一方程模型在植被冠层中的应用

利用建立的一方程模式对植被气象场和湍流场进行了模拟计算。结果表明：在植被叶面积密度最大值处，由植被叶面积产生的阻力使风速急剧减小，而后平缓接近地面风速值。Reynolds应力从植被顶部向下剧烈减小，在Z／Hc值为0．4～0．6之间衰减迅速；冠层下部风速小且切变弱，湍流未能充分发展，因而动量输送甚微。由于在植被内部环境中存在着湍流通量的辐散或辐合的现象，湍流强度从植被底部开始由下至上逐渐增强，在冠层上方则基本保持不变。     

植被冠层湍流代数应力模式

从植被冠层湍流时空平均雷诺应力方程出发，根据植被冠层内有关参数化的准则，建立了湍流代数应力模式，并用玉米和小麦两种作物地的实测资料对模式进行了验证。     

二维均匀剪切湍流的湍涡闭合方案

通过仔细分析Thorpe与Woods等对湍流的典型实验,得到如下结论:(1)提出了湍流的太极模型;(2)指出了湍流串级的内容包括2个内容,一是湍流外圈层(大涡)在内旋到内部时产生新的圈层(小涡),二是在阴阳旋臂相切的锋面由于依然存在较大切变而产生新的次尺度湍涡;(3)指出湍流的耗散机制是通过湍涡,把具有切变的2流场的交界面通过旋转而无限拉长以增大2者的接触面积,加强了交界面之间的分子交换,从而实现了湍流层层混合的高效率;(4)基于湍涡结构,提出了湍涡闭合方案,给出了Reynolds应力的表达式,闭合了雷诺方程;(5)混合长反映了湍涡的大小,Karman常数则反映了湍涡内外半径比的关系;(6)指出Reynolds方程算子是湍流方程推导的关键,提出了湍流算子的概念;(7)推导出了新型的湍流运动方程;(8)提出了关系不变性原理。     

植冠层结构对其湍流场影响的数值试验

根据湍流应力模式的模拟结果，讨论了植被冠层中湍流能量的平衡特征，并通过数值试验就冠层结构对流场结构的影响进行了分析。     

湍流的粘性和频散效应

本文应用Prandtl混合长理论导得了既包含湍流粘性又包含湍流频散的新的Reynolds平均运动方程组.分析指出:(1)湍流不仅存在粘性效应,而且存在频散效应;(2)正是湍流的频散效应可能导致能量逆转(即常说的负粘性现象,实质为一定条件下的负频散现象),并给出了能量逆转的必要条件和充分条件;(3)给出了湍流的KdV-Burgers方程模型.     

大气边界层湍流能量交换特征研究进展

从一维湍流能量平衡方程出发,回顾了近几十年湍流能量平衡方程中的各项以及Karman常数k的研究成果,总结了大气边界层湍流能量交换特征的研究概况和热点问题,并对今后发展趋势做了展望。实验研究表明,湍流能量平衡方程中的各项在不同条件下有不同的形式;传统的能量产生和耗散的局地平衡假设存在不足,特别是在不稳定条件下,垂直的湍流输运和压力脉动对湍流能量收支起了非常重要的作用。Karman常数与Rossby数和Reynolds数无关,在比较光滑的下垫面上k近似等于040±001。     

湍流的频散效应对行星边界层大气平衡运动的影响

本文应用包含湍流粘性和湍流频散效应的新的Reynolds平均运动方程求解了行星边界层中大气的平衡运动,着重分析了湍流频散效应对平衡运动的影响。分析指出:(1)考虑了湍流频散效应后,在γ>0时,Ekman层风速随高度的变化减缓,而Ekman层厚度增大,新的Ekman螺线更加附会于实际;(2)考虑了湍流频散效应后,近地面层中性层结下的风速随高度变化仍呈对数定律,只是Karman常数     

植被内水平风速脉动特征

利用QDF型热线微风仪在农田植被内进行了水平风速脉动的观测。初步认为,植被内水平风速脉动具有比裸地湍流度大、脉动强度小、短周期振动强、湍流积分尺度小的特征,且随时间变化平稳,其时间结构函数基本符合“2/3”幂律(τ≤16秒)。     

植被内水平风速脉动特征

利用QDF型热线微风仪在农田植被内进行了水平风速脉动的观测。初步认为,植被内水平风速脉动具有比裸地湍流度大、脉动强度小、短周期振动强、湍流积分尺度小的特征,且随时间变化平稳,其时间结构函数基本符合“2/3”幂律(τ≤16秒)。     

湍流频散对边界层风廓线的影响

应用包括湍流粘性和频散的新的Reynolds平均动量方程，分析了边界层的垂直风速廓线，发现包含湍流频散的地面层的风速廓线对经典的风廓线指数规律有一个对数规律的修改；而且在不稳定层结下比在稳定层结下，湍流的频散效应更为显；在中性条件下，指数规律退化为对数规律并且Karman常数被另外一个常数所代替，而这个新常数也可以通过相似理论来获得。     

NUMERICAL SIMULATIONS OF TURBULENT FLOW IN AND ABOVE PLANT CANOPIES

The turbulent flow in and above plant canopies is of fundamental importance to the understanding oftransport processes of momentum,heat and mass between plant canopies and atmosphere,and to microme-teorology.The Reynolds stress equation model(RSM)has been applied to calculate the turbulence in cano-pies in this paper.The calculated mean wind velocity profiles,Reynolds stress,turbulent kinetic energy andviscous dissipation rate in a corn canopy and a spruce forest are compared with field observed data and withWilson's and Shaw's model.The velocity profiles and Rynolds stress calculated by both models are in goodagreement,and the length scale of turbulence appears to be similar.     

On the downward transport of turbulent kinetic energy in the surface layer over plant canopies

Downward fluxes of turbulent kinetic energy have been frequently observed in the air layer just above plant canopies. In order to investigate the mechanism for such downward transport, analysis of observational data is attempted. Height-dependency of turbulent kinetic energy flux and turbulence statistics including higher order moments is represented as a function of a non-dimensional height z/H, where z is an observational height and H an average height of plant canopies. Downward fluxes and non-Gaussianity of wind velocity fluctuations are predominant just above plant canopies and decrease with increasing height. The downward flux is closely related to the high intensity of turbulence and the non-Gaussianity of wind velocity fluctuations, especially with a positive skewness in the longitudinal wind and a negative skewness in the vertical wind. The analysis method of conditional sampling and averaging is applied to the present observations. The results show that the predominance of the intermittent inrush phase over the intermittent ejection phase leads to the above-mentioned non-Gaussianity. Finally, a simple explanation is given in order to interpret the turbulent flow structure in the air layer near the plant canopies, which is associated with the downward energy transport process.     

An assessment of mixing-length closure schemes for models of turbulent boundary layers over complex terrain

The effectiveness of closure assumptions implemented in turbulent boundary-layer models is rather uncertain over complex terrain. Different closure schemes for Reynolds shear stress based on the mixing-length concept are compared with data from wind tunnel experiments over complex terrain and the results are analysed on the basis of second-order moment equations. A good estimation of the vertical momentum flux velocity scale turns out to be given by the standard deviation of the vertical velocity while the turbulent kinetic energy scaling gives less satisfactory results in regions where turbulence anisotropy is large. Fairly good results are given by closure models implementing a shear-limited mixing-length already proposed for non-logarithmic wind profiles, while large errors characterize traditional mixing-length formulations.     

The evolution of a convective planetary boundary layer — A higher-order-closure model study

The evolution of the boundary layer on day 33 of the Wangara experiment in southeast Australia is calculated with a higher-order-closure turbulence model. This model, which includes equations for the mean field as well as the second moments of the turbulent field, is described in detail. The mean profiles of wind, temperature, and humidity, the profiles of heat and humidity fluxes, the Reynolds stress distributions, and the height of the boundary layer are shown between 10 a.m. and 6 p.m. The results agree well with those from Deardorff's 3-D simulation and take relatively little computer time.     

Comparison of turbulence statistics within three boreal forest canopies

Three-dimensional sonic anemometers were used to measure velocities and temperatures within three natural boreal forest canopies. Vertical profiles of atmospheric turbulence statistics for a black spruce forest, a jack pine forest, and a trembling aspen forest, all located in southeastern Manitoba, were plotted and compared. The canopy structures were quite different, with total leaf-area indices of 2, 4 and 10, for the pine, aspen, and spruce forests, respectively.The profiles of the first and second moments differed among the canopies, where velocities decreased more rapidly in the top portions of the denser canopies. The velocity distributions were skewed and kurtotic within all canopies, and showed some differences among the canopies. Eulerian time scale profiles were generally similar among the canopies, and the vertical and streamwise time scale profiles were almost mirror images of each other. Eulerian length scale profiles showed some differences among canopies caused by differences in the velocity profiles. Ratios of vertical-to-horizontal time and length scales had a maximum in mid-canopy.Shear stress profiles were similar in the top parts of all canopies, and upward momentum fluxes were occasionally observed within the canopy trunk space. Countergradient heat fluxes were also observed sometimes. The countergradient fluxes and the skewed, kurtotic velocity distributions indicate the contribution of intermittent, large-scale eddies that are important for energy and mass transfer within canopies.     

重庆市区冬季小风条件下近地面层某些湍流特征量的分析

利用１９８９年１２月１６日－１９９０年１月１４日在重庆市用三轴风速仪所测得的风速资料，计算了小风条件下重庆近地面层的湍流参数，欧拉时间自相关系数，湍流时间积分尺度，湍流强度，摩擦速度，湍流应力等湍流特征量，以了解重庆地区小风条件下的近地面层的湍流场特征。     

Effects of precipitation on the eddy exchange in a wind-driven sea

Effects of precipitation on the temperature and salinity profiles in the top 2 or 3 m of the sea are calculated for four models of turbulent exchange: (1) constant eddy exchange with depth; (2) drift-current eddy exchange; (3) eddy exchange produced by shear in the orbital velocity under a surface wave; and (4) drift current and wave turbulence acting together. The surface temperature and salinity are only weakly affected, but the density gradient can become significantly stabler. The resulting influence on the eddy exchange is modeled with empirical formulas employing the Richardson number. Stability strongly damps drift-current eddy exchange but has much less effect on the wave-induced turbulence.The energy budget shows an increase in turbulent dissipation near the surface for weak stability when the surface stress in assumed to remain constant. Beyond a critical Richardson number of about 0.9 energy consumed by turbulent energy dissipation decreases. This occurs more due to a reduction in mechanical energy production than because of energy being used for the redistribution of mass.     

Large-eddy simulation of turbulent flow above and within a forest

A large-eddy simulation has been performed of an atmospheric surface layer in which the lower third of the domain is occupied by a drag layer and heat sources to represent a forest. Subgridscale processes are treated using second-order closure techniques. Lateral boundaries are periodic, while the upper boundary is a frictionless fixed lid. Mean vertical profiles of wind velocity derived from the output are realistic in their shape and response to forest density. Similarly, vertical profiles of Reynolds stress, turbulent kinetic energy and velocity skewness match observations, at least in a qualitative sense. The limited vertical extent of the domain and the artificial upper boundary, however, cause some departures from measured turbulence profiles in real forests. Instantaneous turbulent velocity and scalar fields are presented which show some of the features obtained by tower instrumentation in the field and in wind tunnels, such as the vertical coherence of vertical velocity and the slope of structures revealed by temperature patterns.     

Measurements of turbulence structure in the boundary layer on a rough surface

Mean products of velocity fluctuations up to fourth order have been measured in a wind tunnel at the trailing edge of a flat plate, one side of which was covered with floor-sanding paper to produce a fully rough surface. This set-up permits easy comparison of structural parameters in smooth-wall and rough-wall boundary layers. The Reynolds-stress profiles and second-order parameters are closely the same on the rough and smooth surfaces; in particular the decrease in Reynolds shear stress near the rough surface, encountered by several other laboratory workers, was not found in the present results. The triple products are spectacularly altered for a distance of up to 10 roughness heights from the rough surface, and imply a large net rate of transport of turbulent energy and shear stress towards the surface. Comparison with other published data shows that the behaviour of this modified region depends on roughness geometry as well as on the roughness height itself; for example, the mean cube of the normal-component fluctuation remains positive (energy transport away from the surface) over sand or gravel roughness but goes negative, like the other energy-transport terms, over crop canopies.     

An Investigation of Higher-Order Closure Models for a Forested Canopy

Simultaneous triaxial sonic anemometer velocity measurements vertically arrayed at six levels within and above a uniform pine forest were used to examine two parameterization schemes for the triple-velocity correlation tensor employed in higher-order closure models. These parameterizations are the gradient-diffusion approximation typically used in second-order closure models, and the full budget for the triple-velocity correlation tensor typically employed in third-order closure models. Both second- and third-order closure models failed to reproduce the measured profiles of the triple-velocity correlation within and above the canopy. However, the Reynolds stress tensor profiles (including velocity variances) deviated greatly from the measurements only within the lower levels of the canopy. It is shown that the Reynolds stresses are most sensitive to the parameterization of the triple-velocity correlation in these lower canopy regions where local turbulent production is negligible and turbulence is mainly sustained by the flux transport term. The failure of the third-order closure model to reproduce the measured third moments in the upper layers of the canopy-top contradicts conclusions from a previous study over shorter vegetation but agrees with another study for a deciduous forest. Whether the third-order closure model failure is due to the zero-fourth-cumulant closure approximation is therefore considered. Comparisons between measured and predicted quadruple velocity correlations suggest that the zero-fourth-cumulant approximation is valid close to the canopy-atmosphere in agreement with recent experiments.