Methodological Considerations Regarding the Measurement of Turbulent Fluxes in the Urban Roughness Sublayer: The Role of Scintillometery

We address some of the methodological challenges associated with the measurement of turbulence and use of scintillometers in the urban roughness sublayer (RSL). Two small-aperture scintillometers were located near the roof interface in a densely urbanized part of Basel, Switzerland, as part of the Basel Urban Boundary-Layer Experiment (BUBBLE) in the summer of 2002. Eddy correlation instruments were co-located near the mid-point of each scintillometer path for data verification purposes. The study presents the first values of the inner length scale of turbulence (l ₀) and the refractive index structure parameter of air for a city and demonstrates the influence of mechanical driven turbulence on dissipation. Comparison of dissipation values determined from the two approaches show large scatter that is possibly due to the spatial inhomogeneity of the turbulence statistics within the RSL. Velocity and temperature spectra display a −2/3 slope in the inertial subrange, although the spectral ratio is less than the theoretical prediction of 4/3 expected for isotropy. Conventional Monin–Obukhov equations used to calculate fluxes from the scintillometer were replaced with urban forms of the equations. The results suggest that the scintillometer may be an appropriate tool for the measurement of sensible heat flux (Q _H ) above the rooftops given a suitable determination of the effective measurement height.     

Northern Aegean Earthquake (Mw=6.9): Observations at three seismic downhole arrays in Istanbul

The Northern Aegean Earthquake (M_w=6.9\M_L=6.5) took place on May 24, 2014 between the islands of Gokceada, Turkey and Samothraki, Greece. The tremors were felt as far as in Istanbul, about 300 km on the East – Northeast (ENE) side of the epicenter. Kandilli Observatory and Earthquake Research Institute (KOERI) of Bogazici University, Turkey operate three downhole arrays in Istanbul, namely Atakoy (ATK), Fatih (FTH) and Zeytinburnu (ZYT) arrays. In this study, waveforms and site response observed at the KOERI operated seismic downhole arrays during the May 2014 Northern Aegean Earthquake (NAE2014) are analyzed in detail and presented. Evaluation of the acceleration records have shown low amplitude but long period and long duration motions at Istanbul. Furthermore, the analyses of the recordings suggest that V_s30 alone may not be a sufficient parameter for the characterization of site amplification.     

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.     

直立浮式柱群波浪荷载计算

为了研究类似张力腿平台（ＴＬＰ）等海洋工程结构物的运动,浮式直立柱群的波浪荷载计算是十分重要的。应用匹配渐进和特征函数展开法得到单柱流场速度势的半解析解,再引入大间距假设,将柱群中某一柱体对其它柱体的扰动作用用平面入射波来代替。同时对该问题进行了非平面波修正,计入柱间水动力相互作用,解决了柱群的绕射问题。应用该方法计算了不同条件下的双柱波浪荷载的幅频特征。     

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。