Edge Flow and Canopy Structure: A Large-Eddy Simulation Study

Sharp heterogeneities in forest structure, such as edges, are often responsible for wind damage. In order to better understand the behaviour of turbulent flow through canopy edges, large-eddy simulations (LES) have been performed at very fine scale (2 m) within and above heterogeneous vegetation canopies. A modified version of the Advanced Regional Prediction System (ARPS), previously validated in homogeneous conditions against field and wind-tunnel measurements, has been used for this purpose. Here it is validated in a simple forest-clearing-forest configuration. The model is shown to be able to reproduce accurately the main features observed in turbulent edge flow, especially the “enhanced gust zone” (EGZ) present around the canopy top at a few canopy heights downwind from the edge, and the turbulent region that develops further downstream. The EGZ is characterized by a peak in streamwise velocity skewness, which reflects the presence of intense intermittent wind gusts. A sensitivity study of the edge flow to the forest morphology shows that with increasing canopy density the flow adjusts faster and turbulent features such as the EGZ become more marked. When the canopy is characterized by a sparse trunk space the length of the adjustment region increases significantly due to the formation of a sub-canopy wind jet from the leading edge. It is shown that the position and magnitude of the EGZ are related to the mean upward motion formed around canopy top behind the leading edge, caused by the deceleration in the sub-canopy. Indeed, this mean upward motion advects low turbulence levels from the bottom of the canopy; this emphasises the passage of sudden strong wind gusts from the clearing, thereby increasing the skewness in streamwise velocity as compared with locations further downstream where ambient turbulence is stronger.     

Effect of Vertical Wind Shear on Concentration Fluctuation Statistics in a Point Source Plume

Measurements of concentration fluctuation intensity, intermittency factor, and integral time scale were made in a water channel for a plume dispersing in a well-developed, rough surface, neutrally stable, boundary layer, and in grid-generated turbulence with no mean velocity shear. The water-channel simulations apply to full-scale atmospheric plumes with very short averaging times, on the order of 1–4 min, because plume meandering was suppressed by the water-channel side walls. High spatial and temporal resolution vertical and crosswind profiles of fluctuations in the plume were obtained using a linescan camera laser-induced dye tracer fluorescence technique. A semi-empirical algebraic mean velocity shear history model was developed to predict these concentration statistics. This shear history concentration fluctuation model requires only a minimal set of parameters to be known: atmospheric stability, surface roughness, vertical velocity profile, and vertical and crosswind plume spreads. The universal shear history parameter used was the mean velocity shear normalized by surface friction velocity, plume travel time, and local mean wind speed. The reference height at which this non-dimensional shear history was calculated was important, because both the source and the receptor positions influence the history of particles passing through the receptor position.     

Upgrades to the Boundary-Layer Scheme in the Met Office Numerical Weather Prediction Model

Recent upgrades to the boundary-layer scheme in the UK Met Office operational global Numerical Weather Prediction model are documented. These comprise a reduction in turbulent mixing in stable conditions over the sea, and the inclusion of non-local momentum mixing in convective conditions. The dependence of low-level winds on changing stability is shown to have been significantly improved. Crucially, it is also found that these improvements in local performance have been achieved without degrading the model skill in terms of synoptic evolution—something that has proved difficult to achieve in the past in many operational models. In fact some aspects of the large-scale flow (e.g. zonal mean winds) have been slightly improved.     

Measurements and Modelling of the Wind Speed Profile in the Marine Atmospheric Boundary Layer

We present measurements from 2006 of the marine wind speed profile at a site located 18 km from the west coast of Denmark in the North Sea. Measurements from mast-mounted cup anemometers up to a height of 45 m are extended to 161 m using LiDAR observations. Atmospheric turbulent flux measurements performed in 2004 with a sonic anemometer are compared to a bulk Richardson number formulation of the atmospheric stability. This is used to classify the LiDAR/cup wind speed profiles into atmospheric stability classes. The observations are compared to a simplified model for the wind speed profile that accounts for the effect of the boundary-layer height. For unstable and neutral atmospheric conditions the boundary-layer height could be neglected, whereas for stable conditions it is comparable to the measuring heights and therefore essential to include. It is interesting to note that, although it is derived from a different physical approach, the simplified wind speed profile conforms to the traditional expressions of the surface layer when the effect of the boundary-layer height is neglected.     

一次多回击自然闪电的高速摄像观测

2006年8月1日在广东省从化市利用成像率为5000幅/s的高速摄像系统观测得到了一次包含有13个回击过程的自然负地闪, 其梯级先导的传播速度为106 m/s的量级; 一次企图先导的传播速度随高度的降低而减小; 一次直窜先导的传播速度随高度的降低而增加。有3次继后回击相对积分亮度的峰值大于首次回击相对积分亮度的峰值。研究发现:此次自然闪电的继后回击及紧跟其后的连续电流过程的发光总量与该次继后回击之前闪电通道的截止时间有关, 较大的发光总量对应于较长的截止时间, 较小的发光总量对应于较短的截止时间, 但两者没有固定的比例关系。     

城市化进程对安徽省风速的影响

利用1981-2010年安徽省61个站的逐日风速资料,结合卫星遥感台站分类方法,统计分析了城市化进程对年、季节平均风速、最大风速和小风日数的影响和贡献。结果表明：(1) 近30年安徽省年、季节平均风速和最大风速呈显著减少趋势,小风日数呈显著增加趋势。城市站的变化速率明显大于乡村站,郊区站基本介于二者之间。(2) 2000年开始安徽省城市化进程加快,导致城市站与乡村站平均风速及小风日数距平的差异有明显增大趋势。(3) 城市站与乡村站年平均风速的趋势系数之差为-0.10 (m/s) /10a,城市化对年平均风速减弱的贡献率为40.0%,春季更明显;城市站与乡村站年小风日数的趋势系数之差为15.58 d/10a,城市化对年小风日数增多的贡献率为46.9%,秋、冬季更明显;城市化对年最大风速的影响不明显。     

毛乌素沙地光伏电站项目区风速流场及风蚀防治措施

针对毛乌素沙地光伏电站项目,观测项目区的风速、风速廓线及风速流场特征,探讨有效的风蚀防治措施及其合理布局。结果表明:(1)太阳能板的存在使项目区风速流场的空间分布发生了明显变化,显著增加了板间近地面出风口处的风速,降低了板间远地面进风口处的风速,太阳能板间呈现出明显的掏蚀区和堆积区。(2)与项目区外围相比,太阳能板使20 cm及200 cm高度的风速分别降低了44.06%及63.68%。项目区外围及太阳能板正中间风速随高度的增加均呈上升趋势,而且太阳能板间60 cm高度以下风速增加更明显,不利于地表沙土的固定。(3)项目区外围麦草方格沙障、砾石压盖及红泥覆盖均起到一定的固沙作用,其中,以砾石压盖措施的固沙效果最优。(4)太阳能板间,除麦草方格沙障外,其余各措施均能起到固沙作用,其中以掏蚀区+砾石/堆积区+红泥覆盖的组合措施的固沙效果最优。     

利用断头树估计2016年6月23日阜宁县龙卷风强度与尺度

2016年6月23日在盐城市阜宁县附近发生特大龙卷风事件,人员财产损失惨重。本文利用天气图和多普勒雷达图等气象资料,对此次龙卷风的产生机制进行分析,再根据受灾相对较严重的5个地区断头树情况的调查资料,以“大风中风速达到约42 m/s时,大风经过的树林中折断树的百分比急剧升高到50%”为依据,来估计风速达到42 m/s的龙卷风范围。调查结果显示:大气不稳定层结,冷、暖湿空气交汇,风的垂直风切变是此次龙卷风的重要诱发因子;此次龙卷风风速达到42 m/s的影响范围的长度至少达到20 km,最大宽度至少达到952 m。     

电动变桨式潮流能水轮机获能分析与应用

我国具有丰富的潮流能资源,但是存在潮流流速偏低、难以高效利用等问题。潮流能水轮机变桨距技术的利用,可有效提高潮流能资源利用效率。以提高潮流能水轮机高效获能为目标,分析了水轮机叶片桨距角对潮流能水轮机获能的影响规律,研究了水轮机变桨距技术原理及控制策略。在20 k W潮流能水轮机中运用了电动变桨距技术,根据潮流流速的不同,使用最大功率点追踪控制算法控制桨距角,并对机组运行过程进行实时测试。机组运行数据表明,与非变桨水轮机相比,变桨式潮流能水轮机可有效提高其获能效率。     

Sequential Filtering for Surface Wind Speed Estimation from Ambient Noise Measurement

Many research results show that ocean ambient noise and wind speed are highly relevant, and the surface wind speed can be effectively inverted using ocean noise data. In most deep-sea cases, the ambient noise of medium frequency is mainly determined by the surface wind, and there is a conventional relationship between them. This paper gives an equation which shows this relationship firstly, and then a surface-wind inversion method is proposed. An efficient particle filter is used to estimate the speed distribution, and the results exhibit more focused close to the actual wind speed. The method is verified by the measured noise data, and analysis results showed that this approach can accurately give the trend of sea surface wind speed.