Estimation of sector roughness lengths and the effect on prediction of the vertical wind speed profile

An estimate of roughness length is required by some atmospheric models and is also used in the logarithmic profile to determine the increase of wind speed with height under neutral conditions. The choice of technique for determining roughness lengths is generally constrained by the available input data. Here, we compare sets of roughness lengths derived by different methods for the same site and evaluate their impact on the prediction of the vertical wind speed profile.Wind speed and direction data have been collected at four heights over a three-year period at the North Norfolk Wind Monitoring Site. Wind speed profiles were used to generate sector roughness lengths based on the logarithmic profile formula. This is the only direct way of determining roughness lengths. The simplest and cheapest method is to use maps with published tables giving roughness length estimates for different terrain types. Alternatively Wieringa (1976, 1986) and Beljaars (1987) give formulae for determining roughness lengths from wind speed gusts or standard deviations.The four sets of estimated roughness lengths vary considerably. They were used to estimate 34 m wind speeds from 12.7 m observations. The profile-derived roughnesses are used simply as a check on the prediction of the wind speed profiles. The terrain-derived roughness lengths give reasonable results. Gust-derived and standard deviation roughnesses both predict wind speeds which are lower than the observed ones. The error is greater in the case of standard deviation roughnesses. If stability corrections are applied in the prediction of the vertical wind speed profile, the results are considerably improved.     

Stability Dependence of Height Scales and Effective Roughness Lengths of Momentum and Heat Transfer Over Roughness Changes

It is widely accepted that the correct formulation of an effective roughness length, defined as the area average of the roughness length in heterogeneous terrain, relies upon the appropriate de-termination of a height scale. At this height a meteorological quantity is approximately in equilibrium with local surface conditions and independent of horizontal position. This research note determines explicitly the different height scales from the perturbation solutions of flow velocity and temperature, as well as the fluxes of momentum and heat, in a stratified boundary layer. These solutions are derived from the asymptotic approximation theory and shown to capture major characteristics of momentum and heat transfer over heterogeneous terrain with changes of the underlying roughness lengths. The effective roughness lengths can then be computed by use of these height scales. The dependence of height scales and effective roughness lengths upon stratification is also discussed briefly.     

Representative roughness parameters for homogeneous terrain

Objective requirements are drawn up for experimental determination of the roughness of homogeneous terrain, particularly with respect to matching of the observation array to a terrain situation with given fetch, blending height and displacement length. Some fifty well-documented published experiments over various surfaces, ranging from mobile surfaces (sea, or moving sand or snow) to forests and towns, are shown to meet these criteria and are compiled. It is shown that most presently popular terrain reviews underestimate actual terrain roughness lengths by about a factor two.     

Estimates of effective surface roughness over complex terrain

Turbulence data collected in an area of three-dimensional complex terrain using instruments atteched to the tether cable of a captive balloon together with radiosonde ascents are presented. In addition, data collected using only radiosonde ascents in an area of two-dimensional complex terrain of large slope are also shown. Eddy correlation measurements of the turbulent momentum flux and wind velocity profiles are used to deduce the magnitude of the effective roughness from the drag coefficient and normalised velocity profiles. A relationship connecting the terrain characteristics and the roughness length is compared with the experimental data for both types of terrain plus previous experimental estimates of the roughness length over complex terrain. The formula taken from previous work by Grant and Mason (1990) is found to agree with the data when representing an area of order 100 km².     

Roughness lengths for temperature and momentum over heterogeneous terrain

Measurements are presented describing the behaviour of roughness lengths for momentum and temperature for heterogeneous terrain of low fractional rough cover. The momentum roughness lengths were found to be up to several orders of magnitude larger than that for temperature, behaviour which is characteristic of a bluff-rough type of surface and which is consistent with previous experimental and theoretical studies. This contrasts with observations over uniform, vegetated surfaces, which show only an order-of-magnitude increase of momentum roughness length over that for temperature.     

Wind characteristics at the Boulder Atmospheric Observatory

Wind speeds at the 300 m tower at the Boulder Atmospheric Observatory have been analyzed. This tower is located in slightly rolling farmland. The following conclusions have been reached:

1. For west winds, the terrain is sufficiently uniform for simple surface-layer theory to be adequate without modification even though the air has moved up a small slope to reach the tower. For south and southeast winds, ‘effective’ roughness lengths must be introduced, which are significantly larger than the ‘true’ roughness length.
2. Useful wind estimates up to 150 m can be made from winds at 10 m and stability information, provided the ‘effective’ roughness length is known.
3. The observations are consistent with a von Kármán constant of 0.35.

     

Variances and spectra of vertical velocity just above the surface layer

Variances and spectra of vertical velocity at 75 m and 225 m obtained by aircraft have been analyzed. Given effective roughness lengths for the different terrain types, nomograms for the standard deviations were constructed as functions of wind speed and lapse rate, guided by theoretical considerations.Normalized spectra agree well with analogous spectra obtained elsewhere, and with requirements of similarity theory; however, over mountainous terrain, low-frequency spectra peaks appear, the frequency of which appears to be independent of height.     

Comments and further analysis on effective roughness lengths for use in numerical three-dimensional models

Simple calculations of the apparent roughness length for the areally averaged flow over flat but heterogeneous terrain are presented. These results could be used to specify effective roughness lengths for use in large-scale models. Some of our conclusions differ significantly from those reached recently by André and Blondin (1986).     

A Measure of Inhomogeneity of the Land Surface and Parametrization of Turbulent Fluxes under Natural Conditions

Summary The effects of surface heterogeneity and nonstationarity on turbulent fluxes are parametrized by introducing an additional roughness length that corrects for deviations from Stationarity and Horizontal Homogeneity (SHH). Using data from the experiments of FIFE, KUREX, TARTEX and SADE, a relationship was detected between the underestimation of turbulent fluxes (the lack of balance in the surface energy budget) and terrain inhomogeneity. Based on the assumption that the parametrization obtained from these data is applicable for complex terrain surfaces in general, the paper presents some simple relationships for surface turbulent fluxes, bulk transfer coefficients, and roughness lengths. Received August 8, 1997 Revised March 9, 1998     

Wind profiles,momentum fluxes and roughness lengths at Cabauw revisited

We describe the results of an experiment focusing on wind speed and momentum fluxes in the atmospheric boundary layer up to 200 m. The measurements were conducted in 1996 at the Cabauw site in the Netherlands. Momentum fluxes are measured using the K-Gill Propeller Vane. Estimates of the roughness length are derived using various techniques from the wind speed and flux measurements, and the observed differences are explained by considering the source area of the meteorological parameters. A clear rough-to-smooth transition is found in the wind speed profiles at Cabauw. The internal boundary layer reaches the lowest k-vane (20 m) only in the south-west direction where the obstacle-free fetch is about 2 km. The internal boundary layer is also reflected in the roughness lengths derived from the wind speed profiles. The lower part of the profile (< 40 m) is not in equilibrium and no reliable roughness analysis can be given. The upper part of the profile can be linked to a large-scale roughness length. Roughness lengths derived from the horizontal wind speed variance and gustiness have large footprints and therefore represent a large-scale average roughness. The drag coefficient is more locally determined but still represents a large-scale roughness length when it is measured above the local internal boundary layer. The roughness length at inhomogeneous sites can therefore be determined best from drag coefficient measurements just above the local internal boundary layers directly, or indirectly from horizontal wind speed variance or gustiness. In addition, the momentum and heat fluxes along the tower are analysed and these show significant variation with height related to stability and possibly surface heterogeneity. It appears that the dimensionless wind speed gradients scale well with local fluxes for the variety of conditions considered, including the unstable cases.     

Relations among stability parameters in the surface layer

Observations at 5 sites have been analyzed to determine relations among variables used to assess the effect of stability on dispersion. The variables compared were Monin-Obukhov length, Richardson number, bulk Richardson number and Pasquill and Turner categories. All the relationships differ with different terrain roughness.     

Sensitivity of Turbine-Height Wind Speeds to Parameters in Planetary Boundary-Layer and Surface-Layer Schemes in the Weather Research and Forecasting Model

We evaluate the sensitivity of simulated turbine-height wind speeds to 26 parameters within the Mellor–Yamada–Nakanishi–Niino (MYNN) planetary boundary-layer scheme and MM5 surface-layer scheme of the Weather Research and Forecasting model over an area of complex terrain. An efficient sampling algorithm and generalized linear model are used to explore the multiple-dimensional parameter space and quantify the parametric sensitivity of simulated turbine-height wind speeds. The results indicate that most of the variability in the ensemble simulations is due to parameters related to the dissipation of turbulent kinetic energy (TKE), Prandtl number, turbulent length scales, surface roughness, and the von Kármán constant. The parameter associated with the TKE dissipation rate is found to be most important, and a larger dissipation rate produces larger hub-height wind speeds. A larger Prandtl number results in smaller nighttime wind speeds. Increasing surface roughness reduces the frequencies of both extremely weak and strong airflows, implying a reduction in the variability of wind speed. All of the above parameters significantly affect the vertical profiles of wind speed and the magnitude of wind shear. The relative contributions of individual parameters are found to be dependent on both the terrain slope and atmospheric stability.     

半干旱区榆中地表粗糙度年变化及影响机理

利用2006年6月—2010年12月兰州大学半干旱气候与环境观测站 (SACOL) 观测资料,分析了黄土高原自然植被下垫面榆中地表粗糙度时间变化特征,考虑到地形、植被物理特征以及降水和热力条件的影响,分析了东南风向和西北风向粗糙度年变化规律及其影响机理,并分别给出这两个风向归一化粗糙度与时间的拟合关系式。研究发现:对于非均一下垫面,由于地形起伏和下垫面植被差别造成的不同风向粗糙度差异显著。选取东南风向和西北风向,这两个风向的地表粗糙度无论是量级还是年变化特征都有很大差别,且由于地形和植被的差别,东南风向粗糙度年变化趋势与稳定度年变化趋势一致,粗糙度与稳定度存在一定相关关系,而西北风向粗糙度年变化趋势与降水量年变化趋势一致,粗糙度与降水量相关性较好。     

Charnock’s Roughness Length Model and Non-dimensional Wind Profiles Over the Sea

An analysis tool for the study of wind speed profiles over the water has been developed. The profiles are analysed using a modified dimensionless wind speed and dimensionless height, assuming that the sea surface roughness can be predicted by Charnock’s roughness length model. In this form, the roughness dependency on wind speed is extracted and the variations on the wind profile are due solely to atmospheric stability. The use of the Charnock’s non-dimensional wind profile is illustrated using data collected from a meteorological mast installed in the Danish North Sea. The best fit with the observed mean non-dimensional wind profile under neutral atmospheric conditions is found using a value of 1.2 × 10⁻² for Charnock’s parameter. The stability correction on the neutral wind profile suggested by the Businger-Dyer relations was found to perform well over the sea.     

区域气候模式RegCM2对标量粗糙敏感性试验

在地气通量的计算中,一般没有考虑温度、水汽、动量的相应粗糙度之间的不同。本文将标量粗糙度Z0T和Z0q引入区域气候模式RegCM2的陆面过程BATS中,用1991年6月与7月的观测资料作了三组敏感性试验,并同实况进行了比较。结果表明：在区域气候模式RegCM2中引入标量粗糙度后,提高了地气间感热通量与潜热通量的计算精度,改善了地表温度和地表比湿的模拟,进而改变了降水的模拟,表明区域气候模式对标量粗糙是敏感的,并且在晴天状况下更为敏感。     

青藏高原大气边界层湍流特征量分析

采用１９７９年青藏高原气象科学实验资料,研究高原陆面上总体湍流送系数ＣＤ和ＣＨ的特征及边界层高度受高原地形的影响。结果表明用廓线法计算的高原上局地的ＣＤ和ＣＨ值比同样粗糙度长度条件下平原地区的值大得多,而且日较差也大于平原地区。     

Effective Roughness Calculated from Satellite-Derived Land Cover Maps and Hedge-Information used in a Weather Forecasting Model

In numerical weather prediction, climate and hydrologicalmodelling, the grid cell size is typically larger than the horizontal length scales of variations in aerodynamicroughness, surface temperature and surface humidity. These local land cover variations give rise to sub-gridscale surface flux differences. Especially the roughness variations can give a significantly differentvalue between the equilibrium roughness in each of the patches as compared to the aggregated roughness value,the so-called effective roughness, for the grid cell. The effective roughness is a quantity that secures thephysics to be well-described in any large-scale model. A method of aggregating the roughness step changesin arbitrary real terrain has been applied in flat terrain (Denmark) where sub-grid scale vegetation-drivenroughness variations are a dominant characteristic of the landscape. The aggregation model is a physicaltwo-dimensional atmospheric flow model in the horizontal domain based on a linearized version of theNavier Stoke equation. The equations are solved by the Fast Fourier Transformation technique, hence the codeis very fast. The new effective roughness maps have been used in the HIgh Resolution Limited Area Model(HIRLAM) weather forecasting model and the weather prediction results are compared for a number of casesto synoptic and other observations with improved agreement above the predictions based on currentstandard input. Typical seasonal springtime bias on forecasted winds over land of +0.5 m s^-1 and-0.2 m s^-1 in coastal areas is reduced by use of the effective roughness maps.     

Velocity spectra over different terrains

Power spectra of longitudinal and vertical components of wind velocity have been measured over a variety of terrain types: uniform, inhomogeneous and urban. The result shows that the spectral shapes and peaks of u -spectra are sensitive to local terrain, while those of w -spectra are not affected by changes in roughness surface.     

Update of a Footprint-Based Approach for the Characterisation of Complex Measurement Sites

Horizontal heterogeneity can significantly affect the flux data quality at monitoring sites in complex terrain. In heterogeneous conditions, the adoption of the eddy-covariance technique is contraindicated by the lack of horizontal homogeneity and presence of advective conditions. In addition, uncertainty concerning the sources or sinks influencing a measurement compromises the data interpretation. The consideration of the spatial context of a measurement, defined by a footprint analysis, can therefore provide an important tool for data quality assessment. This study presents an update of an existing footprint-based quality evaluation concept for flux measurement sites in complex terrain. The most significant modifications in the present version are the use of a forward Lagrangian stochastic trajectory model for the determination of the spatial context of the measurements, and the determination of effective roughness lengths with a flux aggregation model in a pre-processing step. Detailed terrain data gathered by remote sensing methods are included. This approach determines spatial structures in the quality of flux data for varying meteorological conditions. The results help to identify terrain influences affecting the quality of flux data, such as dominating obstacles upwind of the site, or slopes biasing the wind field, so that the most suitable footprint regions for the collection of high-quality datasets can be identified. Additionally, the approach can be used to evaluate the performance of a coordinate rotation procedure, and to check to what extent the measured fluxes are representative for a target land-use type.     

A theory for the scalar roughness and the scalar transfer coefficients over snow and sea ice

Although the bulk aerodynamic transfer coefficients for sensible (C _H ) and latent (C _E ) heat over snow and sea ice surfaces are necessary for accurately modeling the surface energy budget, they have been measured rarely. This paper, therefore, presents a theoretical model that predicts neutral-stability values of C _H and C _E as functions of the wind speed and a surface roughness parameter. The crux of the model is establishing the interfacial sublayer profiles of the scalars, temperature and water vapor, over aerodynamically smooth and rough surfaces on the basis of a surface-renewal model in which turbulent eddies continually scour the surface, transferring scalar contaminants across the interface by molecular diffusion. Matching these interfacial sublayer profiles with the semi-logarithmic inertial sublayer profiles yields the roughness lengths for temperature and water vapor. When coupled with a model for the drag coefficient over snow and sea ice based on actual measurements, these roughness lengths lead to the transfer coefficients. C _E is always a few percent larger than CH. Both decrease monotonically with increasing wind speed for speeds above 1 m s^–1, and both increase at all wind speeds as the surface gets rougher. Both, nevertheless, are almost always between 1.0 × 10^–3 and 1.5 × 10^–3.