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.     

On three-dimensionality of shelterbelt structure and its influences on shelter effects

Natural shelterbelts, unlike planar barriers, have a certain width, within which interactions among wind speed, drag force and pressure perturbations create a net sheltering effect. The variations of flow, drag force, permeability, and pressure perturbation for shelterbelts of different widths and different horizontal structures are numerically studied, and their influences on shelter efficiency are discussed. Comparisons are made of fourteen medium-dense shelterbelts, with the same overall leaf-area, that differ only in width or horizontal distribution of leaf-area density. The simulated results are consistent with both field observations and wind-tunnel measurements.The simulations demonstrate that the total drag force of the entire shelterbelt varies little with changes in width and structure. The results also show that shelter distance and the overall average wind speed reduction decrease only by 15–18% as width increases by a factor of 100, and changes even less for different internal structure. However, width greatly affects the location of minimum wind speed, pressure perturbation, and the permeability of shelterbelts. Horizontal changes of wind speed inside the uniform shelterbelts have four different patterns, which depend on shelterbelt width and height. The absolute pressure perturbation significantly decreases with increasing width. A possible cause of the insensitivity of shelter efficiency to width and internal inhomogeneous structure is the compensation between the effects of permeability and pressure perturbation on shelter efficiency.     

“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。     

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.     

地形上空边界层流中低层锋面结构的理论研究 I：冷锋、均匀地转流

文中利用一个包含地形、边界层摩擦作用的二层锋面简化模型 ,讨论了地形、边界层对冷锋锋面结构、环流的动力学影响作用问题。冷锋的倾斜主要与冷锋暖区的地转流分布、锋面移动速度、锋面与地形的相对位置有关。当冷锋位于迎风坡时 ,其坡度减小 ,位于背风坡时 ,其坡度增大。在静止冷锋存在两类不同的环流系 ,即位于大气低层、地面锋附近的反时针环流系和远离地面锋的顺时针环流系。静止冷锋位于背风坡 ,其冷域中的这支闭合环流增强 ,范围增大 ,而位于冷锋界面上的环流转向点沿锋面上移 ,暖域中沿锋面的暖空气运动范围变大。当静止冷锋位于迎风坡时 ,结果相反。冷锋移动较慢时 ,其冷域远离地面锋的这支顺时针环流主体可被地形完全阻塞在迎风坡。当静止冷锋移离地形时 ,由于地形作用可在锋面暖域、地形下游形成一个背风槽。地形对锋区的垂直运动影响主要通过地形与边界层共同作用 ,改变锋区流场辐合辐散的分布及地形强迫抬升两条途径实现。由于边界层摩擦的辐合作用 ,在地面冷锋的前缘可形成一支范围较窄、强度较大的上升运动带 ,当冷锋位于迎风坡时 ,其强度增强 ,当冷锋位于背风坡时 ,其强度减弱。当冷锋位于背风坡时 ,在暖区沿锋面上升的暖空气运动范围增大 ,可以出现沿锋面相间隔的多个上升区     

A New Large-Eddy Simulation Model for Simulating Air Flow and Warm Clouds Above Highly Complex Terrain. Part II: The Moist Model and its Application to Banner Clouds

In Part I the dry version of a new large-eddy simulation (LES) model was presented that is specifically designed to simulate air flow and clouds above highly complex terrain. Here the implemented moisture physics are described and a new method for the generation of turbulent inflow conditions for meteorological LES is proposed. As a typical area of application the new model is applied to simulate banner clouds developing downwind of pyramidal mountain peaks. Banner clouds are shown to be primarily a dynamical phenomenon, and form in the lee of steep mountain peaks as a result of dynamically forced lee upslope flow. Due to the highly asymmetric flow field induced by the extreme orography, banner clouds can form even under horizontally homogeneous initial conditions regarding both moisture and temperature. Thus, additional leeward moisture sources, distinct air masses on both windward and leeward sides, or radiation effects are no prerequisite for banner-cloud formation. The probability of banner-cloud formation increases with increasing obstacle height and steepness and is, to a first approximation, independent of the pyramid’s orientation. Simulations with and without moisture physics reveal that, for the set-up chosen, moisture is of only secondary importance for banner-cloud dynamics. The reinforcement of lee upslope flow and corresponding cloud formation due to latent heat release turns out to be almost negligible. Nevertheless moisture physics are shown to induce a dipole-like structure in the vertical profile of the Brunt-Väisälä frequency, which in turn leads to a moderate increase in leeward turbulence.     

新疆百里风区强风中尺度特征分析

大尺度环流背景和天山山脉大地形共同作用形成新疆百里风区，其风力之大居全疆九大风区之首。为进一步研究百里风区强风中尺度特征及其与局地地形的关系，选取2018年5月6—8日百里风区强风天气过程，使用WRF模式进行中尺度模拟分析，形成以下结论：天山两侧气压梯度力驱动下冷空气翻越天山，经色皮山口狭管效应和过山波水跃下沉接力加速，在背风坡上空形成强风区，强风区接地形成百里风区地面大风；大风过程中，七角井盆地地形强迫引发有限振幅重力波，背风坡上空大风区之上的临界层吸收上层能量并向下传递，增大了大风区的风速，使得低空大风区的接地更加充分。低空大气稳定层结的强度与大风强度相对应。     

Air movement and its consequences around a multiple shelterbelt system under advective conditions in semi-arid Northern Nigeria

Summary Horizontal wind speed patterns above a scarce millet row crop on inhomogeneous sandy soil revealed insufficient protection from hot winds by multiple shelterbelts in semi-arid Northern Nigeria. This appeared mainly due to too high distances between the belts. Marked yield drops occurred with distance between the belts, in what McNaughton defined (under mechanical damage and microclimate disturbance from strong winds) as the unprotected wake zone. These may, in the case of hot winds, mainly be attributed to combined negative effects on soil moisture and crop physiology of the combination of turbulence, worsened by the shelterbelts, and advected heat. Other parameters confirm the picture of the wake zone and the quiet zone, the latter also being present windward of the belts in a reduced form. The results have serious consequences for the design rules of multiple shelterbelts and alternatives under African semi-arid conditions.     

泰山地形对一次局地强降水过程动力作用的数值模拟分析

文章针对2005年9月2日发生在济宁郭楼镇的一次泰山背风下游区局地特大暴雨过程（438 mm/7 h）,利用济南SA雷达资料和WRF中尺度数值模式资料,分析了泰山山区地形造成的对流层中低层大气动力过程及其对暴雨中尺度系统的影响,开展了地形敏感性数值试验。结果表明：对流层低层回流南下的东北气流受鲁中山区地形影响,水平方向发生绕流,垂直方向被迫抬升,从而在泰山背风向的暴雨区附近形成准定常的绕流汇合区和重力波扰动区,两项作用强迫的垂直运动,与天气系统辐合区共同作用触发该地区的对流活动,并使移入该地的对流系统增强和维持。改变地形后引起低层风场和散度场变化,进而影响降雨带中的强降水落区和强度,但对主雨带的分布无明显影响。     

洱海盆地一次冬季大风演变特征及其机制模拟分析

利用WRF模式耦合Noah陆面模式和CLM湖泊模式,对2015年1月23日大理地区洱海盆地的大风天气进行模拟,对大风的发展期、强盛期和减弱期的三维动力热力结构特征进行分析,并得出了洱海盆地大风形成机制:在洱海盆地大风发展期,高空以西风为主,盆地中部上空1km高度处出现局地小气旋,地面以偏东风为主,高空偏西气流翻越苍山形成波动扰动,在背风坡侧形成空腔区和二次涡,低层形成了波不稳定区域,波不稳定区域发生波破碎,波破碎区域湍流运动活跃,把上层的能量往下传播。大风强盛期,盆地南北侧高空为两支西风气流控制,中部变为弱的辐散场,造成高空扰动,苍山东侧近地面浅薄逆温层消失,低空逆温层之上温度廓线几乎垂直上升,大气层结处于不稳定状态,有利于高空动量向下输送。大风减弱期,高空西风减弱,扰动消失,湍流动能耗散,地面风速逐渐减小。     

On fitting the log-linear model to wind speed and temperature profiles over a melting glacier

The profile structure of wind speed and temperature in katabatic flow over a melting glacier is analyzed within the log-linear framework. Similarity between windspeed and temperature profiles is indicated but the log-linear model should be restricted to heights within 1.5 m of the ice. Marked deviation from the model occur at greater heights, probably due to the effects of flux divergence.Unlike results from other stable atmospheres, a decreases with increasing stability. This may arise from the use of the Obukhov length L. When the height H at which the wind speed reaches its maximum value is used instead, does not vary with stability. It has mean values of 4.5 and 4.3 for windspeed and temperature profiles, respectively.     

地形上空边界层流中低层锋面结构的理论研究Ⅰ:冷锋、均匀地转流

文中利用一个包含地形、边界层摩擦作用的二层锋面简化模型,讨论了地形、边界层对冷锋锋面结构、环流的动力学影响作用问题.冷锋的倾斜主要与冷锋暖区的地转流分布、锋面移动速度、锋面与地形的相对位置有关.当冷锋位于迎风坡时,其坡度减小,位于背风坡时,其坡度增大.在静止冷锋存在两类不同的环流系,即位于大气低层、地面锋附近的反时针环流系和远离地面锋的顺时针环流系.静止冷锋位于背风坡,其冷域中的这支闭合环流增强,范围增大,而位于冷锋界面上的环流转向点沿锋面上移,暖域中沿锋面的暖空气运动范围变大.当静止冷锋位于迎风坡时,结果相反.冷锋移动较慢时,其冷域远离地面锋的这支顺时针环流主体可被地形完全阻塞在迎风坡.当静止冷锋移离地形时,由于地形作用可在锋面暖域、地形下游形成一个背风槽.地形对锋区的垂直运动影响主要通过地形与边界层共同作用,改变锋区流场辐合辐散的分布及地形强迫抬升两条途径实现.由于边界层摩擦的辐合作用,在地面冷锋的前缘可形成一支范围较窄、强度较大的上升运动带,当冷锋位于迎风坡时,其强度增强,当冷锋位于背风坡时,其强度减弱.当冷锋位于背风坡时,在暖区沿锋面上升的暖空气运动范围增大,可以出现沿锋面相间隔的多个上升区.     

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.     

Observational Study of Surface Wind along a Sloping Surface over Mountainous Terrain during Winter

The 2018 Winter Olympic and Paralympic Games will be held in Pyeongchang, Korea, during February and March. We examined the near surface winds and wind gusts along the sloping surface at two outdoor venues in Pyeongchang during February and March using surface wind data. The outdoor venues are located in a complex, mountainous terrain, and hence the near-surface winds form intricate patterns due to the interplay between large-scale and locally forced winds. During February and March, the dominant wind at the ridge level is westerly; however, a significant wind direction change is observed along the sloping surface at the venues. The winds on the sloping surface are also influenced by thermal forcing,showing increased upslope flow during daytime. When neutral air flows over the hill, the windward and leeward flows show a significantly different behavior. A higher correlation of the wind speed between upper-and lower-level stations is shown in the windward region compared with the leeward region. The strong synoptic wind, small width of the ridge, and steep leeward ridge slope angle provide favorable conditions for flow separation at the leeward foot of the ridge. The gust factor increases with decreasing surface elevation and is larger during daytime than nighttime. A significantly large gust factor is also observed in the leeward region.     

北太平洋冬季西部发展型天气尺度涡旋对平均流的影响

在中纬度北太平洋大气强斜压区,存在频繁的天气尺度涡旋活动,通过水分、动量和能量输送维持大气环流。为了进一步研究天气尺度涡旋发生发展与大尺度环流之间的联系,利用1981—2013年再分析资料,筛选出西部发展型天气尺度涡旋114个偏强日和87个偏弱日,给出了西部发展型天气尺度涡旋异常导致的动力和热力强迫的变化,同时从能量转换的角度分析了西部发展型天气尺度涡旋与平均流之间的相互作用,并探讨了其与西太平洋遥相关型的关系。结果表明:西部发展型天气尺度涡旋通过动力强迫和热力强迫影响平均流,其中动力强迫主要造成北太平洋中纬度上空的西风气流加速并向北移动;热力强迫的作用则是减弱中纬度大气斜压性。同时,强西部发展型天气尺度涡旋有利于西北太平洋上空对流层低层斜压有效位能向扰动动能的转化增大和扰动动能向平均流的转化增大,有利于中纬度地区对流层高层平均流向扰动动能的转化增大。此外,西部发展型天气尺度涡旋通过与平均流的作用,对维持西太平洋遥相关型的负位相有一定影响。     

过水体气流流速的改变

流场如何适应非均匀地表面是微气象学的一个重要问题。本文通过建立一个包含地面过程参数化和水温预报方案的非静力平衡中小尺度数值模式来研究过水体气流流速改变的空间结构和变化规律。结果表明:由于水体使表面粗糙度有脉冲式跃变,当较强气流流过时,形成两个内边界层,这两个内边界层在空间交汇呈“舌状”分布,在此之内流速增大,并且地面粗糙度愈大,流速增大愈多。大气层结明显地影响过水体气流流速的改变,造成流速改变有明显的日变化和季节性变化。过水体气流流向改变不大。      

Integrated modeling of the dynamic meteorological and sea surface conditions during the passage of Typhoon Morakot

In August 2009, Typhoon Morakot caused massive flooding and devastating mudslides in the southern Taiwan triggered by extremely heavy rainfall (2777 mm in 4 days) which occurred during its passage. It was one of the deadliest typhoons that have ever attacked Taiwan in recent years. In this study, numerical simulations are performed for the storm surge and ocean surface waves, together with dynamic meteorological fields such as wind, pressure and precipitation induced by Typhoon Morakot, using an atmosphere–waves–ocean integrated modelling system. The wave-induced dissipation stress from breaking waves, whitecapping and depth-induced wave breaking, is parameterized and included in the wave–current interaction process, in addition to its influence on the storm surge level in shallow water along the coast of Taiwan. The simulated wind and pressure field captures the characteristics of the observed meteorological field. The spatial distribution of the accumulated rainfall within 4 days, from 00:00 UTC 6 August to 00:00 UTC 10 August 2009, shows similar patterns as the observed values. The 4-day accumulated rainfall of 2777 mm at the A-Li Shan mountain weather station for the same period depicted a high correlation with the observed value of 2780 mm/4 days. The effects of wave-induced dissipation stress in the wave–current interaction resulted in increased surge heights on the relatively shallow western coast of Taiwan, where the bottom slope of the bathymetry ranges from mild to moderate. The results also show that wave-breaking has to be considered for accurate storm surge prediction along the east coast of Taiwan over the narrow bank of surf zone with a high horizontal resolution of the model domain.     

Analysis of Hawaiian diurnal rainfall patterns

Summary We analyzed hourly precipitation data over the period 1965–1998 for 133 stations located throughout the Hawaiian Islands and found distinctive diurnal cycles in the amount and frequency of the rainfall. The diurnal signals were generally strongest on the island of Hawaii and more pronounced in the summer than in the winter. Windward sides of the islands generally showed the time of maximum for frequency or amount to be before sunrise while the leeward sides generally revealed the primary maximum in the afternoon. We conducted the analyses for El Niño years and La Niña years and found the diurnal patterns in rainfall to be unaffected by this Pacific-area oscillation. The results are consistent with others who have argued, based on work on the island of Hawaii, that anabatic and katabatic wind flows are responsible for the observed diurnal rainfall patterns. A semi-diurnal cycle with two maxima and two minima appears to be important in the summer season in the leeward locations. Our results show that processes and patterns described for the island of Hawaii are found throughout the larger islands of the Hawaii chain.     

Orographical modification and large scale forcing of a cold front

Summary The development of a cold front influenced by orography and large scale forcing is examined with a two-dimensional meso-scale model. The model is based on the primitive equations and uses the hydrostatic and anelastic approximations. Gradients of the basic flow and temperature field in the third dimension are taken into account during the simulations. Low diffusive numerical schemes and radiation boundary conditions reduce the numerical errors to an acceptable minimum for a two day simulation and avoid reflections at the upper and lateral boundaries. Frontogenetical forcing is included in the simulations by specifying either a vertically sheared or horizontally convergent basic zonal flow field. Model runs with an idealized cold front were carried out over flat terrain and in the presence of a bell shaped mountain ridge.The simulations show a weakening of the cold front on the windward side of the mountain ridge and a strong reintensification on the leeward side relative to the control runs without topography. Analysis of frontogenesis terms demonstrates the importance of convergence in the ageostrophic circulation and of along-front temperature advection for the development of the cold front. The strong intensification of the cold front on the leeward side of the mountain ridge can only partly be explained by superposition with the mountain induced wave. It is mainly caused by ageostrophic deformation forcing in the strong downward flow of this wave.The results also show that the cold front passage over the mountain ridge is not a continuous process. The formation of a new frontal structure on the leeward side of the mountain ridge, well separated from the primary one, is observed while the initial cold front still exists in the upslope region. Generally nonlinear interactions between the mountain wave and the cold front are the important mechanisms to explain these phenomena.With 18 Figures     

Numerical Simulation of the Airflow Across Trees in a Windbreak

The flow across a three-dimensional (3-D) windbreak comprising individual cypress trees is studied to establish the significance and extent of the 3-D flow patterns. The cypress tree is modelled as a solid cylindrical stem and a conic porous canopy. Cases with a single row of trees or two rows of trees with different distances between the rows are considered; in the case of a single row, several densities of the canopy are used. The steady Reynolds-averaged Navier–Stokes (RANS) approximation is solved using a commercial computational fluid dynamics (CFD) package and a high-resolution mesh. Three-dimensional flow is found in the vicinity of the windbreak up to a leeward distance of 1–2 tree-heights, depending on the density of the canopy, and is manifest as significant lateral variations and reduced vertical flow. At larger leeward distances, a two-dimensional (2-D) flow is established with characteristics similar to existing 2-D studies; the flow leeward of the last row is insensitive to the distance between the rows. Homogeneous 2-D windbreak models are found to be inaccurate in the vicinity of the windbreak. This is exactly the region that needs to be sheltered in many cases, since the inner vegetation is anyway protected by the outer vegetation.