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1.
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.  相似文献   

2.
Aerodynamic roughness of the sea surface at high winds   总被引：2，自引：0，他引：2  
The role of the surface roughness in the formation of the aerodynamic friction of the water surface at high wind speeds is investigated. The study is based on a wind-over-waves coupling theory. In this theory waves provide the surface friction velocity through the form drag, while the energy input from the wind to waves depends on the friction velocity and the wind speed. The wind-over-waves coupling model is extended to high wind speeds taking into account the effect of sheltering of the short wind waves by the air-flow separation from breaking crests of longer waves. It is suggested that the momentum and energy flux from the wind to short waves locally vanishes if they are trapped into the separation bubble of breaking longer waves. At short fetches, typical for laboratory conditions, and strong winds the steep dominant wind waves break frequently and provide the major part of the total form drag through the air-flow separation from breaking crests, and the effect of short waves on the sea drag is suppressed. In this case the dependence of the drag coefficient on the wind speed is much weaker than would be expected from the standard parameterization of the roughness parameter through the Charnock relation. At long fetches, typical for the field, waves in the spectral peak break rarely and their contribution to the air-flow separation is weak. In this case the surface form drag is determined predominantly by the air-flow separation from breaking of the equilibrium range waves. As found at high wind speeds up to 60 m s−1 the modelled aerodynamic roughness is consistent with the Charnock relation, i.e. there is no saturation of the sea drag. Unlike the aerodynamic roughness, the geometrical surface roughness (height of short waves) could be saturated or even suppressed when the wind speed exceeds 30 m s−1.  相似文献   

3.
Surface-based and aircraft measured fluxes over the heterogeneous surface in HAPEX-MOBILHY are analyzed for the ten flight days when cloud cover above the boundary layer was minimal. The fair-weather climatology of the spatial variation of surface fluxes is estimated to provide an assessment of the generality of previous case studies appearing in the literature. For the 10-day averages, greater heating over the forest generates a forest breeze which leads to rising motion and a modest increase of boundary-layer cloud cover at the forest edge. The exchange coefficients and effective roughness lengths are computed for local averages (15 km scale) and for regional averages (100 km scale) intended to represent a range of grid sizes in numerical models of the atmosphere. The effective roughness length for momentum over the mixed agricultural region for both scales is on the order of 1 m, apparently due to bluff roughness effects associated with scattered trees, edges of small woods and other obstacles. This roughness length value is an order of magnitude larger than values used in numerical models for the same region, which are based on the dominant vegetation type. The spatially varying effective roughness length for heat is computed for use in those models which use surface radiation temperature to estimate surface heat flux. The effective roughness lengths for heat are found to be smaller than those typically used in numerical models of the atmosphere.  相似文献   

4.
Tethered balloon data collected over a heterogeneous, semi-rural area of southern England are presented. The terrain within which the observations were taken is dominated by regularly spaced bluff obstacles that form the boundaries to grass fields. The data are compared with previous surface data to assess how representative these fixed-point measurements are of area-averaged values. It is found that there is a vertical height above which the flow approximates to being homogeneous and observations made above it are representative of the area average. The effective roughness length for momentum is then compared with that derived from a model describing the drag partition around isolated obstacles that are either non-porous or porous to the incident flow. It is shown that the method chosen to partition the drag is of the right order to produce reasonable predictions of the effective roughness length for different roughness densities.  相似文献   

5.
Both large-eddy simulations (LES) and water-tunnel experiments, using simultaneous stereoscopic particle image velocimetry and laser-induced fluorescence, have been used to investigate pollutant dispersion mechanisms in regions where the surface changes from rural to urban roughness. The urban roughness was characterized by an array of rectangular obstacles in an in-line arrangement. The streamwise length scale of the roughness was kept constant, while the spanwise length scale was varied by varying the obstacle aspect ratio l / h between 1 and 8, where l is the spanwise dimension of the obstacles and h is the height of the obstacles. Additionally, the case of two-dimensional roughness (riblets) was considered in LES. A smooth-wall turbulent boundary layer of depth 10h was used as the approaching flow, and a line source of passive tracer was placed 2h upstream of the urban canopy. The experimental and numerical results show good agreement, while minor discrepancies are readily explained. It is found that for \(l/h=2\) the drag induced by the urban canopy is largest of all considered cases, and is caused by a large-scale secondary flow. In addition, due to the roughness transition the vertical advective pollutant flux is the main ventilation mechanism in the first three streets. Furthermore, by means of linear stochastic estimation the mean flow structure is identified that is responsible for street-canyon ventilation for the sixth street and onwards. Moreover, it is shown that the vertical length scale of this structure increases with increasing aspect ratio of the obstacles in the canopy, while the streamwise length scale does not show a similar trend.  相似文献   

6.
A two-dimensional numerical mesoscale model is used to determine the pressure drag of sinoidal mountains and valleys in a neutral atmosphere. In the first part, pressure distributions and flow patterns for isolated obstacles are considered. For large aspect ratios, the pressure drag exerted by valleys becomes small compared to that of mountains. In the second part, interactions between several obstacles are investigated. For mountains, the drag on downstream obstacles is reduced considerably by the first obstacle when the obstacles are close together. For valleys there is a slight increase of the average drag exerted by each obstacle. In the limit for a large number of obstacles, average drag exerted by one mountain is equal to average drag for one valley. For smaller aspect ratios, this average drag can be entered into the resistence law from the Rossby number similarity theory to yield an effective roughness length.  相似文献   

7.
Land Surface Processes Experiment (LASPEX) was conducted over semi-arid region of western India in 1997. As a part of this program, wind and temperature observations were taken using slow as well as fast response sensors over a semi-arid station Anand (22°35′N, 72°55′E) situated in Gujarat state of India. Turbulent parameters such as drag coefficient and sensible heat flux were estimated using eddy correlation method and aerodynamic roughness length was estimated using wind profiles. The analysis has been carried out for the data representing summer, monsoon and winter seasons. It was found that the wind speed does not exceed 5 ms− 1 during the observational period considered in this study. Relationship of aerodynamic drag coefficient and roughness length with wind speed and stability has been investigated. Aerodynamic roughness length was greater in the stable conditions when the wind speed was low and it reduced drastically during convective conditions. The resulting values of aerodynamic roughness length and drag coefficient for the monsoon period agree well with values reported in literature over Indian subcontinent for homogeneous grass covered surfaces.  相似文献   

8.
绿洲-沙漠复合地表条件下的局地和有效粗糙度   总被引：11，自引：0，他引：11  
地表空气动力学粗糙度是研究水平非均匀条件下陆面过程的基础,文中对黑河试验区各不同下垫面上地表空气动力学粗糙度进行了估算和分析,分别得到了地表空气动力学粗糙度ｚ０ｍ的有效值和局地值,发现各不同典型下垫面的局地ｚ０ｍ值明显不同,尤其在有作物存在时,绿洲的局地ｚ０ｍ值比沙漠、戈壁的值明显要大;而对同一测站,有效空气动力学粗糙度比局地空气动力学粗糙度值明显要大,运用应力分解理论对这种差异进行了解释,认为有效空气动力学粗糙度与复合表面的总应力有关,其代表了较大水平尺度内的表面和较高障碍物的综合作用,当涉及对陆表通量参数化问题时,必须首先确定水平尺度及相应的地表粗糙度参数  相似文献   

9.
Flow in the urban boundary layer is strongly influenced by the surface roughness, which is composed principally of isolated buildings or groups of buildings. Previous research has shown that the flow regime depends on the characteristic height of these obstacles (H), and the spacing between them (W). In reality, the urban boundary layer contains roughness elements with a wide range of length scales; in many practical situations these can be classified into large-scale roughness—buildings, or groups of buildings—and small-scale roughness, such as street furniture and elements on the façades and roofs. It is important to understand how the small-scale roughness might modify mass and momentum transfer in the urban boundary layer, but relatively little information is available concerning the potential interaction between large- and small-scale roughness elements in the different flow regimes. This problem has been studied using wind-tunnel experiments, by measuring vertical velocity profiles over a two-dimensional obstacle array, adding small-scale roughness elements to the top of larger parallel square bars. The experiments were performed for different cavity aspect ratios: the results show that the small-scale roughness increases the turbulence intensities and the momentum transfer when the large-scale obstacles are closely packed (H/W > 1) but it has very little effect for more widely-spaced obstacles (H/W < 1).  相似文献   

10.
The parameterization of friction velocity, roughness length, and the drag coefficient over coastal zones and open water surfaces enables us to better understand the physical processes of air-water interaction. In context of measurements from the Humidity Exchange over the Sea Main Experiment (HEXMAX), we recently proposed wave-parameter dependent approaches to sea surface friction velocity and the aerodynamic roughness by using the dimensional analysis method. To extend the application of these approaches to a range of natural surface conditions, the present study is to assess this approach by using both coastal shallow (RASEX) and open water surface measurements (Lake Ontario and Grand Banks ERS-1 SAR) where wind speeds were greater than 6.44 m s-1. Friction velocities, the surface aerodynamic roughness, and the neutral drag coefficient estimated by these approaches under moderate wind conditions were compared with the measurements mentioned above. Results showed that the coefficients in these approaches for coastal shallow water surface differ from those for open water surfaces, and that the aerodynamic roughness length in terms of wave age or significant wave height should be treated differently for coastal shallow and open water surfaces.  相似文献   

11.
We present analytical and numerical calculations of the effective roughness length (ERL) over a flat surface with varying roughness elements, for use in large-scale models. It is shown that ERL is mostly determined by the roughest elements present inside the averaging domain and that, more surprisingly, the ERL increases as the first level of the numerical model gets closer to the surface and its altitude approaches the value of the largest local roughness length. This effect further increases the drag coefficient, in addition to the well-known increase due to the lowering of the first model level.  相似文献   

12.
Wind flows and forces in a model spruce forest   总被引：3，自引：1，他引：3  
Wind tunnel tests have been conducted on a 1:75 scale model of a Sitka spruce forest in a correctly scaled turbulent boundary-layer flow. 12000 tree models were manufactured with mass, flexibility and aerodynamic drag characteristics chosen to give dynamical similarity with typical 15 m trees in a 30ms–1 gale. To measure the dynamic response of a sample tree, set within this model forest, a miniature, fast response strain-gauge balance was designed and built. Linked to a computer for on-line data sampling, this balance provided measurements of the fluctuating along-wind and acrosswind components of the overturning moment at ground level, leading to values of mean and extreme moments and the frequency spectrum of the sway motion. Associated measurements of local wind flow characteristics were made with hot-wire anemometers and a laser anemometer. The response of the tree has the characteristics of classical lightly damped vibration and there is evidence that resonant sway motion increases the extreme overturning moments significantly above the values produced by wind gust forces alone.  相似文献   

13.
Surface Flux Parameterization in the Tibetan Plateau   总被引：9，自引：1，他引：9  
This study investigates some basic aspects related to surface-flux parameterization in the Tibetan Plateau, based on the measurement at three sites. These sites are essentially flat and covered by very sparse and short grasses in the monsoon season. The main contributions include: (1) an optimization technique is proposed to estimate aerodynamic roughness length based on wind and temperature profiles. The approach is not sensitive to random measurement errors if the number of data samples is large enough. The optimized values reasonably vary with surface characteristics. (2) At the three sites, kB-1 (the logarithm of the ratio of aerodynamic roughness length to thermal roughness length) experiences seasonal and diurnal variations in addition to a dependence on surface types. The mean values for the individual sites vary over a range of 2.7 to 6.4 with large standard deviations. (3) A formula for estimatingthe value of kB-1 isproposed to account for the effect of seasonal variation of aerodynamic roughness length and diurnal variation of surface temperature. With the formula, the flux parameterization with surface temperature estimates sensible heat flux better than profile parameterization for all the sites.  相似文献   

14.
Determination of the Drag Coefficient over the Tibetan Plateau   总被引：7，自引：0，他引：7  
In this paper,a preliminary study is given on the drag (i.e.bulk transfer for momentum) coefficient,on the basis of data from four sets of AWS in Tibet during the first observational year from July 1993 to July 1994 according to China Japan Asian Monsoon Cooperative Research Program.The results show that the drag coefficient over the Tibetan Plateau is 3.3 to 4.4×103.In addition,monthly and diurnal variations of drag coefficient and the relationship among the drag coefficients and the bulk Richardson number,surface roughness length and wind speed at 10 m height are discussed in detail.  相似文献   

15.
Field data collected over land and water near Point Lay, Alaska, yielded determinations of the aerodynamic roughness lengths and drag coefficients over three locations on an arctic barrier island and over the Chukchi Sea. The agreement between the results over the sea and previous published results from data taken at approximately the same latitude in the Beaufort Sea is excellent.  相似文献   

16.
Drag of the sea surface   总被引：6，自引：1，他引：6  
It is shown how the drag of the sea surface can be computed from the wind speed and the sea state. The approach, applicable both for fully developed and for developing seas, is based on conservation of momentum in the boundary layer above the sea, which allows one to relate the drag to the properties of the momentum exchange between the sea waves and the atmosphere.The total stress is split into two parts: a turbulent part and a wave-induced part. The former is parameterized in terms of mixing-length theory. The latter is calculated by integration of the wave-induced stress over all wave numbers. Usually, the effective roughness is given in terms of the empirical Charnock relation. Here, it is shown how this relation can be derived from the dynamical balance between turbulent and wave-induced stress. To this end, the non-slip boundary conditions is assigned to the wave surface, and the local roughness parameter is determined by the scale of the molecular sublayer.The formation of the sea drag is then described for fully developed and developing seas and for light to high winds.For the Charnock constant, a value of about 0.018–0.030 is obtained, depending on the wind input, which is well within the range of experimental data.It is shown that gravity-capillary waves with a wavelength less than 5 cm play a minor role in the momentum transfer from wind to waves. Most of the momentum is transferred to decimeter and meter waves, so that the drag of developing seas depends crucially on the form of the wave spectrum in the corresponding high wavenumber range.The dependence of the drag on wave age depends sensitively on the dependence of this high wavenumbertail on wave age. If the tail is wave-age independent, the sea drag appears to be virtually independent of wave age. If the tail depends on wave age, the drag also does. There is contradictory evidence as to the actual dependence. Therefore, additional experiments are needed.The investigation was in part supported by the Netherlands Geosciences Foundation (GOA) with financial aid from the Netherlands Organization for Scientific Research (NWO).  相似文献   

17.
To understand the response of the Greenland ice sheet to climate change the so-called ablation zone is of particular importance, since it accommodates the yearly net surface ice loss. In numerical models and for data analysis, the bulk aerodynamic method is often used to calculate the turbulent surface fluxes, for which the aerodynamic roughness length (z 0) is a key parameter. We present, for the first time, spatial and temporal variations of z 0 in the ablation area of the Greenland ice sheet using year-round data from three automatic weather stations and one eddy-correlation mast. The temporal variation of z 0 is found to be very high in the lower ablation area (factor 500) with, at the end of the summer melt, a maximum in spatial variation for the whole ablation area of a factor 1000. The variation in time matches the onset of the accumulation and ablation season as recovered by sonic height rangers. During winter, snow accumulation and redistribution by snow drift lead to a uniform value of z 0≈ 10−4 m throughout the ablation area. At the beginning of summer, snow melt uncovers ice hummocks and z 0 quickly increases well above 10−2 m in the lower ablation area. At the end of summer melt, hummocky ice dominates the surface with z 0 > 5  ×  10−3 m up to 60 km from the ice edge. At the same time, the area close to the equilibrium line (about 90 km from the ice edge) remains very smooth with z 0 = 10−5 m. At the beginning of winter, we observed that single snow events have the potential to lower z 0 for a very rough ice surface by a factor of 20 to 50. The total surface drag of the abundant small-scale ice hummocks apparently dominates over the less frequent large domes and deep gullies. The latter results are verified by studying the individual drag contributions of hummocks and domes with a drag partition model.  相似文献   

18.
Geometric and aerodynamic roughness of sea ice   总被引：2，自引：0，他引：2  
The aerodynamic drag of Arctic sea ice is calculated using surface data, measured by an airborne laser altimeter and a digital camera in the marginal ice zone of Fram Strait. The influence of the surface morphology on the momentum transfer under neutral thermal stratification in the atmospheric boundary layer is derived with the aid of model concepts, based on the partitioning of the surface drag into a form drag and a skin drag. The drag partitioning concept pays attention to the probability density functions of the geometric surface parameters. We found for the marginal ice zone that the form drag, caused by floe edges, can amount to 140% of the skin drag, while the effect of pressure ridges never exceeded 40%. Due to the narrow spacing of obstacles, the skin drag is significantly reduced by shadowing effects on the leeward side of floe edges. For practical purposes, the fractional sea-ice coverage can be used to parameterize the drag coefficientC dn, related to the 10 m-wind. C dnincreases from 1.2 · 10-3 over open water to 2.8 · 10-3 for 55% ice coverage and decreases to 1.5 · 10-3 for 100% ice coverage.Aircraft turbulence measurements are used to compare the model values of C dnwith measureents. The correlation between measured and modelled drag coefficients results in r 2 = 0.91, where r is the correlation coefficient.  相似文献   

19.
We simulate the microscale heterogeneities of turbulent variables observed at a complex site for different wind directions. The atmospheric computational fluid dynamics (CFD) results are compared with an ensemble of 36 months of data collected at the experimental site SIRTA “Site Instrumental de Recherche par Télédétection Atmosphérique”, located near Paris (France) in a semi-urban environment. The experimental data show that the normalized turbulent kinetic energy (TKE) k/U 2 (where k is TKE and U is the wind speed) at 10-m height, for two different locations, is highly dependent on wind direction and strongly influenced by trees. These measurements show a strong increase of the normalized TKE downstream of the forest canopies with a large variability within the 36-month period in part due to the variation of the tree foliage. The numerical simulations are carried out using the CFD code Code_Saturne with the standard k?ε closure, in neutral stratification. The buildings are taken into account explicitly in the mesh and the forested areas are modelled with two approaches: the classical roughness wall law and a drag porosity. A comparison has been performed between the calculated values and the median of measured values of the normalized TKE and the normalized friction velocity, for each wind sector of 10°. A very good agreement is obtained with the drag porosity model, whereas the classical roughness law leads to a strong underestimation downstream of the forested areas. However, this large improvement of the results using the drag porosity model can only be obtained with a refinement of the grid, especially in forested areas, and an accurate land-use map.  相似文献   

20.
Near Wall Flow over Urban-like Roughness   总被引：3，自引：35，他引：3  
In this study, comprehensive measurements over a number of urban-type surfaces with the same area density of 25% have been performed in a wind tunnel. The experiments were conducted at a free stream velocity of 10 m s-1 and the main instrumentation was 120 ° x-wire anemometry, but measurement accuracy was checked using laser Doppler anemometry.The results haveconfirmed the strong three-dimensionalityof the turbulent flow inthe roughness sublayer and the depths of the inertial sublayer (log-law region) and roughness sublayer for each surface have been determined. Spatial averaging has been used to remove the variability of the flow in the roughness sublayer due to individual obstacles and it is shown that the spatially averaged mean velocity in the inertial sublayer and roughness sublayer can,together, be described by a single log-law with a mean zero-plane displacement and roughness length for the surface, provided that the proper surface stress is known. The spatially averaged shear stresses in the inertial sublayer and roughness sublayer are compared with the surface stress deduced from form drag measurements on the roughness elements themselves.The dispersive stress arising from the spatial inhomogeneity in the mean flow profiles was deduced from the data and is shown to be negligible compared with the usual Reynolds stresses in the roughness sublayer. Comparisons have been made between a homogeneous (regular element array) surface and one consisting of random height elements of the same total volume. Although the upper limits of the inertial sublayer for both surfaces were almost identical at equivalent fetch, the roughness sublayer was much thicker for the random surface than for the uniform surface, the friction velocity and the roughness length were significantly larger and the `roughness efficiency' was greater. It is argued that the inertial sublayer may not exist at all in some of the more extreme rough urban areas. These results will provide fundamental information for modelling urban air quality and forecasting urban wind climates.  相似文献   

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