at the same time, interact with each other. In this context, the major findings made in recent two decades are summarized below: (1) The earliest onset of the Asian summer monsoon occurs in most of cases in the central and southern Indochina Peninsula. The onset is preceded by development of a BOB (Bay of Bengal) cyclone, the rapid acceleration of low-level westerlies and significant increase of convective activity in both areal extent and intensity in the tropical East Indian Ocean and the Bay of Bengal. (2) The seasonal march of the East Asian summer monsoon displays a distinct stepwise northward and northeastward advance, with two abrupt northward jumps and three stationary periods. The monsoon rain commences over the region from the Indochina Peninsula-the SCS-Philippines during the period from early May to mid-May, then it extends abruptly to the Yangtze River Basin, and western and southern Japan, and the southwestern Philippine Sea in early to mid-June and finally penetrates to North China, Korea and part of Japan, and the topical western West Pacific. (3) After the onset of the Asian summer monsoon, the moisture transport coming from Indochina Peninsula and the South China Sea plays a crucial “switch” role in moisture supply for precipitation in East Asia, thus leading to a dramatic change in climate regime in East Asia and even more remote areas through teleconnection. (4) The East Asian summer monsoon and related seasonal rain belts assumes significant variability at intraseasonal, interannual and interdecadal time scales. Their interaction, i.e., phase locking and in-phase or out-phase superimposing, can to a greater extent control the behaviors of the East Asian summer monsoon and produce unique rythem and singularities. (5) Two external forcing i.e., Pacific and Indian Ocean SSTs and the snow cover in the Eurasia and the Tibetan Plateau, are believed to be primary contributing factors to the activity of the East Asian summer monsoon. However, the internal variability of the atmospheric circulation is also very important. In particular, the blocking highs in mid-and high latitudes of Eurasian continents and the subtropical high over the western North Pacific play a more important role which is quite different from the condition for the South Asian monsoon. The later is of tropical monsoon nature while the former is of hybrid nature of tropical and subtropical monsoon with intense impact from mid-and high latitudes.