A numerical investigation on the interaction of turbulent and long-wave radiative fluxes in the surface layer

The interaction of turbulent and radiative transfer applied to a number of plausible atmospheric situations in the surface layer under the stably stratified condition is discussed.The calculated results show that the long-wave radiative flux has a great influence upon the thermal structure of the surface layer, and that it usually acts in such a way as to weaken the thickness of the constant turbulent heat flux layer. In the case of low wind velocities and strongly stable stratifications, the thickness of the turbulent heat flux layer will become very thin and/or inexistent.     

Phase-averaged results of wave-follower measurements of turbulent shear flow above water waves

Average air velocities and turbulent Reynolds stresses were calculated at fixed heights above 1 Hz progressive sinusoidal water waves by reanalyzing the experimental data of Chao and Hsu (1978).     

On convective turbulence and the influence of rotation

A series of experiments were performed in a rotating cylindrical tank over a wide range of rotation rates in which convective turbulence was generated by a bottom-mounted heated plate in both homogeneous and stratified fluids. Measurements were made of the turbulent velocities in all three axes over the full depth of the chamber, and of the temperatures at the mid-depth near the centre of the tank. For even small rotation rates, the measurements showed that the turbulent velocities were weakly affected by rotation at all depths, but as the rotation rate increased, the deviation from the non-rotational scaling slowly and progressively increased until eventually the turbulent velocities were fully rotationally controlled. The results indicated that there was no sudden transition of the turbulent field from the non-rotational state (a function only of the surface buoyancy flux B and the depth z) to the rotational state (where the strength of the turbulent field is a function of only B and the Coriolis parameter f). Rather the transition was a smooth asymptotic one from one state to the other. Nevertheless, it was possible to parametrize this transition by a single value of the turbulent or small scale Rossby number, defined by Ro = (B/f³z²)^1/3. Our measurements suggested a critical value of Ro_c ≈ 0.1, below which the turbulence was fully rotationally controlled and which was equivalent to a critical depth z_c = (35 ± 15)(B/f³)^1/2. Using typical oceanic values for B and f, the oceanic turbulence driven by surface cooling events becomes rotationally controlled only for depths greater than about 10 km, a depth which is greater than that of the bulk of the world's oceans. Thus, convective turbulence actively being generated by cooling of the ocean surface is best described by non-rotating turbulent velocity and length scales and is a function only of the surface buoyancy flux and the depth.     

Using Temperature Fluctuation Measurements to Estimate Meteorological Inputs for Modelling Dispersion During Convective Conditions in Urban Areas

We examine the performance of several methods to estimate meteorological inputs for modelling dispersion in urban areas during convective conditions. Sensible heat flux, surface friction velocity and turbulent velocities are estimated from measurements of mean wind speed and the standard deviation of temperature fluctuations at a single level on a tower at two suburban sites and at one urban site in Riverside, California. These estimates are compared with observations made at these sites during a field study conducted in 2007. The sensible heat flux is overestimated in the urban area, while it is underestimated at a suburban site when temperature fluctuations are used in the free convection formulation to estimate heat flux. The bias in heat flux estimates can be reduced through a correction that depends on stability. It turns out that the bias in heat flux estimates has a minor effect on the prediction of surface friction velocity and turbulent velocities. Estimates of sensible heat flux, surface friction velocity and turbulent velocities are sensitive to estimates of aerodynamic roughness length, and we suggest estimating the aerodynamic roughness length through detailed micrometeorological measurements made during a limited field study. An examination of the impact of the uncertainty in estimating surface micrometeorology on concentrations indicates that, at small distances from a surface release, ground-level concentrations computed using estimates of heat flux and surface friction compare well with the those based on observed values: the bias is small and the 95% confidence interval of the ratio of the two concentrations is 1.7. However, at distances much larger than the Obukhov length, this confidence interval is close to 2.3 because errors in both friction velocity and heat flux affect plume spread. Finally, we show that using measurements of temperature fluctuations in estimating heat flux is an improvement on that based on the surface energy balance, even when net radiation measurements are available.     

Prediction of wind properties in urban environments using artificial neural network

Artificial neural network (ANN) modeling has been performed to predict turbulent boundary layer characteristics for rough terrain based on experimental tests conducted in a boundary-layer wind tunnel to simulate atmospheric boundary layer using passive roughness devices such as spires, barriers, roughness elements on the floor, and slots in the extended test section. Different configurations of passive devices assisted to simulate urban terrains. A part of the wind tunnel test results are used as training sets for the ANN, and the other part of the test results are used to compare the prediction results of the ANN. Two ANN models have been developed in this study. The first one has been used to predict mean velocity, turbulence intensity, and model length scale factor. Results show that ANN is an efficient, accurate, and robust modeling procedure to predict turbulent characteristics of wind. In particular, it was found that the ANN-predicted wind mean velocities are within 4.7%, turbulence intensities are within 6.2%, and model length scale factors are within 3.8% of the actual measured values. In addition, another ANN model has been developed to predict instantaneous velocities that enables calculating the power spectral density of longitudinal velocity fluctuations. Results show that the predicted power spectra are in a good agreement with the power spectra obtained from measured instantaneous velocities.     

Large-Eddy Simulation of the Wind Field and Plume Dispersion Within Different Obstacle Arrays Using a Dynamic Mixing Length Subgrid-Scale Model

A novel dynamic mixing length (DML) subgrid-scale (SGS) model is proposed to improve the large-eddy simulations of the wind field and contaminant dispersion around a group of buildings. Wind field and contaminant dispersion in two kinds of building array geometries are simulated using the model, with wind-tunnel experimental data used to validate the model. The relative errors in the lateral profiles of the streamwise mean velocities behind the sixth row of the buildings of the staggered obstacle array and the aligned obstacle array at the half height of the building are 15 and 9%, respectively. The DML velocity fluctuations in the staggered and aligned obstacle arrays are in agreement with those of the experiment. The results indicate that the DML model can make a more accurate prediction of the mean velocity and velocity fluctuations. The DML model is highly suitable for the simulation of multi-scale turbulent flow in urban canyons, of high Reynolds number turbulent flow and of complex turbulent flow.     

A model for eddy diffusivity in a stable boundary layer

Expressions for the vertical and the lateral diffusivity coefficients were derived from the Local Similarity Theory and the Statistical Diffusion Theory. For such, the spectral density energies for the turbulent velocities were used. The expressions here derived are compared with the diffusivity coefficients for momentum and heat suggested by Sorbjan (from the Minnesota experiments) and Nieuwstadt (from the Cabauw experiments). This comparison allows us to conclude that turbulence is equally efficient in transporting momentum, heat and contaminants in an ideally stable boundary layer.     

Large-Eddy Simulation for the Mechanism of Pollutant Removal from a Two-Dimensional Street Canyon

Large-eddy simulation (LES) is conducted to investigate the mechanism of pollutant removal from a two-dimensional street canyon with a building-height to street-width (aspect) ratio of 1. A pollutant is released as a ground-level line source at the centre of the canyon floor. The mean velocities, turbulent fluctuations, and mean pollutant concentration estimated by LES are in good agreement with those obtained by wind-tunnel experiments. Pollutant removal from the canyon is mainly determined by turbulent motions, except in the adjacent area to the windward wall. The turbulent motions are composed of small vortices and small-scale coherent structures of low-momentum fluid generated close to the plane of the roof. Although both small vortices and small-scale coherent structures affect pollutant removal, the pollutant is largely emitted from the canyon by ejection of low-momentum fluid when the small-scale coherent structures appear just above the canyon where the pollutant is retained. Large-scale coherent structures also develop above the canyon, but they do not always affect pollutant removal.     

Turbulent transport from an arctic lead: A large-eddy simulation

The upward transfer of heat from ocean to atmosphere is examined for an Arctic lead, a break in the Arctic ice which allows contact between the cold atmosphere and the relatively warm ocean. We employ a large-eddy model to compute explicitly the three-dimensional turbulent response of the atmosphere to a lead of 200 m width. The surface heat flux creates a turbulent plume of individual quasi-random eddies, not a continuous updraft, which penetrate into the stable atmosphere and transport heat upward.Maximum updraft velocities and turbulence occur downwind of the lead rather than over the lead itself, because the development time of an individual thermal eddy is longer than its transit time across the lead. The affected vertical region, while shallow over the lead itself, grows to a height of 65m at 600 m downwind of the lead; beyond that, the depth of the turbulent region decreases as the eddies weaken. The maximum vertical turbulent heat flux occurs at the downwind edge of the lead, beyond which a relative maximum extends upward into the plume. Negative surface heat flux immediately downwind of the lead creates a growing stable layer, but above that internal boundary layer the turbulent heat flux is still positive. Updraft maxima are typically 28 cm/s, but compensating downdrafts result in time-averaged vertical velocities of less than 1 cm/s in the plume. Conditional sampling separates the updraft and downdraft contributions. Formulas for the horizontal eddy development distance and for the vertical plume penetration height are presented. The relative importance of mean and turbulent transport is compared for both vertical and horizontal heat transfer: turbulence dominates the vertical heat transport whereas mean advection dominates the horizontal transport, these offsetting transports producing a quasi-stationary state.     

Numerical studies on flow fields around buildings in an Urban street canyon and cross-road

The questions on how vortices are constructed and on the relationship between the flow patterns and concentration distributions in real street canyons are the most pressing questions in pollution control studies. In this paper, the very large eddy simulation (VLES) and large eddy simulation (LES) are applied to calculate the flow and pollutant concentration fields in an urban street canyon and a cross-road respectively. It is found that the flow separations are not only related to the canyon aspect ratios, but also with the flow velocities and wall temperatures. And the turbulent dispersions are so strongly affected by the flow fields that the pollutant concentration distributions can be distinguished from the different aspect ratios, flow velocities and wall temperatures.     

The measurement of turbulent surface-layer fluxes by use of bichromatic scintillation

Scintillation measurements with a HeNe and a CO₂ laser were used to derive turbulent fluxes of heat and momentum in the surface layer. This was achieved by the structure constant or dissipation technique, i.e., by relating the measured structure constants and inner scales of refractive index fluctuations to structure constants of temperature fluctuations and dissipation rates of turbulent kinetic energy, respectively, and then assuming Monin-Obukhov similarity.The resulting heat fluxes agree well with measurements using the eddy correlation technique but for averaging periods of 10 min, the optical data show a much smoother and physically more plausible behaviour. The optically derived friction velocities are in good agreement with estimates derived from wind velocity and surface roughness. It was also observed that for stationary conditions, 1-min averaged optical measurements already provide good estimates for longer averaged heat and momentum fluxes.Even though some uncertainty remains about the empirical constants and Monin-Obukhov similarity expressions used, the method clearly proves to be of great value for monitoring surface-layer turbulence.     

The influence of buoyancy on third-order turbulent velocity statistics within a deciduous forest

The influence of atmospheric stability on the behaviour of the third moment of flow velocities observed inside a deciduous forest canopy is examined. Results suggest that buoyancy plays a dominant role in dictating the magnitude of gusts observed inside tall vegetation. Furthermore, an examination of the turbulence recorded throughout leaf fall inside the same forest indicates that larger velocity skewnesses are observed inside a canopy in full leaf than inside a sparse canopy. The behaviour of the measured terms in the non-dimensionalized rate equation of the third moment of canopy flow velocities is also examined. Turbulent diffusion and turbulence gradient interaction terms are largest in stable conditions in the upper canopy layer while these are most important in unstable conditions in the lower canopy layer. In all stability regimes, the turbulent diffusion term is the main source of skewness. The turbulence gradient interaction term, the residual and buoyant production terms all contribute to destroy skewness in stable conditions.     

Turbulence Characteristics In The Surface Boundary Layer Over The South American Pampa

This paper presents results of turbulence measurementsmade in the south of Brazil in the Pampa region.Data collected at 1Hz are used to calculatestandard deviations of temperature and velocities. Onthe other hand data collected at 10Hz areused to study the behaviour of spectra and cospectraof turbulence in the surface layer. Dimensionless dissipation rates of turbulent kinetic energy and temperature variance are also presented. The frameworkof Monin–Obukhov Similarity theory is used and allresults are compared with other experimentalstudies.     

A three-dimensional model for calculating the concentration distribution in inhomogeneous turbulence

A new approach for calculating the concentration distribution in inhomogeneous turbulence is suggested. The model is a 3-D model, constrained to describe incompressible flow. The model requires a knowledge of the covariance matrix of the Eulerian velocities and the two-point third moments. The model is applied for three types of turbulent field: homogeneous isotropic turbulence, constant flux neutral boundary layer and free convective turbulence. The required Eulerian moments are calculated using the eddy model of the turbulent field. Concentration moments are calculated and results are compared to experimental data. Other model predictions which have no experimental support can be compared to measurements when available.     

大涡模式垂直分辨率在夹卷及示踪物垂直传输模拟结果分析中的作用

基于敦煌野外观测资料和大涡模式,研究了垂直方向不同尺度湍涡对夹卷及示踪物垂直传输的影响,明确了模式垂直分辨率在模拟结果分析中的作用。结果表明:垂直方向上小尺度湍涡对夹卷作用贡献更大,小尺度湍涡较多时夹卷层相对更暖,而夹卷层厚度、夹卷强度和风速变化受垂直方向湍涡尺度影响较小。当垂直分辨率为50 m时,越往夹卷层上部,上升气流和下沉气流分布较多且强度较大;分辨率为10、20和30 m时,夹卷层各高度垂直速度、位温和示踪物浓度分布较接近。另外,垂直方向湍涡尺度对示踪物垂直传输高度影响不大,而对示踪物的空间分布有一定作用。当大尺度湍涡较多且强度较强时,越有利于将高浓度的示踪物向上传输。综合考虑到模式采用较高分辨率模拟时产生的噪音及计算时间等问题,认为模式采用30 m的垂直分辨率,既能较好地模拟出夹卷层平均结构特征,又能模拟出夹卷层湍流的精细分布,是较为理想的选择。     

A miniature three-dimensional anemometer for use within and above plant canopies

A miniature anemometer has been designed for the measurement of turbulent transport within canopies. The sensing element utilizes a relatively new concept in hot-film anemometry, in which the angular measurement is derived from the non-uniformity in heat transfer coefficient around the circumference of the cylindrical hot-film. The element is split along its length to form two separate conducting films and the relative magnitudes of the heat convected from each side are used to calculate the elevation angle of the wind. An electromechanical servosystem operated by a second split-film keeps the sensing head facing into the wind. The anemometer measures all three components of velocity over the complete solid angle without octant ambiguity and at velocities as low as 20 cm s^–1. It is a research instrument and because of its non-linear response characteristics, data handling is best accomplished by digital computer.The response of the split-film elements extends to high frequencies. The servo-system follows turbulent fluctuations up to approximately 5 Hz and keeps the probe within a few degrees of the wind at all times. In field tests, total wind speed and wind component measurements compared well with more conventional anemometers; eddy-correlation measurements of shear-stress with the split-film anemometer were in good agreement with measurements from a shear stress lysimeter and from a pressure-sphere anemometer.     

A numerical study of the influence of an air temperature-inversion layer and a seawater density-jump layer on the structure of interacting boundary layers

The effects of an air-temperature inversion in the atmosphere and a seawater density jump in the ocean on the structure of the atmospheric and oceanic boundary layers are studied by use of a coupled model. The numerical model consists of a closed system of equations for velocities, turbulent kinetic energy, turbulent exchange coefficient, local turbulent length scale, and stratification expressions for both air and sea boundary layers. The effects of the temperature inversion and the density jump are incorporated into the equations of turbulent kinetic energy of the atmosphere and ocean by a parameterization. A series of numerical experiments was conducted to determine the effects of various strengths of the inversion layer and surface heat fluxes in the atmosphere and of the density-jump layer in the ocean on the structure of the interacting boundary layers.The numerical results show that the temperature inversion in the atmosphere and density jump in the ocean have strong influences on turbulent structure [especially on the turbulent exchange coefficient (TEC) and turbulent kinetic energy (TKE)] and on air-sea interaction characteristics. Maxima of TKE and TEC strongly decrease with increasing strength of the inversion layer, and they disappear for strong inversions in the atmosphere. Certain strengths (density differences between the upper and the lower layers) of the density-jump layer in the ocean (₂ 0.1 g/cm³) produce double maxima in TEC-profiles and TKE-profiles in the ocean. The magnitudes of air-sea interaction characteristics such as geostrophic drag coefficient, and surface drift current increase with increasing strength of the density-jump layer in the ocean. The density-jump layer plays the role of a barrier that limits vertical mixing in the ocean. The numerical results agree well with available observed data and accepted quantitive understanding of the influences of a temperature inversion layer and a density-jump layer on the interacting atmospheric and oceanic boundary layers.     

A numerical method for determining the dry deposition of atmospheric trace gases

A diagnostic deposition model based on generally accepted micrometeorological ideas on the transfer of momentum, sensible heat and matter near the Earth's surface is presented. The parameterization of fluxes is based on the flux-gradient relationships in the turbulent region of the surface layer and the sublayer Stanton number as well as the Reynolds analogy between concentration, temperature and wind velocity distributions in the underlying sublayer. The model requires only vertical profile data of wind velocity, dry- and wet-bulb temperatures and trace gas concentrations from the turbulent part of the surface layer.The method has been applied to vertical profile data collected in field experiments such as the GREIV I 1974 project and the Great Plains Turbulence Project. In order to illustrate the way in which the model can be used to evaluate deposition fluxes and velocities of reactive trace gases, it has been applied to observed concentrations of NO, NO₂ and ozone.Presented at the Symposium on Environmental Meteorology, Würzburg, FRG, September 29 to October 1, 1987.     

Near-surface wind estimates using statistics from a planetary boundary-layer model

This paper shows the possibilities of a procedure for estimating near-surface wind statistics, by means of the numerical integration of a simple boundary-layer model with a second-order turbulent closure. Standard and easily available synoptic data are used as initial and boundary conditions. The development of this methodology is impelled by increasing requirements of a quick and precise knowledge of the wind characteristics in many regions of South America, which confronts the serious limitation of a reduced number of extended observational series, scattered over a vast continent. In order to evaluate the methodology, near-surface wind statistics from observed data at two locations are compared with model output statistics. Relative errors are about 0.2 for daily mean velocities and about 0.1 for weekly mean velocities, observed and computed time series being highly correlated in both cases. Calculated frequency distribution of wind directions is in good agreement with the observed one, and the absolute mean error in the daily mean wind direction is about 20 deg. Even though a wide variety of large-scale synoptic situations has been indirectly considered through boundary conditions, basic model output statistics resemble fairly well those observed at different levels between the surface and 100 m.