Influence of Obstacles on the Aerodynamic Roughness of the Netherlands

The aim of this study was to analyse the influence of large- and small-scale obstacles (orography, tree lines, and dikes) on the effective aerodynamic roughness of the Netherlands, a relatively flat, small-scale landscape. The roughness averaging approach was based on drag coefficients. The effective roughness was locally dominated by small-scale obstacles such as tree lines and dikes. Even at a regional scale (40,000 km2), the small-scale obstacle drag was of the same order of magnitude as the shear stress due to landuse. The neglect of those obstacles on a regional scale would result in approximately 10% overestimated averaged windspeed at 10~m above the surface. It was concluded that small-scale obstacles need to be taken into account to calculate the aerodynamic roughness of flat landscapes. Orography was of minor importance in this lowland country.     

Aerodynamic Variables in the Bulk Formulation of Turbulent Fluxes

Aerodynamic variables are required to apply Monin–Obukhov similarity theory in the bulk formulation of surface fluxes. In the literature, these aerodynamic variables are commonly misinterpreted. In this paper, we review the concept of the aerodynamic variable, its connection to surface-layer similarity theory and how and why the aerodynamic variable is replaced with other variables.Observed mean variables below the surface layer, such as the surface radiation temperature, or the air temperature at canopy height, are often used in place of the extrapolated aerodynamic variables in the bulk formula, requiring empirical relationships between aerodynamic and observed variables, or requiring empirical adjustments of bulk resistances. The present study examines the validity of these relationshi Experiment (CODE). The results indicate that using a measured substitute for an aerodynamic variable can lead to significant errors in estimates of turbulent surface fluxes.     

冬季北京城市近地层的气象特征

运用2001年1～3月北京大气边界层和大气化学综合试验期间,中国科学院大气物理研究所铁塔上所获得的8～320 m 15层风、温度和湿度梯度资料,对冬季北京城市边界层特征进行了诊断分析.结果表明,在冬季北京城市边界层中,平均而言地表粗糙度为1.34、零平均位移约为20 m;温度基本上随高度呈线性变化;风速随高度的变化并不总遵循对数关系,尤其是在午前和夜间,风速与高度之间对数关系的不显著率可达30%～40%.不能简单套用在Monin-Obuhov相似理论中由Businger-Dve风廓线层结订正获得的近地面层动量、热量和水汽湍流输送计算公式.     

A Flume Experiment on the Adjustment of the Mean and Turbulent Statistics to a Transition from Short to Tall Sparse Canopies

Water-flume experiments are conducted to study the structure of turbulent flow within and above a sparse model canopy consisting of two rigid canopies of different heights. This difference in height specifies a two-dimensional step change from a rough to a rougher surface, as opposed to a smooth-to-rough transition. Despite the fact that the flow is in transition from a rough to a rougher surface, the thickness of the internal boundary layer scales as x ^4/5, consistent with smooth-to-rough boundary layer adjustment studies, where x is the downstream distance from the step change. However, the analogy with smooth-to-rough transitions no longer holds when the flow inside the canopy and near the canopy top is considered. Results show that the step change in surface roughness significantly increases turbulence intensities and shear stress. In particular, there is an adjustment of the mean horizontal velocity and shear stress as the flow passes over the rougher canopy, so that their vertical profiles adjust to give maximum values at the top of this canopy. We also observe that the magnitude and shape of the inflection in the mean horizontal velocity profile is significantly affected by the transition. The horizontal and vertical turbulence spectra compare well with Kolmogorov’s theory, although a small deviation at high frequencies is observed in the horizontal spectrum within the canopy. Here, for relatively low leaf area index, shear is found to be a more effective mechanism for momentum transfer through the canopy structure than vortex shedding.     

旋转扰动流体中壁面湍流不稳定及其拟序结构研究Ⅰ:实验

重点介绍和讨论了中性条件下旋转扰动流体中边界层强迫不稳定及其相关的一些问题,阐述了旋转体系中切变驱动边界层不稳定的动力学特征.这些不稳定状态的研究在大气物理学、流体动力学、海洋学等多个领域中引起科学家极大的兴趣,近年来在实验和理论研究中都得到了不断的发展.意大利都灵大学基础物理系地球科学实验组通过水槽旋转实验方法,不断改变水槽启动或结束时的旋转运动速度,以及底部壁面粗糙度等要素,所得到的实验结果与SDBL理论非常一致.     

粗糙度研究的现状及展望

略述边界层剪切湍流各区域速度廓线规律和地面粗糙分类后,对各种地面粗糙度(包括沙质粗糙度、动力粗糙度、植被粗糙度、复杂地面粗糙度、有效粗糙度)的研究现状和计算方法作了简要综述,认为动力粗糙度算式中系数A和植被或有效粗糙度算式中位移高度d将会是进一步研究的重点。     

Landscape Roughness Parameters for Sherwood Forest – Validation of Aggregation Models

Various models for calculating the effective or area-averaged roughness length zoe have been tested for a partly forested area. Three types of model are considered: the tile approach for very large scales of inhomogeneity (> 20 km), drag models for very small scales (up to 1 km), and surface-layer methods such as the blending-height method for intermediate scales. Over partly forested areas, where both pressure effects and roughness sub-layer effects may become significant, small-scale models are expected be the most suitable type of model. The various model types were tested against new experimental data that were obtained over the partly forested Sherwood Forest area (UK). The best fit with the data was obtained with the blending-height method, rather than with the different small-scale models. This is remarkable as the surface-layer assumptions of the blending-height method were clearly violated: the calculated blending height was 7 m, as compared to the mean tree height of 20 m. Of the small-scale models, a sparse-canopy approach compared poorly with the experimental data. The drag models overestimated the area-averaged roughness to a lesser degree, but a major point of concern remains the choice of the model parameters. Therefore, suggestions are made for an improved choice of these parameters.     

表面粗糙度对海浪成长率的影响

本文在Miles准层流风生波模型基础上，引入表面粗糙度滑波面不均匀分布的线性模型，通过不稳定性分析，导出了平均压力随表面粗糙度变化和海浪成长率之间的关系，表明表面粗糙度的不均匀分布是影响海浪成长率的一个重要因素。     

SURFACE-LAYER SCALING FOR THE CONVECTION-INDUCED STRESS REGIME

The Prandtl, Obukhov, and Monin andObukhov similarity theories are widely used todescribe the structure of turbulence in theatmospheric surface layer. Currently it isunderstood that in strong convection with no or veryweak mean wind the traditional theory breaks down.In particular, the traditional theory implies asingle-valued correspondence between localturbulence statistics and local properties ofthe flow. In very strong convection, this is nottrue because of large-scale ( 10 3 m) coherentstructures, embracing the entire convective boundarylayer (CBL). These structures produce random guststhat crucially affect surface-layer turbulence andmake it dependent on global properties of theflow, such as the CBL depth. In the present paperthe limits of validity of the traditional surface-layer similarity theory are determined and a revisedtheory of fair weather convection in the surface layeris developed by considering the effect of gustiness. It is shownthat the theoretical predictions are consistent withfield data from the TOGA COARE and SCOPEexperiments.