MOMENTUM BUDGET AND SHELTER MECHANISM OF BOUNDARY-LAYER FLOW NEAR A SHELTERBELT

We use a nonhydrostatic shelterbelt boundary-layer turbulence model with Mellor–Yamada second-order closure to evaluate quantitatively the dynamic processes of surface boundary-layer flow perturbed by shelterbelts of different densities and to understand the shelter mechanism. We first analyze the drag exerted on air by shelterbelts of different densities, a root cause of any shelter function, and the resulting wind reduction. The results show that the effectiveness of a shelter is determined not only by its total drag but also by the distribution of the drag-generated momentum deficit in the sheltered area, and that medium-dense shelterbelts have the maximum shelter effect. We also analyze the horizontal momentum budget and find that the shelter mechanism is the product of several processes. The results reveal that strong vertical mean transport and the pressure gradient also play important roles in shelter efficiency. The pressure perturbation caused by the shelter extends far downstream of the shelter, and combines with advective transport to provide the larger shelter efficiency of medium-dense shelterbelts. We finally analyze the changes of perturbed pressure, turbulence, and vertical velocity with shelterbelt density to further clarify the shelter mechanism.     

Shelter effects of vegetation belts — Results of field measurements

The results of full-scale measurements of the wind reduction behind four different types of shelterbelts are reported. Two of the shelterbelts in question were of the single-row type. The remaining two consisted of at least two rows of deciduous trees and in both cases some conifers.Wind measurements were made under various weather conditions, both in winter and in summer. The minimum relative wind speed on the lee side varied from ca 0.1 behind the most dense shelterbelt to 0.4 behind the most porous one when the wind was blowing at right angles in summer. In the winter, the shelter effect was much less than in the summer but the two most dense shelterbelts did reduce the wind speed considerably also then.When the wind was blowing at an angle to the shelterbelt, the lee maximum occurred closer to the belt and the wind speed recovered faster than when it blew at right angles. The wind reduction just behind a shelterbelt always tended to be greater in oblique than in perpendicular wind. The wind reduction behind a leafy vegetation belt decreased as the reference wind speed increased. The approach wind speed affected wind reduction much more at a porous shelterbelt than at a dense one.The difference between reference and local wind directions could be great just behind a shelterbelt but this difference disappeared within a few h downstream of the belt. With further increasing distance (up to 4–11h), the wind continued to veer so that the direction became more parallel with the shelterbelt than in the case of the undisturbed wind. Then the wind slowly veered back to the direction of the approaching wind again.The present data give a good idea of the amount of wind reduction which can be expected in the lee of some common types of shelterbelts.     

A numerical simulation of boundary-layer flows near shelterbelts

We have developed a shelterbelt boundary-layer numerical model to study the patterns and dynamic processes relating to flow interaction with shelterbelts. The model simulates characteristics of all three zones of airflow passing over and through shelterbelts: the windward windspeed-reduction zone, the overspeeding zone above the shelterbelt, and the leeward windspeed-reduction zone. Locations of the maximum windspeed reduction and recirculation zone, as well as the leeward windspeed-recovery rate are well simulated by the model. Where comparisons with field measurements and wind-tunnel experiments were possible, the model demonstrated good performance for flows over and through shelters ranging from almost completely open to almost solid. The dynamic pressure resulting from the convergence and divergence of the flow field alters the perturbation pressure field. The disturbed pressure controls not only the formation of the separated flow but also the location of maximum windspeed reduction, streamline curvature, speed-up over the shelterbelt, and leeward windspeed recovery rate. The interaction of pressure with the flow produces complex flow patterns, the characteristics of which are determined, to a great extent, by shelterbelt structure.     

Reduction of horizontal wind speed in a boundary layer with obstacles

The reduction of horizontal wind speed at hub height in an infinite cluster of wind turbines is computed from a balance between a loss of horizontal momentum due to the drag and replenishment from above by turbulent fluxes. This reduction is derived without assumptions concerning the vertical wind profile above or below hub height, only some basic assumptions on turbulent exchange have been made. Two applications of the result are presented, one considering wind turbines and one pressure drag on orographic obstacles in the atmospheric boundary layer. Both applications are basically governed by the same kind of momentum balance.     

Wind profiles,momentum fluxes and roughness lengths at Cabauw revisited

We describe the results of an experiment focusing on wind speed and momentum fluxes in the atmospheric boundary layer up to 200 m. The measurements were conducted in 1996 at the Cabauw site in the Netherlands. Momentum fluxes are measured using the K-Gill Propeller Vane. Estimates of the roughness length are derived using various techniques from the wind speed and flux measurements, and the observed differences are explained by considering the source area of the meteorological parameters. A clear rough-to-smooth transition is found in the wind speed profiles at Cabauw. The internal boundary layer reaches the lowest k-vane (20 m) only in the south-west direction where the obstacle-free fetch is about 2 km. The internal boundary layer is also reflected in the roughness lengths derived from the wind speed profiles. The lower part of the profile (< 40 m) is not in equilibrium and no reliable roughness analysis can be given. The upper part of the profile can be linked to a large-scale roughness length. Roughness lengths derived from the horizontal wind speed variance and gustiness have large footprints and therefore represent a large-scale average roughness. The drag coefficient is more locally determined but still represents a large-scale roughness length when it is measured above the local internal boundary layer. The roughness length at inhomogeneous sites can therefore be determined best from drag coefficient measurements just above the local internal boundary layers directly, or indirectly from horizontal wind speed variance or gustiness. In addition, the momentum and heat fluxes along the tower are analysed and these show significant variation with height related to stability and possibly surface heterogeneity. It appears that the dimensionless wind speed gradients scale well with local fluxes for the variety of conditions considered, including the unstable cases.     

“天气-风障-产量”系统中林带结构的马氏决策

“天气-风障-产量”是一个复杂巨系统, 若想通过该系统取得最佳效益, 必须对林带结构做出优化决策。该文论述了马尔可夫决策过程 (MDP) 在该决策中的应用问题。利用在沈阳市康平县对农田防护林带考察中所取得的资料, 依据概率论的基本原理, 计算出不同结构林带对玉米气象产量影响这一生态控制系统中有关状态的转移概率; 用MDP折扣模型计算出折扣率β=0.9条件下的最优策略并加以改进。结果表明:在当地天气、作物状况下所形成的复杂系统中, 采用透风系数为0.35的疏透结构农田防护林带具有最优决策结果。     

复杂地形强降雪过程中垂直运动诊断分析

利用WRF模式对2018年11月30日伊犁河谷和天山北坡强降雪过程进行数值模拟,并分析复杂地形强降雪过程垂直速度和垂直动能变化机制。研究表明,冷锋过境造成地表气压升高,干空气气柱质量增大,从而导致垂直气压梯度力和干空气气柱浮力发生变化,进而引起垂直运动发生发展。垂直速度局地时间变化主要取决于扰动垂直气压梯度力、水物质拖曳力和扰动干空气浮力。在天山北坡,气流过山时,迎风坡的扰动垂直气压梯度力较大,扰动干空气浮力较小,二者合力促进上升运动;在背风坡,扰动垂直气压梯度力和扰动空气浮力形成向下的合力,产生下沉加速度,导致背风坡下沉大风。扰动垂直气压梯度力做功和扰动干空气浮力做功情况基本相反,背风坡扰动垂直气压梯度力和综合强迫做功项抑制垂直动能,扰动干空气浮力和水物质拖曳力做功项增强垂直动能。此外,扰动垂直气压梯度力和扰动干空气浮力做功项主要出现在中低层,水物质拖曳力做功项主要位于低层,平缓地形处的综合强迫做功明显小于地形复杂处。     

Meso-beta scale numerical simulations of terrain drag-induced along-stream circulations Part I: Midtropospheric frontogenesis

Summary The problem of along-stream ageostrophic frontogenesis is studied by employing a numerical model at meso-alpha and meso-beta scales in simulations of the downstream circulations over the Front Range of the Rocky Mountains. Three-dimensional real data simulations at these two scales of motion are used to diagnose the transition from semigeostrophic cross-stream frontogenesis accompanying a propagating baroclinic upper-level jet streak to midtropospheric along-stream ageostrophic frontogenesis. This along-stream ageostrophic frontogenesis results from the perturbation of the jet streak by the Rocky Mountain range. The case study represents an example of internal wave dynamics which are forced by the drag of the Rocky Mountains on a strong jet streak in the presence of a low-level inversion.The simulation results indicate that, unlike semi-geostrophic frontogenesis, a front (which is alligned perpendicular to the axis of the jet stream) may form when significant adiabatic heating occurs within a stratified shear flow over horizontal length scales shorter than the Rossby radius of deformation. The mechanism responsible for the frontogenesis is the growth of the divergent along-stream wind velocity component which becomes coupled to the front's along-stream pressure gradient force. This nonlinear interaction produces hydrostatic mesoscale frontogenesis as follows: 1) vertical wind shear in the along-stream plane strengthens resulting in the increasingly nonuniform vertical variation of horizontal temperature advection as the ageostrophic wind component grows in magnitude downstream of the meso-scale terrain-induced adiabatic heating, 2) increasing along-stream differential vertical motions (i.e., along-stream thermally indirect circulation with warm air sinking to the west and cold air rising to the east) tilt the vertical gradient of isentropes into the horizontal as the vertical temperature gradient increases due to the previous process in proximity to horizontal gradients in the along-stream component of the ageostrophic wind, 3) as tilting motions act to increase the along-stream horizontal temperature gradient, the along-stream confluence acts to nonuniformly increase the along-stream frontal temperature gradient which increases the along-stream pressure gradient force resulting in further accelerations, ageostrophy, and frontal steepening as part of a scale contraction process.The evolution of the aforementioned processes results in the three-dimensional hydrostatic frontogenesis accompanying the overturning of isentropic surfaces. These adjustments act to turn air parcels to the right of the southwesterly geostrophic wind vector at successively lower atmospheric levels as the scale contraction continues. This simulated along-stream front is verified from diagnostic analysis of the profiler-derived temperature and wind fields.With 17 Figures     

Numerical studies on the two-dimensional flow in horizontally homogeneous canopy layers

The two-dimensional equation of motion containing the pressure gradient and Coriolis force is numerically solved for the wind field in and above the layers of a horizontally homogeneous canopy with a vertical distribution of leaf-area densities. The solution shows that, in the case of descending through the canopy, the wind vector turns with an angle which depends on the profile of leaf-area densities. In particular, for the canopy of a forest consisting of upper layers with higher densities and lower layers with smaller densities, the turning is striking; a secondary maximum in wind profile appears in the lower layers.Variations of the aerodynamic parameters for the flow above the canopy are indicated with respect to the leaf-area density. The roughness length varies in such a manner that a maximum appears in intermediate density values, depending on the shape of the profile of leaf-area density. In the case of very dense canopies, the shearing stress acting on the flow above the canopy is determined by the contribution from only the upper canopy elements, but not by that from the lower parts of the canopy.     

A TWO-DIMENSIONAL NUMERICAL STUDY OF WIND SHELTERING EFFECTS OF SHELTERBELTS

Based on the dynamic and thermodynamic equations in the SBL,the protection characteristics and mech-anism of shelterbelts are numerically studied in this paper.The results are in consistent with those ob-served in fields and wind-tunnels.Research shows that the belts with a permeability of 0.3—0.4 have themaximum sheltering effects.The distance where wind speed reduces at least by 20% is at 18—19 times oftree height(H)behind belts.The protection distance reduces fast with the increase of the permeability andslowly with its decrease.The belts have weaker sheltering effects in upstream,generally beyond 6H infront of belts there are no sheltering effects.In addition,the belts with low permeability at top and highpermeability at bottom have much better sheltering effects than the belts with high permeability at top andlow permeability at bottom.The belts with 50% overall permeability reduce wind speed at least,by 20%at the distance of 5H on the leeward side and by 10% at 11H.When the air is unstable,the protectingeffects will be reduced.For the uniform belts with a permeability of 0.35,the distance where wind speedreduces at least by 20% is 15H under the unstable air and 13H under very unstable air.The belts reducethe turbulent diffusion coefficient at the bottom of belts,but increase it at the top.     

飑线传播与发展及其引发地面强风过程个例分析

针对2006年4月28日发生在江苏境内的一次飑线过程,分析了过程期间的边界层特征、垂直结构和飑线的传播演变特征,讨论了飑线造成地面强风的物理原因。结果表明,在近地面层中,飑线首先在地面辐合线上激发并发展起来,当它向前传播时,边界层水平风场产生扰动,使飑线在近地面层的传播过程中表现出明显的波动特征,飑线后部降水区中降水质点的拖曳和下沉气流的共同作用是地面强风产生的关键。     

Low-level wind response to mesoscale pressure systems

Observations are presented which show a strong correlation between low-level wind behaviour (e.g., rotation near the surface) and the passage of mesoscale pressure systems. The latter are associated with frontal transition zones, are dominated by a pressure-jump line and a mesoscale high pressure area, and produce locally large horizontal pressure gradients. The wind observations are simulated by specifying a time sequence of perturbation pressure gradient and subsequently solving the vertically-integrated momentum equations with appropriate initial conditions. Very good agreement is found between observed and calculated winds; in particular, (i) a 360 ° rotation in wind on passage of the mesoscale high; (ii) wind-shift lines produced dynamically by the pressure-jump line; (iii) rapid linear increase in wind speed on passage of the pressure jump.     

A technique of radiosonde launch under the surface wind of high speed

Upper-air sounding is impacted by the surface wind of high speed in polar regions and in the areas affected by tropical cyclones. Considered are the main existing methods of radiosonde launching under such conditions. It is suggested to use the cone tower in order to provide a greater initial height of launching of the instrumental box as compared with the routine launching method. The employement of the tower allows using the wind force jointly with the lifting force of the balloon to deliver the instrumental box up to the top of the tower. Installation of the tower into the wind shelter secures the operator and provides the radiosonde launching even in the case of the very strong wind. The field model experiment corroborated the efficiency of the proposed scheme of radiosonde launching.     

塔克拉玛干沙漠公路人工防护林植被的C, N化学计量特征及其固存能力研究

本研究以塔克拉玛干沙漠公路沿线生物防护林为研究对象,研究防护林三种主要建林植物不同器官的C,N含量及其生物量,进而估算其C, N固存能力及固碳释氧价值。结果显示：沙拐枣每个器官的生物量均显著高于梭梭和柽柳(P<0.05),个体的总生物量是梭梭和柽柳的2-3倍。整个防护林在建林8年后的总生物量达到116786.4 t。C,N含量在不同器官中的含量不尽相同,三种植物均表现为凋落物C含量最低。而植物叶中的N含量均显著高于枝,凋落物和根系(P<0.05)。建林8年后梭梭单株的固碳量为1404.6g,固氮量为201.5g,柽柳单株的固碳量为1449.7g,固氮量为195.4g,沙拐枣单株的固碳量和固氮量显著高于其他两种,分别为3979.8g,为520.9g,(P<0.05)。建林8年后整个防护林的总固碳植为35886.3 t,总固氮值为4917.7 t。整个防护林的固碳总价值为22555.3万元,释氧价值为50320万元,总的固碳释氧价值达72875.3万元。     

水平风作用下雨滴水平速度的数值仿真

为了研究雨滴在水平风作用下的水平移动情况,在分析水平风作用下雨滴受力情况的基础上,通过对曳力系数与雷诺数对应关系的研究,采用数值仿真的方法分别对海平面大气条件和不同大气条件下雨滴水平运动速度进行仿真.结果表明:当有水平风作用时,雨滴的水平运动速度不等于风的速度,而是随雨滴直径和水平风速的变化而变化;在水平风作用下,雨滴的水平移动速度可以在较短的时间内(一般小于15 s)达到一个稳定的值;在水平风速相同的情况下,气压越高、温度越低,雨滴达到平衡时的水平移动末速度相对越大,反之则越小.这些结论对基于图像采集原理的光学降水自动观测仪器进行雨滴图像拼合有重要的应用价值.     

Laboratory Study of Topographic Effects on the Near-surface Tornado Flow Field

To study topographic effects on the near-surface tornado flow field, the Iowa State University tornado simulator was used to simulate a translating tornado passing over three different two-dimensional topographies: a ridge, an escarpment and a valley. The effect of the translation speed on maximum horizontal wind speeds is observed for translation speeds of 0.15 and 0.50 \(\hbox {m}\,\hbox {s}^{-1}\), with the lower value resulting in a larger maximum horizontal wind speed. The tornado translation over the three topographies with respect to flat terrain is assessed for changes in: (a) the maximum horizontal wind speeds in terms of the flow-amplification factor; (b) the maximum aerodynamic drag in terms of the tornado speed-up ratio; (c) the maximum duration of exposure at any location to high wind speeds of a specific range in terms of the exposure amplification factor. Results show that both the maximum wind amplification factor of 14%, as well as the maximum speed-up ratio of 14%, occur on the ridge. For all topographies, the increase in aerodynamic drag is observed to be maximized for low-rise buildings, which illustrates the importance of the vertical profiles of the horizontal wind speed near the ground. The maximum exposure amplification factors, estimated for the range of wind speeds corresponding to the EF2 (50–60 \(\hbox {m}\,\hbox {s}^{-1}\)) and EF3 (61–75 \(\hbox {m}\,\hbox {s}^{-1})\) scales, are 86 and 110% for the ridge, 4 and 60% for the escarpment and ? 6 and 47% for the valley, respectively.     

2018年北京城区和远郊区低层大气风场特征分析

利用2018年1月1日至12月31日在北京国家综合气象观测实验基地获得的风廓线雷达资料和同时期在河北香河的华北香河全大气层野外科学观测研究站获得的多普勒声雷达资料,比较分析北京城区和远郊区的低层（0～600 m）大气风场特征。结果表明:水平风速随高度增加而增大,同一高度层,远郊区的平均水平风速大于城区,且受湍流活动影响,城区和远郊区水平风速日变化趋势均为白天小于夜间。春、夏季城区风向受局地山谷风影响显著,以偏西南偏南气流为主,城区和远郊区秋冬季受冷空气活动影响,以西北风为主,且水平风向日变化特征具有季节性差异。远郊区低层大气垂直速度分布特征四季相同,正、负速度出现频率相当,日变化趋势为单峰型;城区冬春季有差异,在390 m高度以下正速度出现频率明显大于负速度,且日变化趋势在四季差异较大。北京城区和郊区风场特征差异与其他特大城市相比无特殊性,主要受大气环流、局地地形、下垫面环境等因素影响。     

多层城市冠层模式的建立及数值试验研究

为在城市气象数值模拟中更好地体现由城市发展引起的下垫面土地利用改变及人为活动对大气过程的影响,建立了基于建筑物三维分布的多层城市冠层模式,冠层内动力方程组考虑了建筑物冠层拖曳力的作用及雷诺应力的影响,通过引入建筑物宽度、间距以及垂直分布密度指数等建筑物形态特征参数,以更好地体现城市复杂地表对大气温度、湿度及动量方程的影响.同时,该模式分屋顶、4个侧壤及地面分别考虑辐射及能量平衡求解表面温度,计算各表面与大气的通量交换,并考虑辐射阴影效应、冠层内部各个面之间的可视因子、以及与冠层内建筑物密度指数、可视因子等相关的多重反射辐射导致的辐射截陷作用.模式的离线检验结果表明:(1)冠层模式计算风廓线与风洞实验测量数据吻合良好;(2)离线冠层模式能够模拟实际小区的风速、温度垂直廓线,并能够较好地体现小区内气温日变化.冠层模式与区域边界层模式耦合检验结果表明:(1)耦合模拟的近地面(2 m处)气温及地表温度的结果明显优于传统的水泥平板方案,尤其是在夜间,水泥平板方案与实测气温最大偏差4 K左右,耦合模拟方案为1-2 K;(2)耦合模拟方案考虑了建筑物对冠层之上的拖曳力影响以及建筑物形态结构对雷诺应力的影响,风速(10 m处)计算结果与观测值相差约在1 m/s,水泥平板方案偏差3 m/s左右.     

Equilibrium Atmospheric Boundary-Layer Flow: Computational Fluid Dynamics Simulation with Balanced Forces

Forcing relationships in steady, neutrally stratified atmospheric boundary-layer (ABL) flow are thoroughly analyzed. The ABL flow can be viewed as balanced between a forcing and a drag term. The drag term results from turbulent stress divergence, and above the ABL, both the drag and the forcing terms vanish. In computational wind engineering applications, the ABL flow is simulated not by directly specifying a forcing term in the ABL but by specifying boundary conditions for the simulation domain. Usually, these include the inflow boundary and the top boundary conditions. This ‘boundary-driven’ ABL flow is dynamically different from its real counterpart, and this is the major reason that the simulated boundary-driven ABL flow does not maintain horizontal homogeneity. Here, first a dynamical approach is proposed to develop a neutrally stratified equilibrium ABL flow. Computational fluid dynamics (CFD) software (Fluent 6.3) with the standard \(k\) – \(\varepsilon \) turbulence model is employed, and by applying a driving force profile, steady equilibrium ABL flows are simulated by the model. Profiles of wind speed and turbulent kinetic energy (TKE) derived using this approach are reasonable in comparison with the conventional logarithmic law and with observational data respectively. Secondly, the equilibrium ABL profiles apply as inflow conditions to simulate the boundary-driven ABL flow. Simulated properties between the inlet and the outlet sections across a fetch of 10 km are compared. Although profiles of wind speed, TKE, and its dissipation rate are consistently satisfactory under higher wind conditions, a deviation of TKE and its dissipation rate between the inlet and outlet are apparent (7–8 %) under lower wind-speed conditions (2 m s \(^{-1}\) at 10 m). Furthermore, the simulated surface stress systematically decreases in the downwind direction. A redistribution of the pressure field is also found in the simulation domain, which provides a different driving pattern from the realistic case in the ABL.     

考虑背景风压场影响的边界层数值模式

本文提出一个在中性情况下边界层数值模式中考虑背景风压场影响的方法。背景风压场的水平非均匀影响了边界层运动方程中的惯性力项及边界层的上界风速。本文把三维问题用一种线性化方法简化为一维问题，只要知道背景气压场的空间分布，即可求出考虑了背景场影响的边界层风场。