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1.
Analysis of data collected during the Prairie Grass, Kansas and Minnesota experiments reveals the following empirical relationship between the Monin-Obukhov length L and the friction velocity u *: L = Au * 2, A = 1.1 × 103s2m-1. This result combined with the formulation for the height of the stable boundary layer h suggested by Zilitinkevich (1972) leads to h u * 3/2 f–1/2 where f is the Coriolis parameter. Data from the Minnesota study (Caughey et al., 1979) provide ample support for this expression.These empirical equations for L and h are useful for routine dispersion estimates during stable conditions.  相似文献   

2.
A wind-tunnel experiment has been used to investigate momentum absorption by rough surfaces with sparse random and clustered distributions of roughness elements. An unusual (though longstanding) method was used to measure the boundary-layer depth δ and friction velocity u * and thence to infer the functional relationship z 0/h = f(λ) between the normalised roughness length z 0/ h and the roughness density λ (where z 0 is the roughness length and h the mean height of the roughness elements). The method for finding u * is based on fitting the velocity defect in the outer layer to a functional form for the dimensionless velocity-defect profile in a canonical zero-pressure-gradient boundary layer. For the conditions investigated here, involving boundary layers over sparse roughness with strong local heterogeneity, this velocity-defect-law method is found to be more robust than several alternative methods for finding u * (uw covariance, momentum integral and slope of the logarithmic velocity profile).The experimental results show that, (1) there is general agreement in the relationship z 0/h = f(λ) between the present experiment with random arrays and other wind-tunnel experiments with regular arrays; (2) the main effect of clustering is to increase the scatter in the z 0/h = f(λ) relationship, through increased local horizontal heterogeneity; (3) this scatter obscures any trend in the z 0/h = f(λ) relationship in response to clustering; and (4) the agreement between the body of wind-tunnel data (taken as a whole) and field data is good, though with scatter for which it is likely that a major contribution stems from local horizontal heterogeneity in the field.  相似文献   

3.
We use a conceptual model to investigate how randomly varying building heights within a city affect the atmospheric drag forces and the aerodynamic roughness length of the city. The model is based on the assumptions regarding wake spreading and mutual sheltering effects proposed by Raupach (Boundary-Layer Meteorol 60:375?C395, 1992). It is applied both to canopies having uniform building heights and to those having the same building density and mean height, but with variability about the mean. For each simulated urban area, a correction is determined, due to height variability, to the shear stress predicted for the uniform building height case. It is found that u */u *R , where u * is the friction velocity and u *R is the friction velocity from the uniform building height case, is expressed well as an algebraic function of ?? and ?? h /h m , where ?? is the frontal area index, ?? h is the standard deviation of the building height, and h m is the mean building height. The simulations also resulted in a simple algebraic relation for z 0/z 0R as a function of ?? and ?? h /h m , where z 0 is the aerodynamic roughness length and z 0R is z 0 found from the original Raupach formulation for a uniform canopy. Model results are in keeping with those of several previous studies.  相似文献   

4.
Summary In this paper the results of an urban measurement campaign are presented. The experiment took place from July 1995 to February 1996 in Basel, Switzerland. A total of more than 2000 undisturbed 30-minute runs of simultaneous measurements of the fluctuations of the wind vector u′, v′, w′ and the sonic temperature θ s ′ at three different heights (z=36, 50 and 76 m a.g.l.) are analysed with respect to the integral statistics and their spectral behaviour. Estimates of the zero plane displacement height d calculated by the temperature variance method yield a value of 22 m for the two lower levels, which corresponds to 0.92 h (the mean height of the roughness elements). At all three measurement heights the dimensionless standard deviation σ w /u * is systematically smaller than the Monin-Obukhov similarity function for the inertial sublayer, however, deviations are smaller compared to other urban turbulence studies. The σθ/θ* values follow the inertial sublayer prediction very close for the two lowest levels, while at the uppermost level significant deviations are observed. Profiles of normalized velocity and temperature variances show a clear dependence on stability. The profile of friction velocity u * is similar to the profiles reported in other urban studies with a maximum around z/h=2.1. Spectral characteristics of the wind components in general show a clear dependence on stability and dimensionless measurement height z/h with a shift of the spectral peak to lower frequencies as thermal stability changes from stable to unstable conditions and as z/h decreases. Velocity spectra follow the −2/3 slope in the inertial subrange region and the ratios of spectral energy densities S w (f)/S u (f) approach the value of 4/3 required for local isotropy in the inertial subrange. Velocity spectra and spectral peaks fit best to the well established surface layer spectra from Kaimal et al. (1972) at the uppermost level at z/h=3.2. Received September 26, 1997 Revised February 15, 1998  相似文献   

5.
With observational data collected and interpreted by Crane et al. (1977), the adequacy of the O'Brien polynomial to represent the exchange profile of heat and pollution in a convective boundary layer is examined and a refinement suggested. Also, it is shown that the height of the surface layer, h=0.04 z i , developed by Blackadar and Tennekes (1968) for a neutrally stratified boundary layer (with z z =0.25u */f) appears to be equally valid for the convective boundary layer where z i , defined as the top of the mixed layer, is used.  相似文献   

6.
The height of the atmospheric boundary layer (ABL) obtained with lidar and radiosondes is compared for a data set of 43 noon (12.00 GMT) cases in 1984. The data were selected to represent the synoptic circulation types appropriately. Lidar vertical profiles at 1064 nm were used to obtain three estimates for the ABL height (h lid), based on the first gradient in the back-scatter profile, namely, at the beginning, middle and top of the gradient. The boundary-layer height obtained with the radiosondes (h s) was determined with the dry-parcel-intersection method in unstable conditions. As a first guess for near-neutral and stable conditions, the height of the first significant level in the potential temperature profile was taken. Overall, the boundary-layer thickness estimates agree surprisingly well (regression lineh lidb=hs:cc.=0.93 and the standard error=121 m). However, in 10% of the cases, the lidar estimate was significantly lower (difference>400 m) than the routinely inferredh s. These outliers are discussed separately. For stable conditions, an estimate of ABL height (h N) is also made based on the friction velocity and the Brunt-Väisälä frequency. The agreement betweenh Nandh lidbis good. Discrepancies between the two methods are caused by:
  1. rapid growth of the boundary layer arround the measurement time;
  2. the presence of a deep entrainment layer leading to a large zone in which quantities are not well mixed;
  3. a large systematic error of 100–200 m in the estimate of boundary-layer height obtained from the radiosonde due to the way that profiles are recorded, as a series of significant points.
  相似文献   

7.
The linear normal mode instabilities of a parallel shear flow which varies both vertically (z) and meridionally (y) in a quasigeostrophic, rotating, stratified fluid are considered. The β effect (variation of Coriolis parameter with y) is included. Both two-layer and continuous fluids are treated. Attention is concentrated on the types of instability possible for a given shear flow. It is found that the instability can be described adequately by three nondimensional parameters: Λ, the ratio of the horizontal length scale of the shear to the internal deformation radius: δ, which is either the ratio of layer depths in the two-layer fluid or the fractional depth of variation of the stratification in the continuous fluid; and β, suitably nondimensionalized.Asymptotic analyses, confirmed by direct numerical solutions, are performed for conditions in which various parameters become large or small. The β effect is essentially quantitative, whereas Λ and δ define the type of instability as barotropic (if the kinetic energy of the mean flow feeds the growing perturbations), baroclinic (if the available potential energy of the mean flow feeds the perturbations) or mixed (a combination of the two).The case of large Λ (the most relevant for oceanographic applications) is treated in detail. It is shown that y-independent problems have only limited relevance. For a fixed deformation radius and the y scale of the mean flow increasing without limit, the asymptote is not the case of no y variation in the mean flow.  相似文献   

8.
The presence of a low-level, capping inversion layer will affect the height and structure of the planetary boundary layer (PBL). Results from models of varying levels of sophistication, including analytical, turbulent kinetic energy (TKE), second-order closure (SOC), large-eddy simulation (LES) and direct numerical simulation (DNS) models, are used to investigate this influence for the neutral, barotropic PBL. Predicted and observed profiles of stress and geostrophic departure components, and integral measures, such as the parameters of Rossby-number similarity theory, are compared for the KONTUR, Marine Stratocumulus, JASIN, Leipzig, Pre-Wangara and Upavon field experiments.Analytical models of the equilibrium value of inversion height zi, which depend on the surface friction velocity u*, and both the Coriolis parameter f and the free-flow Brunt-Väisälä frequency N, are found to give reasonable estimates of the PBL height. They also indicate that only the KONTUR and Marine Stratocumulus experiments were strongly influenced by N. More quantitative comparisons would require larger, more comprehensive datasets. The effects of the presence of a capping inversion on the profile structure were found to be insignificant for h* = |f|zi/u* > 0.15.The simple analytical model performed quite well over all values of h*; it predicted the profiles of the longitudinal stress component (in the direction of the surface stress) better than the lateral component. The more advanced models performed well for small values of h* (for flow over the sea), but systematically underestimated the cross-isobaric angle for flow over land. These models predicted the profiles of the lateral stress component better than the longitudinal component. The profiles of the analytical model agreed with those of the advanced models when the constant eddy viscosity of the outer layer was increased.Agreement with DNS was achieved by increasing the eddyviscosity of the analytical model by a factor of 5.Zilitinkevich and Esau(2002, Boundary-Layer Meteorology, 104, 371–379)suggest that the neutral, barotropic values of A and B of Rossby-numbersimilarity theory are not universal constants, but depend on the ratio N/|f|. The dependence for A and B is calculated using the analytical model and TKE models. Over the sea (h* 0.1; N/|f| 100, where we have used the Zilitinkevich-Esau relation to convert between h* and N/|f|) there is agreement between the model predictions and observations; however over land where the equilibrium boundary-layer height is greater (h* 0.35; N/|f| 10) the inconsistency between the advanced model predictions (TKE, SOC, LES, and DNS) and observations, as noted previously by Hess and Garratt, still exists. We attribute this disagreement to violations of the strict assumptions of steady, horizontally homogeneous, neutral, barotropic conditions implicit in the observations. At small values of zi and a strongly stable background stratification (h* 0.04; N/|f| 1000) both the TKE and analytical models predict that A and B depend significantly on h*, however observations are unavailable to confirm these predictions. Zilitinkevich and Esau call this case the `long-lived near-neutral PBL', and state that it is found in cold weather at high latitudes.  相似文献   

9.
林乃实  孙立潭 《气象学报》1997,55(3):306-317
本文应用Ｈｏｗａｒｄ（１９６１）所用的精巧积分方法研究了三维非静力平衡层结大气随高度变化基流上重力内波的不稳定性质。给出了不稳定产生的必要条件和相速度在相速复平面上的分布规律；发展了Ｍｉｌｅｓ定理和Ｈｏｗａｒｄ半圆定理；得到了有限深度气层的半椭圆定理。并指出在随高度变化的基流中，重力内波的不稳定范围与基流的垂向结构及波传播方向有关；半椭圆的位置和长轴分别重合于Ｈｏｗａｒｄ半圆的位置和直径，而短轴则依赖于与Ｒｉｃｈａｒｄｓｏｎ数相关的参数Ｊ０，Ｒ０和波数｜ｋ｜及流场厚度参数ｋ０  相似文献   

10.
A theory is offered for the drag and heat transfer relations in the statistically steady, horizontally homogeneous, diabatic, barotropic planetary boundary layer. The boundary layer is divided into three regionsR 1,R 2, andR 3, in which the heights are of the order of magnitude ofz 0,L, andh, respectively, wherez 0 is the roughness length for either momentum or temperature,L is the Obukhov length, andh is the height of the planetary boundary layer. A matching procedure is used in the overlap zones of regionsR 1 andR 2 and of regionsR 2 andR 3, assuming thatz 0 L h. The analysis yields the three similarity functionsA(),B(), andC() of the stability parameter, = u */fL, where is von Kármán's constant,u * is the friction velocity at the ground andf is the Coriolis parameter. The results are in agreement with those previously found by Zilitinkevich (1975) for the unstable case, and differ from his results only by the addition of a universal constant for the stable case. Some recent data from atmospheric measurements lend support to the theory and permit the approximate evaluation of universal constants.  相似文献   

11.
This paper describes a wind-tunnel experiment on the dispersion of trace heat from an effectively planar source within a model plant canopy, the source height being h s = 0.80 h c , where h c is the canopy height. A sensor assembly consisting of three coplanar hot wires and one cold wire was used to make simultaneous measurements of the temperature and the streamwise and vertical velocity components. It was found that:
  1. The thermal layer consisted of two parts with different length scales, an inner sublayer (scaling with h s and h c ) which quickly reached streamwise equilibrium downstream of the leading edge of the source, and an outer sublayer which was self-preserving with a length scale proportional to the depth of the thermal layer.
  2. Below 2h c , the vertical eddy diffusivity for heat from the plane source (K HP ) was substantially less than the far-field limit of the corresponding diffusivity for heat from a lateral line source at the same height as the plane source. This shows that dispersion from plane or other distributed sources in canopies is influenced, near the canopy, by turbulence ‘memory’ and must be considered as a superposition of both near-field and far-field processes. Hence, one-dimensional models for scalar transport from distributed sources in canopies are wrong in principle, irrespective of the order of closure.
  3. In the budgets for temperature variance, and for the vertical and streamwise components of the turbulent heat flux, turbulent transport was a major loss between h s and h c and a principal gain mechanism below h s , as also observed in the budgets for turbulent energy and shear stress.
  4. Quadrant analysis of the vertical heat flux showed that sweeps and ejections contributed about equal amounts to the heat flux between h s and h c , though among the more intense events, sweeps were dominant. Below h s , almost all the heat was transported by sweeps.
  相似文献   

12.
We examine the unsteady response of a neutral atmospheric boundary layer (ABL) of depth h and friction velocity u * when a uniform surface heat flux is applied abruptly or decreased rapidly over a time scale t<inf>θ</inf> less than about h /(10u *). Standard Monin–Obukhov (MO) relationships are used for the perturbed eddy viscosity profile in terms of the changes to the heat flux and mean shear. Analytical solutions for changes in temperature, mean wind and shear stress profile are obtained for the surface layer, when there are small changes in h /|LMO| over the time scale tMO~|L MO|/(10u*) (where L MO and t MO are the length and time scales, respectively). They show that a maximum in the wind speed profile occurs at the top of the thermal boundary layer for weak surface cooling, i.e. a wind jet, whereas there is a flattening of the profile and no marked maximum for weak surface heating. The modelled profiles are approximately the same as those obtained from the U.K. Met Office Unified Model when operating as a mesoscale model at 12-km horizontal resolution. The theoretical model is modified when strong surface heating is suddenly applied, resulting in a large change in h /|L MO| (>>1), over the time scale t MO. The eddy structure is predicted to change significantly and the addition of convective turbulence increases the shear turbulence at the ground. A low-level wind jet can form, with convective turbulence adding to the mean momentum of the flow. This was verified by our laboratory experiment and direct numerical simulations. Additionally, it is shown that the effects of Coriolis acceleration diminish (rather than as suggested in the literature, amplify) the formation of the wind jets in the situations considered here. Hence, only when the surface heat flux changes over time scales greater than 1/f (where f is the Coriolis parameter) does the ABL adjust monotonically between its equilibrium states. These results are also applicable to the ABL passing over spatially varying surface heat fluxes.  相似文献   

13.
The friction velocity (u*) and the sensible heat flux density (H) determined with a displaced-beam small aperture scintillometer (DBSAS) and a hot-film eddy correlation system are compared. Random errors in the DBSAS are relatively small, compared to scatter found with two eddy-correlation systems. Assuming that the hot-film system yields the true fluxes, theDBSAS appears to overestimate u* when u* is less than 0.2 m s-1 and to underestimate u* at high wind speeds. This implies that the DBSAS measurements of theinner scale length of turbulence, l0, a direct measure for the dissipation rate of kinetic turbulent energy, are biased. Possible causes for these results are discussedin detail. A correction procedure is presented to account for effects of random noise and of so-called inactive turbulence or sensor vibrations. The errors in u* cause errors in the DBSAS measurements of the structure parameter of temperature CT 2. The derived H appears to be less sensitive to errors in l0 and CT 2, because errors in these quantities tend to cancel out.  相似文献   

14.
The standard deviation of vertical two-point longitudinal velocity fluctuation differences is analyzed experimentally with eleven sets of turbulence measurements obtained at the NASA 150-m ground-winds tower site at Cape Kennedy, Florida. It is concluded that /u *0 is proportional to (fz/u *0)0.22, where the coefficient of proportionality is a function of fz/u *0 and u *0/fL 0. The quantities f and L0 denote the Coriolis parameter and the surface Monin-Obukhov stability length, respectively; u *0 is the surface friction velocity; z is the vertical distance between the two points over which the velocity difference is calculated; and zz is the mean height of the mid-point of the interval z above natural grade. The results of the analysis are valid for 20<-u *0/fL 0<2000.  相似文献   

15.
Wind speed was measured at a height of 1 cm above the ground and at several other heights in and above a canopy of tall fescue grass (Festuca arundinacea) using single hot-wire and triple hot-film anemometers. The plant area density in the canopy was concentrated close to the ground, with 75% of the plant area standing belowz=15 cm, wherez is height above the ground. The frequency distributions of horizontal wind speeds,s, were sharply skewed towards positive values at all measurement heights, but were most highly skewed near the ground where the coefficient of skewness ranged from 1.6 to 2.9. Above mid-canopy height, the frequency distribution ofs was described reasonably well by a Gumbel extreme value distribution. Average wind speed,S, decreased exponentially with depth into the canopy with an exponential scale length of abouth/2.8, whereh is the height of the canopy. Atz=1 cm, the value ofS was about 11% of the surface-layeru *. The standard deviation of the fluctuations of the vertical and horizontal components of the wind speed also decreased exponentially with depth inside the canopy with a scale length of abouth/2.5.Inside the canopy, the Eulerian integral time scales for the vertical ( w ) and horizontal ( u ) components of wind speed were about 0.1 s and 1.0 s, respectively, and were approximately constant with height. Above the canopy, these time scales increased sharply and, atz=2.25h, w and u were approximately 1.0 and 3.0s, respectively. Turbulence length scales in the vertical and downwind directions, u and w ·U, respectively, were approximately 1 cm for heights between 1 to 10 cm above the ground inside the canopy, while atz=2.25h, they were about 55 cm and 277 cm. Relatively quiescent periods (lulls) in the air close to the ground were interrupted frequently by gusts. The frequency of occurrence of gusts appears to be correlated with the value of the local shear near the top of the canopy.  相似文献   

16.
Sodar observations from three nights of the HAPEX-MOBILHY experiment have been used to compute covariances between single measurements of the three velocity components. From these, estimates of a low frequency friction velocityu * are derived which show better correlation with observed values of the stable boundary layer (SBL) height,h, than directu *-measurements by an ultrasonic anemometer. On the contrary, interdiurnal variability ofh is better correlated with directu *-measurements. These findings should be mainly due to the problem of different spectral and spatial representativity of the twou *-values.  相似文献   

17.
Fluctuations of the horizontal wind under unstable conditions   总被引：1，自引：0，他引：1  
The similarity relations for u/u* proposed by Panofskyet al. (1977) and Højstrup (1982) have been verified using eddy-correlation data collected during the EFEDA-experiment, conducted over the extensive plain of La Mancha (Spain), where vine plants form a primary crop. Also, the standard Monin-Obukhov relation is considered. It is found that the expressions by Panofskyet al. and Højstrup both yield almost identical results, and are better than the Monin-Obukhov expression. Also, u measured with a cup anemometer obeys the similarity expressions well, provided that the effect of the variation of wind direction on propellor wind speed is accounted for. The relationship of Panofskyet al. works rather well even when the boundary-layer height scaleh is replaced by a fixed height,h c. Best results were obtained forh c=1800 m. This height scale is possibly associated with the horizontal variability in the surface sensible heat flux pattern.  相似文献   

18.
A Forest SO2 Absorption Model (ForSAM) was developed to simulate (1) SO2 plume dispersion from an emission source, (2) subsequent SO2 absorption by coniferous forests growing downwind from the source. There are three modules: (1) a buoyancy module, (2) a dispersion module, and (3) a foliar absorption module. These modules were used to calculate hourly abovecanopy SO2 concentrations and in-canopy deposition velocities, as well as daily amounts of SO2 absorbed by the forest canopy for downwind distances to 42 km. Model performance testing was done with meteorological data (including ambient SO2 concentrations) collected at various locations downwind from a coal-burning power generator at Grand Lake in central New Brunswick, Canada. Annual SO2 emissions from this facility amounted to about 30,000 tonnes. Calculated SO2 concentrations were similar to those obtained in the field. Calculated SO2 deposition velocities generally agreed with published values.Notation c air parcel cooling parameter (non-dimensional) - E foliar absorption quotient (non-dimensional) - f areal fraction of foliage free from water (non-dimensional) - f w SO2 content of air parcel - h height of the surface layer (m) - H height of the convective mixing layer (m) - H stack stack height (m) - k time level - k drag coefficient of drag on the air parcel (non-dimensional) - K z eddy viscosity coefficient for SO2 (m2·s–1) - L Monin-Obukhov length scale (m) - L A single-sided leaf area index (LAI) - n degree-of-sky cloudiness (non-dimensional) - N number of parcels released with every puff (non-dimensional) - PAR photosynthetically active radiation (W m–2) - Q emission rate (kg s–2) - r b diffusive boundary-layer resistance (s m–1) - r c canopy resistance (s m–1) - r cuticle cuticular resistance (s m–1) - r m mesophyllic resistance (s m–1) - r s stomatal resistance (s m–1) - r exit smokestack exit radius (m) - R normally distributed random variable with mean of zero and variance of t (s) - u * frictional velocity scale, (m s–1) - v lateral wind vector (m s–1) - v d SO2 dry deposition velocity (m s–1) - VCD water vapour deficit (mb) - z can mean tree height (m) - Z zenith position of the sun (deg) - environmental lapse rate (°C m–1) - dry adiabatic lapse rate (0.00986°C m–1) - von Kármán's constant (0.04) - B vertical velocities initiated by buoyancy (m s–1) - canopy extinction coefficient (non-dimensional) - ()a denotes ambient conditions - ()can denotes conditions at the top of the forest canopy - ()h denotes conditions at the top of the surface layer - ()H denotes conditions at the top of the mixed layer - ()s denotes conditions at the canopy surface - ()p denotes conditions of the air parcels  相似文献   

19.
Past work on analyzing ground-source diffusion data in terms of surface-layer similarity theory is reviewed; these analyses assume that z /L orh/L is a function of u * x/L (where h = Q/ dy). It is argued that an alternative scaling, h */L versus x/L, is nearly as universal in that it is very weakly influenced by surface roughness, except for a modest influence in the free convective case (h * = Q/u * dy); the advantage of this scaling is that it eliminates the need to reassess as vertical diffusion progresses. The Prairie Grass data set is adjusted for the difference in source and sampling heights, and is plotted with this scaling. Simple analytic equations are suggested that fit the resultant data plots for stable and unstable conditions, and suggestions are made towards practical application of these results.On assignment from the National Oceanic and Atmospheric Administration, U.S. Department of Commerce.  相似文献   

20.
The stationary, Ekman-layer equations have been solved in closed form for two expressions of the eddy viscosity as a function of height, z: v τ=cu*z(1?z/h)and v τ=cu*z(1?z/h) 2, where u* is the friction velocity, h the boundary-layer height and c a constant. The main difference between both solutions is that the quadratic K-profile leads to a velocity discontinuity at the top of the boundary layer, while the solution for the cubic profile approaches the geostrophic wind at z=h smoothly. We discuss the characteristics of the solutions in terms of a dimensionless parameter C=fh/cu*, where f is the Coriolis parameter. The dependence on C can be interpreted in terms of a varying boundary-layer height or in terms of stability. The results for C ~ 1 are related to a neutral boundary layer. They agree well with results of a second-order model. The limit C → 0 is investigated in detail. We find that the stress profile becomes linear. The velocity profile shows different characteristics depending on whether we consider a shallow or a very unstable boundary layer. The results agree with observations. Finally we consider the influence of baroclinicity on the wind and stress profiles.  相似文献   

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