Scalar Transport over Forested Hills     

Andrew N. Ross 《Boundary-Layer Meteorology》2011,141(2):179-199

Numerical simulations of scalar transport in neutral flow over forested ridges are performed using both a 1.5-order mixing-length closure scheme and a large-eddy simulation. Such scalar transport (particularly of CO₂) has been a significant motivation for dynamical studies of forest canopy–atmosphere interactions. Results from the 1.5-order mixing-length simulations show that hills for which there is significant mean flow into and out of the canopy are more efficient at transporting scalars from the canopy to the boundary layer above. For the case with a source in the canopy this leads to lower mean concentrations of tracer within the canopy, although they can be very large horizontal variations over the hill. These variations are closed linked to flow separation and recirculation in the canopy and can lead to maximum concentrations near the separation point that exceed those over flat ground. Simple scaling arguments building on the analytical model of Finnigan and Belcher (Q J Roy Meteorol Soc 130:1–29, 2004) successfully predict the variations in scalar concentration near the canopy top over a range of hills. Interestingly this analysis suggests that variations in the components of the turbulent transport term, rather than advection, give rise to the leading order variations in scalar concentration. The scaling arguments provide a quantitative measure of the role of advection, and suggest that for smaller/steeper hills and deeper/sparser canopies advection will be more important. This agrees well with results from the numerical simulations. A large-eddy simulation is used to support the results from the mixing-length closure model and to allow more detailed investigation of the turbulent transport of scalars within and above the canopy. Scalar concentration profiles are very similar in both models, despite the fact that there are significant differences in the turbulent transport, highlighted by the strong variations in the turbulent Schmidt number both in the vertical and across the hill in the large-eddy simulation that are not represented in the mixing-length model.  相似文献   

A diagnostic study of two heavy rainfall events in South China     

Liji Wu  Yaping Shao  Andrew Y. S. Cheng 《Meteorology and Atmospheric Physics》2011,111(1-2):13-25

South China is prone to heavy rainfall which may occur both in the pre-monsoon and the monsoon season. The responsible synoptic systems and the water-vapor sources, however, can be substantially different for different seasons. In this study, we aim to develop conceptual models for typical heavy rainfall events in South China through diagnostic case studies. A number of events have been analyzed using the NCEP/NCAR data, but the discussions here are focused on two representative events, one for the pre-monsoon season and the other for the monsoon season. Both events are found to be associated with extensive moist convective instability in the lower part of the troposphere. For the pre-monsoon case the instability was much weaker and the uplift of the warm moist air was provided by the cold air intrusion related to a weak cold front. The moist potential vorticity theory can be used to explain the increase of vorticity and vertical velocity in the lower part of the troposphere. For the monsoon event, the lower troposphere was extensively and strongly unstable and the upward motion was provided by the shear of a southwesterly low-level jet (LLJ) which appeared to be driven, at least partially driven, by the upper-level jet. In both events, LLJ played a major role in not only providing the dynamic conditions but also the supply of water vapor for heavy rainfall in South China. The diagnostic results presented in this study provide a useful guidance for future numerical simulations.  相似文献   

Global and regional coupled climate sensitivity to the parameterization of rainfall interception   总被引：1，自引：0，他引：1  

Jiafu Mao  Andrew J. Pitman  Steven J. Phipps  Gab Abramowitz  YingPing Wang 《Climate Dynamics》2011,37(1-2):171-186

A coupled land?Catmosphere model is used to explore the impact of seven commonly used canopy rainfall interception schemes on the simulated climate. Multiple 30-year simulations are conducted for each of the seven methods and results are analyzed in terms of the mean climatology and the probability density functions (PDFs) of key variables based on daily data. Results show that the method used for canopy interception strongly affects how rainfall is partitioned between canopy evaporation and throughfall. However, the impact on total evaporation is much smaller, and the impact on rainfall and air temperature is negligible. Similarly, the PDFs of canopy evaporation and transpiration for six selected regions are strongly affected by the method used for canopy interception, but the impact on total evaporation, temperature and precipitation is negligible. Our results show that the parameterization of rainfall interception is important to the surface hydrometeorology, but the seven interception parameterizations examined here do not cause a statistically significant impact on the climate of the coupled model. We suggest that broad scale climatological differences between coupled climate models are not likely the result of how interception is parameterized. This conclusion is inconsistent with inferences derived from earlier uncoupled simulations, or simulations using very simplified climate models.  相似文献   

The effect of environmental change on human migration     

Richard Black  W. Neil Adger  Nigel W. Arnell  Stefan Dercon  Andrew Geddes  David Thomas 《Global Environmental Change》2011

The influence of the environment and environmental change is largely unrepresented in standard theories of migration, whilst recent debates on climate change and migration focus almost entirely on displacement and perceive migration to be a problem. Drawing on an increasing evidence base that has assessed elements of the influence of the environment on migration, this paper presents a new framework for understanding the effect of environmental change on migration. The framework identifies five families of drivers which affect migration decisions: economic, political, social, demographic and environmental drivers. The environment drives migration through mechanisms characterised as the availability and reliability of ecosystem services and exposure to hazard. Individual migration decisions and flows are affected by these drivers operating in combination, and the effect of the environment is therefore highly dependent on economic, political, social and demographic context. Environmental change has the potential to affect directly the hazardousness of place. Environmental change also affects migration indirectly, in particular through economic drivers, by changing livelihoods for example, and political drivers, through affecting conflicts over resources, for example. The proposed framework, applicable to both international and internal migration, emphasises the role of human agency in migration decisions, in particular the linked role of family and household characteristics on the one hand, and barriers and facilitators to movement on the other in translating drivers into actions. The framework can be used to guide new research, assist with the evaluation of policy options, and provide a context for the development of scenarios representing a range of plausible migration futures.  相似文献   

Knowing, farming and climate change adaptation in North-Central Namibia   总被引：1，自引：0，他引：1  

Andrew J. Newsham  David S.G. Thomas 《Global Environmental Change》2011,21(2):761-770

The agro-ecological knowledge held by Ovambo farmers in North Central Namibia has, for centuries, given them resilience to high levels of climate variability and associated impacts. New research, conducted in North Central Namibia, suggests that knowledge co-production between farmers and agricultural extension workers may, in addition, strengthen adaptive capacity to future climate change. However, this useful kind of knowledge co-production is far from automatic, and indeed the conditions which make it more likely to happen are not well understood. This paper explores agro-ecological knowledge in North Central Namibia as adaptive capacity, and suggests avenues for better conceptualising and understanding the conditions for adaptive capacity-enhancing knowledge co-production.  相似文献   

Climate change, powerlessness and the commons dilemma: Assessing New Zealanders’ preparedness to act     

Christopher AitkenRalph Chapman  John McClure 《Global Environmental Change》2011,21(2):752-760

Mitigating climate change is recognized as an increasingly urgent task that requires understanding a range of different strategies, including voluntary behavior change. Among the psychological barriers to behavior change are perceptions of powerlessness and the commons dilemma. This paper examines the association between these factors in a sample of New Zealand citizens and clarifies their importance in relation to other barriers to action to mitigate climate change, including uncertainty and perceived risk. Stronger perceptions of powerlessness and the commons dilemma were related to lower levels of action to mitigate climate change and lesser importance being placed upon climate change as an influence on individual actions. The perceived risk of climate change and the perception that humans influence climate change were the strongest predictors of mitigation action.  相似文献   

Occurrence, distribution and characteristics of beached plastic production pellets on the island of Malta (central Mediterranean)     

Turner A  Holmes L 《Marine pollution bulletin》2011,62(2):377-381

The distribution, abundance and chemical characteristics of plastic production pellets on beaches of the island of Malta have been determined. Pellets were observed at all locations visited and were generally most abundant (>1000 m⁻² at the surface) on the backshores of beaches with a westerly aspect. Most pellets were disc-shaped or flattened cylinders and could be categorised as white, yellow, amber or brown. The polymeric matrix of all pellets analysed by infrared spectroscopy was polyethylene and the degree of yellowing or darkening was associated with an increase in the carbonyl index, hence extent of photo-oxidation or aging. Qualitatively, pellets are similar to those reported for other regions of the Mediterranean in surveys spanning three decades, suggesting that they are a general and persistent characteristic of the region.  相似文献   

Can we improve the spatial predictions of seabed sediments? A case study of spatial interpolation of mud content across the southwest Australian margin     

Jin Li  Andrew D. HeapAnna Potter  Zhi HuangJames J. Daniell 《Continental Shelf Research》2011,31(13):1365-1376

Spatially continuous data of environmental variables is often required for marine conservation and management. However, information for environmental variables is usually collected by point sampling, particularly for the marine region. Thus, methods generating such spatially continuous data by using point samples to estimate values for unknown locations become essential tools. Such methods are, however, often data- or even variable-specific and it is difficult to select an appropriate method for any given dataset. In this study, 14 methods (37 sub-methods) are compared using samples of mud content with five levels of sample density across the southwest Australian margin. Bathymetry, distance-to-coast, slope and geomorphic province were used as secondary variables. Ten-fold cross validation with relative mean absolute error (RMAE) and visual examination were used to assess the performance of these methods. A total of 1850 prediction datasets are produced and used to assess the performance of the methods and the effects of other factors considered. Considering both the accuracy and the visual examination, we found that a combined method (i.e., random forest and ordinary kriging: RKrf) is the most robust method. This method is novel, with a RMAE up to 17% less than that of the control. No threshold in sample density was detected in relation to prediction accuracy. No consistent patterns are observed between the performance of the methods and data variation. The RMAE of three most accurate methods is about 30% lower than that of the best methods in previous publications, highlighting the robustness of the methods selected in this study. The implications and limitations of this study are discussed and a number of suggestions are provided for further studies.  相似文献   

The settling dynamics of flocculating mud-sand mixtures: Part 1��Empirical algorithm development     

Andrew James Manning  John V. Baugh  Jeremy R. Spearman  Emma L. Pidduck  Richard J. S. Whitehouse 《Ocean Dynamics》2011,61(2-3):311-350

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地震各向异性流体检测技术在胜利油田垦71井区的应用(英文)     

毕丽飞  钱忠平  张峰  陈晓燕  韩世春  Mark Chapman 《应用地球物理》2011,8(2):117-124

根据Chapman理论模型,在各向异性介质(如HTI介质)中,当入射角在0-45。范围内,慢横波会发生较大的衰减和频散,且对流体粘度敏感,而P波和快横波则比较小。对于沿裂隙法向传播的慢横波,其振幅受流体影响很大。因此,在P波响应对流体不敏感的情况下,可利用慢横波来获得裂隙型油气藏的流体信息。本文分析了胜利油田垦71地区三维三分量地震数据,检测出的慢横波振幅和旅行时异常与该区的测井资料十分吻合。分析结果还发现,与含油区相比,含水区会产生更高的横波分裂。在含水区,慢横波振幅会产生明显变化,而在含油区则几乎没有变化。  相似文献