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    Impact of Pacific Decadal Oscillation on Frequency Asymmetry of El Niño and La Niña Events
    Soon-Il AN
    DOI: 10.1007/s00376-018-8024-7
    Abstract   ( 11 ) PDF (649KB) (13)
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    Development and Pull-through of Climate Science to Services in China
    Chris HEWITT, Nicola GOLDING
    DOI: 10.1007/s00376-018-7255-y
    Abstract   ( 17 ) PDF (843KB) (16)
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    Seasonal Forecasts of the Summer 2016 Yangtze River Basin Rainfall
    Philip E. BETT, Adam A. SCAIFE, Chaofan LI, Chris HEWITT, Nicola GOLDING, Peiqun ZHANG, Nick DUNSTONE, Doug M. SMITH, Hazel E. THORNTON, Riyu LU, Hong-Li REN
    DOI: 10.1007/s00376-018-7210-y
    Abstract   ( 33 ) PDF (1356KB) (13)
    The Yangtze River has been subject to heavy flooding throughout history, and in recent times severe floods such as those in 1998 have resulted in heavy loss of life and livelihoods. Dams along the river help to manage flood waters, and are important sources of electricity for the region. Being able to forecast high-impact events at long lead times therefore has enormous potential benefit. Recent improvements in seasonal forecasting mean that dynamical climate models can start to be used directly for operational services. The teleconnection from El Niño to Yangtze River basin rainfall meant that the strong El Niño in winter 2015/16 provided a valuable opportunity to test the application of a dynamical forecast system. This paper therefore presents a case study of a real-time seasonal forecast for the Yangtze River basin, building on previous work demonstrating the retrospective skill of such a forecast. A simple forecasting methodology is presented, in which the forecast probabilities are derived from the historical relationship between hindcast and observations. Its performance for 2016 is discussed. The heavy rainfall in the May--June--July period was correctly forecast well in advance. August saw anomalously low rainfall, and the forecasts for the June--July--August period correctly showed closer to average levels. The forecasts contributed to the confidence of decision-makers across the Yangtze River basin. Trials of climate services such as this help to promote appropriate use of seasonal forecasts, and highlight areas for future improvements.
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    Interannual Variability of Late-spring Circulation and Diabatic Heating over the Tibetan Plateau Associated with Indian Ocean Forcing
    Yu ZHAO, Anmin DUAN, Guoxiong WU
    DOI: 10.1007/s00376-018-7217-4
    Abstract   ( 4 ) PDF (3462KB) (4)
    The thermal forcing of the Tibetan Plateau (TP) during boreal spring, which involves surface sensible heating, latent heating released by convection and radiation flux heat, is critical for the seasonal and subseasonal variation of the East Asian summer monsoon. Distinct from the situation in March and April when the TP thermal forcing is modulated by the sea surface temperature anomaly (SSTA) in the North Atlantic, the present study shows that it is altered mainly by the SSTA in the Indian Ocean Basin Mode (IOBM) in May, according to in-situ observations over the TP and MERRA reanalysis data. In the positive phase of the IOBM, a local Hadley circulation is enhanced, with its ascending branch over the southwestern Indian Ocean and a descending one over the southeastern TP, leading to suppressed precipitation and weaker latent heat over the eastern TP. Meanwhile, stronger westerly flow and surface sensible heating emerges over much of the TP, along with slight variations in local net radiation flux due to cancellation between its components. The opposite trends occur in the negative phase of the IOBM. Moreover, the main associated physical processes can be validated by a series of sensitivity experiments based on an atmospheric general circulation model, FAMIL. Therefore, rather than influenced by the remote SSTAs of the northern Atlantic in the early spring, the thermal forcing of the TP is altered by the Indian Ocean SSTA in the late spring on an interannual timescale.
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    Collating Historic Weather Observations for the East Asian Region: Challenges, Solutions, and Reanalyses
    Fiona WILLIAMSON, Rob ALLAN, Guoyu REN, Tsz-cheung LE, Wing-hong LUI, Hisayuki KUBOTA, Jun MATSUMOTO, Jürg LUTERBACHER, Clive WILKINSON, Kevin WOOD
    DOI: 10.1007/s00376-017-7259-z
    Abstract   ( 69 ) PDF (1424KB) (27)
    Historic instrumental weather observations are critical in extending our knowledge of past weather, climate and extremes and for comparison with paleo-proxy data. Importantly, it means that we can now extract extra value from historical observations when they are assimilated into “dynamical reanalyses”. This enables us to both produce and visualize detailed information about spatial and temporal fluctuations in atmospheric circulation patterns, weather and climate variations and extreme events—useful for helping to understand the contemporary vulnerability of societies, and the infrastructures supporting them, to weather and climate change within a long historical context. Such instrumental observations have been recorded from as early as the 17th century in Italy, but there are still many that are not available in the form required for modern-day use. The Climate Science for Service Partnership (CSSP) China project includes a dedicated effort—Atmospheric Circulation Reconstructions over the Earth (ACRE) China—which tackles the challenges of recovering, collating, digitizing and working with long-term instrumental weather observations. CSSP China (supported by the UK Business, Energy & Industrial Strategy UK–China Research Innovation Partnership Fund) is enabling this work to go ahead under ACRE China as part of a collaboration between the China Meteorological Administration, the Institute of Atmospheric Physics (IAP) at the Chinese Academy of Sciences, the UK Meteorological Office, and other key institutes within China and the UK. These activities are also linked closely with wider regional projects and programs under ACRE Southeast Asia. This article therefore discusses the components of the project in more detail, and reveals how ACRE China will enable us to develop a better understanding of how East Asian climate has both fluctuated and changed over time, and how it might change in the future.
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    Modeling the Warming Impact of Urban Land Expansion on Hot Weather Using the Weather Research and Forecasting Model: A Case Study of Beijing, China
    Xiaojuan LIU, Guangjin TIAN, Jinming FENG, Bingran MA, Jun WANG, Lingqiang KONG
    DOI: 10.1007/s00376-017-7137-8
    Abstract   ( 41 ) PDF (2975KB) (18)
    The impacts of three periods of urban land expansion during 1990–2010 on near-surface air temperature in summer in Beijing were simulated in this study, and then the interrelation between heat waves and urban warming was assessed. We ran the sensitivity tests using the mesoscale Weather Research and Forecasting model coupled with a single urban canopy model, as well as high-resolution land cover data. The warming area expanded approximately at the same scale as the urban land expansion. The average regional warming induced by urban expansion increased but the warming speed declined slightly during 2000–2010. The smallest warming occurred at noon and then increased gradually in the afternoon before peaking at around 2000 LST—the time of sunset. In the daytime, urban warming was primarily caused by the decrease in latent heat flux at the urban surface. Urbanization led to more ground heat flux during the day and then more release at night, which resulted in nocturnal warming. Urban warming at night was higher than that in the day, although the nighttime increment in sensible heat flux was smaller. This was because the shallower planetary boundary layer at night reduced the release efficiency of near-surface heat. The simulated results also suggested that heat waves or high temperature weather enhanced urban warming intensity at night. Heat waves caused more heat to be stored in the surface during the day, greater heat released at night, and thus higher nighttime warming. Our results demonstrate a positive feedback effect between urban warming and heat waves in urban areas.
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    Evaluating and Improving Wind Forecasts over South China: The Role of Orographic Parameterization in the GRAPES Model
    Shuixin ZHONG, Zitong CHEN, Daosheng XU, Yanxia ZHANG
    DOI: 10.1007/s00376-017-7157-4
    Abstract   ( 36 ) PDF (3001KB) (12)
    Unresolved small-scale orographic (SSO) drags are parameterized in a regional model based on the Global/Regional Assimilation and Prediction System for the Tropical Mesoscale Model (GRAPES_TMM). The SSO drags are represented by adding a sink term in the momentum equations. The maximum height of the mountain within the grid box is adopted in the SSO parameterization (SSOP) scheme as compensation for the drag. The effects of the unresolved topography are parameterized as the feedbacks to the momentum tendencies on the first model level in planetary boundary layer (PBL) parameterization. The SSOP scheme has been implemented and coupled with the PBL parameterization scheme within the model physics package. A monthly simulation is designed to examine the performance of the SSOP scheme over the complex terrain areas located in the southwest of Guangdong. The verification results show that the surface wind speed bias has been much alleviated by adopting the SSOP scheme, in addition to reduction of the wind bias in the lower troposphere. The target verification over Xinyi shows that the simulations with the SSOP scheme provide improved wind estimation over the complex regions in the southwest of Guangdong.
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    Report on IAMAS Activity since 2015 and the IAPSO-IAMAS-IAGA Scientific Assembly ---Good Hope for Earth Sciences
    John TURNER, Teruyuki NAKAJIMA
    DOI: 10.1007/s00376-017-7240-x
    Abstract   ( 34 ) PDF (1249KB) (17)
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    Interannual Weakening of the Tropical Pacific Walker Circulation Due to Strong Tropical Volcanism
    Jiapeng MIAO, Tao WANG, Huijun WANG, Jianqi SUN
    DOI: 10.1007/s00376-017-7134-y
    Abstract   ( 162 ) PDF (4277KB) (69)
    In order to examine the response of the tropical Pacific Walker circulation (PWC) to strong tropical volcanic eruptions (SVEs), we analyzed a three-member long-term simulation performed with HadCM3, and carried out four additional CAM4 experiments. We found that the PWC shows a significant interannual weakening after SVEs. The cooling effect from SVEs is able to cool the entire tropics. However, cooling over the Maritime Continent is stronger than that over the central-eastern tropical Pacific. Thus, non-uniform zonal temperature anomalies can be seen following SVEs. As a result, the sea level pressure gradient between the tropical Pacific and the Maritime Continent is reduced, which weakens trade winds over the tropical Pacific. Therefore, the PWC is weakened during this period. At the same time, due to the cooling subtropical and midlatitude Pacific, the Intertropical Convergence Zone (ITCZ) and South Pacific convergence zone (SPCZ) are weakened and shift to the equator. These changes also contribute to the weakened PWC. Meanwhile, through the positive Bjerknes feedback, weakened trade winds cause El Niño-like SST anomalies over the tropical Pacific, which in turn further influence the PWC. Therefore, the PWC significantly weakens after SVEs. The CAM4 experiments further confirm the influences from surface cooling over the Maritime Continent and subtropical/midlatitude Pacific on the PWC. Moreover, they indicate that the stronger cooling over the Maritime Continent plays a dominant role in weakening the PWC after SVEs. In the observations, a weakened PWC and a related El Niño-like SST pattern can be found following SVEs.
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    Regional Features and Seasonality of Land--Atmosphere Coupling over Eastern China
    Chujie GAO, Haishan CHEN, Shanlei SUN, Bei XU, Victor ONGOMA, Siguang ZHU, Hedi MA, Xing LI
    DOI: 10.1007/s00376-017-7140-0
    Abstract   ( 9 ) PDF (2483KB) (6)
    Land--atmosphere coupling is a key process of the climate system, and various coupling mechanisms have been proposed before based on observational and numerical analyses. The impact of soil moisture (SM) on evapotranspiration (ET) and further surface temperature (ST) is an important aspect of such coupling. Using ERA-Interim data and CLM4.0 offline simulation results, this study further explores the relationships between SM/ST and ET to better understand the complex nature of the land--atmosphere coupling (i.e., spatial and seasonal variations) in eastern China, a typical monsoon area. It is found that two diagnostics of land--atmosphere coupling (i.e., SM--ET correlation and ST--ET correlation) are highly dependent on the climatology of SM and ST. By combining the SM--ET and ST--ET relationships, two “hot spots” of land--atmosphere coupling over eastern China are identified: Southwest China and North China. In Southwest China, ST is relatively high throughout the year, but SM is lowest in spring, resulting in a strong coupling in spring. However, in North China, SM is relatively low throughout the year, but ST is highest in summer, which leads to the strongest coupling in summer. Our results emphasize the dependence of land--atmosphere coupling on the seasonal evolution of climatic conditions and have implications for future studies related to land surface feedbacks.
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