The effect of extreme rainfall on summer succession and vertical distribution of phytoplankton in a lacustrine part of a eutrophic reservoir     

Petr Znachor  Eliška Zapomělová  Klára Řeháková  Jiří Nedoma  Karel Šimek 《Aquatic Sciences - Research Across Boundaries》2008,70(1):77-86

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Airside velocity measurements over the wind-sheared water surface using particle image velocimetry     

Nasiruddin Shaikh  Kamran Siddiqui 《Ocean Dynamics》2008,58(1):65-79

An experimental investigation of the airflow structure in the near surface region over the wind-sheared air–water interface is reported. The two-dimensional velocity fields in a plane perpendicular to the water surface were measured using particle image velocimetry (PIV) technique over a wind speed range from 1.5 to 4.4 m s⁻¹. The results show a reduction in the mean velocity magnitudes and the tangential stresses when gravity waves appear on the surface. An enhanced vorticity layer was observed immediately above the water surface that extended to a height of approximately 2 cm. The vorticity was enhanced by an order of magnitude, and the energy dissipation rate was enhanced by a factor of 7 in this layer at all wind speeds. The vertical profiles of Reynolds stress, energy production, and dissipation indicate the contribution of surface waves in the enhanced transfer of momentum and energy between the two fluids. The results in this study show that the flow dynamics in a layer immediately adjacent to the water surface whose thickness is of the order of the significant wave height is significantly different from that at greater heights. Thus, it is concluded that the quantitative investigation of the flow in the immediate vicinity of the interface is vital for an improved understanding of the heat, mass, and momentum exchange between air and water. The present study demonstrates that PIV is an effective technique to accurately measure the velocity fields in this region.  相似文献   

地震波传播的褶积微分算子法数值模拟   总被引：2，自引：0，他引：2  

李信富  李小凡 《地球科学》2008,33(6):861-866

为了解决地震波场数值模拟中的速度与精度的匹配及局部信息与全局信息的耦合等问题, 该文借助新推出的基于Forsyte广义正交多项式的褶积微分算子, 将计算数学中的Forsyte多项式褶积微分算子应用到地震波传播的数值模拟中.复杂非均匀介质模型数值模拟结果说明了该方法的可行性和优越性.该方法同时具有广义正交多项式褶积微分算子的高精度和有限差分短算子算法的高速度.通过对算子长度的调节及算子系数的优化, 可同时兼顾波场解的全局信息与局部信息.   相似文献   

Cross-shelf transport into nearshore waters due to shoaling internal tides in San Pedro Bay, CA     

Marlene Noble  Burt Jones  Peter Hamilton  Jingping Xu  George Robertson  Leslie Rosenfeld  John Largier   《Continental Shelf Research》2009,29(15):1768-1785

In the summer of 2001, a coastal ocean measurement program in the southeastern portion of San Pedro Bay, CA, was designed and carried out. One aim of the program was to determine the strength and effectiveness of local cross-shelf transport processes. A particular objective was to assess the ability of semidiurnal internal tidal currents to move suspended material a net distance across the shelf. Hence, a dense array of moorings was deployed across the shelf to monitor the transport patterns associated with fluctuations in currents, temperature and salinity. An associated hydrographic program periodically monitored synoptic changes in the spatial patterns of temperature, salinity, nutrients and bacteria. This set of measurements show that a series of energetic internal tides can, but do not always, transport subthermocline water, dissolved and suspended material from the middle of the shelf into the surfzone. Effective cross-shelf transport occurs only when (1) internal tides at the shelf break are strong and (2) subtidal currents flow strongly downcoast. The subtidal downcoast flow causes isotherms to tilt upward toward the coast, which allows energetic, nonlinear internal tidal currents to carry subthermocline waters into the surfzone. During these events, which may last for several days, the transported water remains in the surfzone until the internal tidal current pulses and/or the downcoast subtidal currents disappear. This nonlinear internal tide cross-shelf transport process was capable of carrying water and the associated suspended or dissolved material from the mid-shelf into the surfzone, but there were no observation of transport from the shelf break into the surfzone. Dissolved nutrients and suspended particulates (such as phytoplankton) transported from the mid-shelf into the nearshore region by nonlinear internal tides may contribute to nearshore algal blooms, including harmful algal blooms that occur off local beaches.  相似文献   

Extreme wave event along the Guyana coastline in October 2005     

Mathijs van Ledden  Geoffrey Vaughn  Joost Lansen  Frank Wiersma  Mewburn Amsterdam 《Continental Shelf Research》2009

The objective of the research is to analyze in detail the causes and consequences of the unusual event at the coast of Guyana (South America) during October 16–19, 2005. Several sea defense structures were damaged and flooding of low-lying areas occurred. A data analysis of offshore wave and water level characteristics shows an abrupt change in wave direction from east to almost north on October 16, 2005 and a sudden increase in the offshore peak period up to extreme values. The offshore significant wave height was also relatively high, and these wave characteristics coincided with springtide conditions. The long-wave periods and the sharp transition in wave direction indicate that this event is associated with swell waves generated by a depression far away. An analysis of hurricanes and depressions reveals that a severe depression in the Northern Atlantic Ocean during October 11–15 was the origin of this swell event. Numerical computations with SWAN have been carried out to investigate the propagation of the offshore wave characteristics towards the shoreline. The SWAN model includes wave damping due to the presence of soft mud deposits. A calibration of the parameters has been carried out using joint offshore and onshore wave data from November 2006. The numerical simulations of the event in October 2005 clearly demonstrate that the mud banks damp the wave heights, but have almost no effect on the peak period. The resulting waves at the steep sea walls can be classified as surging waves causing severe runup and overtopping. The obtained insights are translated into practical recommendations for the Guyana Sea and River Defence Division in Guyana to build a sustainable management and maintenance of the sea defenses in the future.  相似文献   

Plasmaspheric hiss overview and relation to chorus     

Jacob Bortnik  Richard M. Thorne  Nigel P. Meredith   《Journal of Atmospheric and Solar》2009,71(16):1636-1646

A brief overview is given of the history of plasmaspheric hiss research, particularly in the context of the recent work by Bortnik et al. (2008) indicating that chorus could be the likely source of plasmaspheric hiss. Previous suggestions given in the literature for this theory are reviewed and then the mechanism itself is outlined, focusing on the characteristic cyclical trajectories executed by typical ray paths that enter into the plasmasphere. A number of directional propagation studies performed in the past are then discussed as well as other work which bears relevance to the present mechanism.  相似文献   

Assessing conditional extremal risk of flooding in Puerto Rico     

Beatriz Vaz de Melo Mendes  Luis Raúl Pericchi 《Stochastic Environmental Research and Risk Assessment (SERRA)》2009,23(3):399-410

We model multivariate hydrological risks in the case that at least one of the variables is extreme. Recently, Heffernan JE, Tawn JA (2004) A conditional approach for multivariate extremes. J R Stat Soc B 66(3):497–546 (thereafter called HT04) proposed a conditional multivariate extreme value model which applies to regions where not all variables are extreme and simultaneously identifies the type of extremal dependence, including negative dependence. In this paper we apply this modeling strategy and provide an application to multivariate observations of five rivers in two clearly distinct regions of Puerto Rico Island and for two different seasons each. This effective dimensionality of ten-dimensions cannot be handled by the traditional models of multivariate extremes. The resulting fitted model, following HT04 model and strategies of estimation, is able to make long term estimation of extremes, conditional than other rivers are extreme or not. The model shows considerable flexibility to address the natural questions that arise in multivariate extreme value assessments. In the Puerto Rico 5 rivers application, the model clearly puts together two regions one of two rivers and another of three rivers, which show strong relationships in the rainy season. This corresponds with the geographical distribution of the rivers.

Beatriz Vaz de Melo MendesEmail:

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不耦合装药爆破对硬岩应力场影响的数值分析   总被引：7，自引：1，他引：6  

姜鹏飞  唐德高  龙源 《岩土力学》2009,30(1):275-279

爆破地震勘探石油是一种重要的方法,但爆破地震效应与爆破参数、地质条件等密切相关。采用动力有限元软件ANSYS/LS-DYNA,对柱状炸药与药孔壁之间为空气或其他介质以及空隙间距变化时碳酸盐岩岩石中爆炸应力波的传播规律和爆炸地震波能量的衰减特性进行了数值模拟研究,得到了不耦合装药爆炸时岩石应力、振动速度的衰减规律以及与不耦合系数、间隙介质的关系,分析了不耦合效应对爆炸地震波能量的影响。研究表明,不耦合或耦合不好时会使岩体中爆炸应力波的强度大大降低;耦合状态对岩体应力及速度的衰减系数和衰减指数影响较大;在空隙中注水或灌满泥浆会改善它们的耦合关系,增大下传的爆破能量。所得成果可为我国西南地区优选适合碳酸盐地层地震勘探的激发因素提供技术途径和方法。  相似文献   

长江上游全新世特大洪水对西南 季风变化的响应   总被引：1，自引：1，他引：0  

葛兆帅 《地理研究》2009,28(3):592-600

利用24次全新世特大洪水序列与川渝地区近两千年的洪灾史料,对长江上游特大洪水事件和气候变化的响应机制进行了分析。从长江上游特大洪水序列与阿拉伯海记录的西南季风气候变化对比分析来看,特大洪水事件与西南季风变化具有很好的响应关系,特大洪水事件多发生于西南季风较弱的阶段,这些阶段以西南季风气候快速变化为特征,是气候剧烈波动期或气候的转型期。与北大西洋、贵州董歌洞石笋所记录的气候变化比较,特大洪水有的与全新世气候突变一致,有的则不一致,可能特大洪水事件更多表现为地方事件。与文献记载的历史洪灾事件相比,长江上游低频高量级的特大洪水事件对西南季风的变化具有更好的响应关系,而高频低量级的洪灾事件具有更高的随机性,由于历史文献记载的洪水量级差异较大,如果将他们笼统地放在一起分析,可能会掩盖洪水事件对气候变化响应机制。  相似文献   

近45年雅鲁藏布江流域极端气候事件趋势分析   总被引：23，自引：2，他引：21  

游庆龙  康世昌  闫宇平  徐彦伟  张拥军  黄杰 《地理学报》2009,64(5)

利用雅鲁藏布江流域10个气象台站1961-2005年逐日最高气温、最低气温和日降水量资料,分析了该流域气温和降水等气候极端事件的变化趋势.研究表明:近45年以来,雅鲁藏布江流域夜间和白天极端低温日数分别以1.94和0.97天/10年的趋势在显著减少,夜间极端低温日数减少在冬季最明显,白天极端低温日数在秋季减少最明显:夜间极端高温日数和白天极端高温日数分别以3.03和1.26天/10年的速度显著增加,夜间极端高温日数增加在夏季最明显,白天极端高温日数增加在冬季最明显;日较差以0.11℃/10a的速度在显著减少,主要发生在冬季:最大的1天降水总量和逐年连续无降水天数有减少趋势,最大的5天降水总量、中雨天数、逐年平均降水强度和逐年连续降水天数有增加趋势,90年代以来增加趋势明显,与该地区经向风与水汽通量增加有关.  相似文献