On the profiles of wind velocity in the roughness sublayer above a coniferous forest

Wind velocity and temperature measurements from a 200 m tower, locatedin a forest near Karlsruhe were used to investigatethe modified profile function of the wind velocity in theroughness sublayer.To avoid determination of the friction velocity we introduced analternative analysis with the expression instead of From the observed Fm* profiles we evaluated the profile function m*. The wind profiles observed under neutral conditions were well representedby a modified non-dimensional profile function with physically based boundary values at the top and at the bottom of theroughness sublayer.The results of our analysis can be used to take into consideration themomentum exchange between the atmosphere and a forest in mesoscaleatmospheric models in a refined way.     

Spatio-temporal Variability of Surface-layer Turbulent Fluxes Over the Bay of Bengal and Arabian sea During the ICARB Field Experiment

We report the spatio-temporal variability of surface-layer turbulent fluxes of heat, moisture and momentum over the Bay of Bengal (BoB) and the Arabian Sea (AS) during the Integrated Campaign for Aerosols, gases Radiation Budget (ICARB) field experiment. The meteorological component of ICARB conducted during March – May 2006 onboard the oceanic research vessel Sagar Kanya forms the database for the present study. The bulk transfer coefficients and the surface-layer fluxes are estimated using a modified bulk aerodynamic method, and then the spatio-temporal variability of these air-sea interface fluxes is discussed in detail. It is observed that the sensible and latent heat fluxes over the AS are marginally higher than those over the BoB, which we attribute to differences in the prevailing meteorological conditions over the two oceanic regions. The values of the wind stress, sensible and latent heat fluxes are compared with those obtained for the Indian Ocean Experiment (INDOEX) period. The variation of drag coefficient (C _D ), exchange coefficients of sensible heat and moisture (C _H = C _E ) and neutral drag coefficient (C _DN ) with wind speed is also discussed.

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Subfilter-Scale Fluxes over a Surface Roughness Transition. Part II: A priori Study of Large-Eddy Simulation Models

The ability of subfilter-scale (SFS) models to reproduce the statistical properties of SFS stresses and energy transfers over heterogeneous surface roughness is key to improving the accuracy of large-eddy simulations of the atmospheric boundary layer. In this study, several SFS models are evaluated a priori using experimental data acquired downwind of a rough-to-smooth transition in a wind tunnel. The SFS models studied include the eddy-viscosity, similarity, non-linear and a mixed model consisting of a combination of the eddy-viscosity and non-linear models. The dynamic eddy-viscosity model is also evaluated. The experimental data consist of vertical and horizontal planes of high-spatial-resolution velocity fields measured using particle image velocimetry. These velocity fields are spatially filtered and used to calculate SFS stresses and SFS transfer rates of resolved kinetic energy. Coefficients for each SFS model are calculated by matching the measured and modelled SFS energy transfer rates. For the eddy-viscosity model, the Smagorinsky coefficient is also evaluated using a dynamic procedure. The model coefficients are found to be scale dependent when the filter scales are larger than the vertical measurement height and fall into the production subrange of the turbulence where the flow scales are anisotropic. Near the surface, the Smagorinsky coefficient is also found to decrease with distance downwind from the transition, in response to the increase in mean shear as the flow adjusts to the smooth surface. In a priori tests, the ability of each model to reproduce statistical properties of the SFS stress is assessed. While the eddy-viscosity model has low spatial correlation with the measured stress, it predicts mean stresses with the same accuracy as the other models. However, the deficiency of the eddy-viscosity model is apparent in the underestimation of the standard deviation of the SFS stresses and the inability to predict transfers of kinetic energy from the subfilter scales to the resolved scales. Overall, the mixed model is found to have the best performance.     

Mean Flow and Turbulence Statistics Over Groups of Urban-like Cubical Obstacles

Direct numerical simulations of turbulent flow over regular arrays of urban-like, cubical obstacles are reported. Results are analysed in terms of a formal spatial averaging procedure to enable interpretation of the flow within the arrays as a canopy flow, and of the flow above as a rough wall boundary layer. Spatial averages of the mean velocity, turbulent stresses and pressure drag are computed. The statistics compare very well with data from wind-tunnel experiments. Within the arrays the time-averaged flow structure gives rise to significant ‘dispersive stress’ whereas above the Reynolds stress dominates. The mean flow structure and turbulence statistics depend significantly on the layout of the cubes. Unsteady effects are important, especially in the lower canopy layer where turbulent fluctuations dominate over the mean flow.     

稻麦轮作农田近地层湍流通量计算方案对比研究

稻麦轮作农田是我国典型农田类型,其模拟效果对我国农田气候模拟具有重要参考价值。气候中尺度模拟结果对近地层通量极为敏感,选择合适的通量计算方案对模拟效果至关重要。因此,对比分析稻麦轮作农田下不同的通量计算方案具有重要意义。本文选取了8种具有代表性的近地层湍流通量计算方案,采用寿县国家气候观象台实测资料对比分析了各方案在稻麦轮作农田的计算特征和差异。结果表明,在不同稳定度和不同风速情况下,各方案的误差特征各异。本文基于归一化标准差综合评价了各方案的准确度,总体而言,所有方案的动量通量总体平均归一化平均差为0.536,其中SS14(Sharan和Srivastava,2014年)方案最大为0.575,SS20(Sharan和Srivastava等,2020年)方案最小为0.517;所有方案的感热通量总体平均归一化标准差为0.638,其中GLGS20(Gryanik等,2020年)方案最大为0.871,SS14方案最小为0.476。此外,本研究还给出了稻麦轮作农田不同稳定层结和不同风速情况下各通量计算方案的误差特征。本文的研究结论,可为准确计算近地层湍流通量提供支撑。     

盘锦芦苇湿地空气动力学参数动态特征及其影响因素分析

应用盘锦芦苇生态系统观测场的涡度相关通量观测系统和小气候梯度系统的观测资料,估算了芦苇下垫面的空气动力学参数,并分析了其影响因素。结果表明:芦苇湿地零平面位移d和粗糙度Z0的季节变化总体呈现先增加后减少的单峰曲线变化规律。Z0和d均在9月达到最大,分别为0.24 m和1.85 m;12月达到最小,分别为0.03 m和0.02 m。芦苇湿地的Z0和d的季节变化主要受叶面积指数(LAI)、株高(h)及风速的影响。d随h的增加而单调增加,Z0随着h的变化遵循二次曲线,d/h、Z0/h与LAI之间的复相关系数分别为0.99和0.78。研究亦表明,大气稳定度和风向也对d和Z0有一定的影响。     

黄河下游动床阻力变化及其计算方法

动床阻力在冲积河道洪水演进与河床冲淤计算中具有十分重要的作用。黄河下游不同水沙条件下的床面形态变化较大,动床阻力变化规律十分复杂,因此需要研究动床阻力的计算方法。利用黄河下游花园口、高村、利津等7个水文站1958—1990年的686组实测数据,确定了影响动床阻力变化的关键水沙因子——水流弗劳德数（Fr）与相对水深（h/D₅₀）,前者表示水流强度,后者表示床面相对粗糙度;建立了基于水流能态分区的动床阻力计算公式,并采用这些实测数据率定了公式中的相关参数;利用黄河下游各水文站1991—2016年的2 288组实测资料,进一步验证了公式的计算精度。计算结果表明:动床阻力的大小随弗劳德数或相对水深的增加而减小;基于水流能态分区动床阻力公式的计算精度明显高于未分区的公式及其他4个动床阻力公式,且决定系数（R2）总体接近0.80,说明水流强度与床面相对粗糙度对动床阻力影响十分显著。     

不同形态磷酸盐对铜绿微囊藻和蛋白核小球藻生长的影响

地表糙度是衡量地表径流形成的一个重要指标．以东北区典型泥炭沼泽——金川泥炭沼泽为例,考虑地表坡度影响,用链条法对不同群落中糙度系数指标进行计算,并以泥炭沼泽中物种多样性、踏头个数以及踏头形态差异变率系数为主,初步分析泥炭泥泽地表糙度的变化机制,结果表明：在芦苇-臌囊苔草群落、臌囊苔草群落和油桦-臌囊苔草-泥炭藓群落三个植物群落中,糙度系数指标的变化与群落中物种丰富度和生物多样性变化相一致;糙度系数变化与踏头个数呈负相关关系,与踏头形态差异变率系数呈正相关关系说明物种多样性越小,生态优势度越大,优势种植物越少,踏头个数越多,踏头形态变率系数越小,微地貌形态发育相对较一致,糙度系数越小;反之,则地表糙度系数越大。     

城市下垫面空气动力学参数的估算

为了定量描述北京城市下垫面的空气动力学特征,为模式提供准确的下垫面参数.利用气象塔大气湍流观测资料,结合Martano(2000)由单层超声风速、温度资料估算非均匀下垫面空气动力学参数的方法,计算了中国科学院大气物理研究所气象塔附近的下垫面空气动力粗糙度z0和零平面位移d,即z0为1.75 m和d为40.12 m.该结果与前人研究成果的比较结果表明最近的8年内,该处的零平面位移和空气动力粗糙度明显增大,这与该塔周围城市建设状况吻合.     

空气动力学粗糙度的物理与实践意义

空气动力学粗糙度是风沙边界层的一个重要参数。文章回顾了空气动力学粗糙度概念的由来;用流体力学的量纲分析法和数学方法,从两个不同的角度推导了空气动力学粗糙度;阐述了空气动力学粗糙度的物理与实践意义;简明介绍了空气动力学粗糙度的分类和各种不同下垫面空气动力学粗糙度的主要研究成果,就目前研究中存在的问题、研究趋势等方面做了探讨。