Showing papers in "Boundary-Layer Meteorology in 1987"
TL;DR: In this article, a two-layer model of soil hydrology and thermodynamics is combined with a one-dimensional model of the planetary boundary layer to study various interactions between evolution of the boundary layer and soil moisture transport.
Abstract: A two-layer model of soil hydrology and thermodynamics is combined with a one-dimensional model of the planetary boundary layer to study various interactions between evolution of the boundary layer and soil moisture transport. Boundary-layer moistening through surface evaporation reduces the potential and actual surface evaporation as well as the boundary-layer growth. With more advanced stages of soil drying, the restricted surface evaporation allows greater sensible heat flux which enhances boundary-layer growth and entrainment drying.
568 citations
TL;DR: In this article, the authors present a theoretical model that predicts neutral-stability values of CH and CE as functions of the wind speed and a surface roughness parameter, which is used to establish the interfacial sublayer profiles of the scalars, temperature and water vapor, over aerodynamically smooth and rough surfaces.
Abstract: Although the bulk aerodynamic transfer coefficients for sensible (CH) and latent (CE) 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 CH and CE 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. CE 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.
355 citations
TL;DR: The Askervein Hill project as mentioned in this paper was a collaborative study of boundary-layer flow over low hills carried out under the auspices of the International Energy Agency Programme of R & D on Wind Energy Conversion Systems.
Abstract: The Askervein Hill project was a collaborative study of boundary-layer flow over low hills carried out under the auspices of the International Energy Agency Programme of R & D on Wind Energy Conversion Systems Two field experiments were conducted during September-October 1982 and 1983 on and around Askervein, a 116 m high hill on the west coast of the island of South Uist in the Outer Hebrides of Scotland During the experiments, over 50 towers were deployed and instrumented for wind measurements The majority were simple 10 m posts bearing cup anemometers but, in the 1983 study, two 50 m towers, a 30 m tower, a 16 m tower, and thirteen 10 m towers were instrumented for 3-component turbulence measurement The present paper provides an overview of the project as a whole, including details of the instrumentation and a summary of the data obtained Additional papers in the series, which are to appear in this journal
Subject to acceptable peer review
, will consider different aspects of the experimental data and related numerical-model and wind-tunnel studies
240 citations
TL;DR: A review of recent field experiments that have been conducted on boundary-layer flow over low hills and what they have learnt from them can be found in this article, where the authors summarize the results of these experiments.
Abstract: In 1975 Jackson and Hunt observed that there had, until that time, been few attempts to relate measurements of wind velocity on hills to the local topography. The succeeding ten years have seen substantial research effort aimed at rectifying this omission. The field measurements, in conjunction with theoretical, numerical and wind tunnel studies now provide a good basis for a preliminary description of neutrally stratified boundary-layer flow over low hills. There are still gaps in the description and understanding of the flow but many more data are now available. The present review attempts to summarize recent field experiments that have been conducted on boundary-layer flow over low hills and to review what we have learnt from them.
195 citations
TL;DR: In this article, the hourly heat storage changes of a suburban area in Vancouver B.C. are evaluated by residual after determining all other terms in the surface energy balance, and the resulting diurnal cycle of heat storage uptake and release appears to behave in a fashion similar to that of less complex systems.
Abstract: The hourly heat storage changes of a suburban area in Vancouver B.C. are evaluated by residual after determining all other terms in the surface energy balance. Despite the errors and uncertainties associated with this approach, the resulting diurnal cycle of heat storage uptake and release appears to behave in a fashion similar to that of less complex systems. When plotted against the net all-wave radiation, the heat storage values display a hysteresis loop with the peak storage preceding that of radiation. Therefore, although a linear parameterization of heat storage is shown to produce reasonable first-order estimates of storage, a form which includes the time derivative of the net radiation is probably better.
165 citations
TL;DR: In this paper, a linear model for neutral surface-layer flow over complex terrain is presented, which combines the simplicity and computational efficiency of linear methods with flexibility for closure schemes of finite-difference methods.
Abstract: A linear model for neutral surface-layer flow over complex terrain is presented. The spectral approach in the two horizontal coordinates and the finite-difference method in the vertical combines the simplicity and computational efficiency of linear methods with flexibility for closure schemes of finite-difference methods. This model makes it possible to make high-resolution computations for an arbitrary distribution of surface roughness and topography. Mixing-length closure as well as E − e closure are applied to two-dimensional flow above sinusoidal variations in surface roughness, the step-in-roughness problem, and to two-dimensional flow over simple sinusoidal topography. The main difference between the two closure schemes is found in the shear-stress results. E − e has a more realistic description of the memory effects in length and velocity scales when the surface conditions change. Comparison between three-dimensional model calculations and field data from Askervein hill shows that in the outer layer, the advection effects in the shear stress itself are also important. In this layer, an extra equation for the shear stress is needed.
160 citations
TL;DR: In this article, simple calculations of the apparent roughness length for the areally averaged flow over flat but heterogeneous terrain are presented, which can be used to specify effective roughness lengths for use in large-scale models.
Abstract: 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 Andre and Blondin (1986).
108 citations
TL;DR: In this paper, a three-dimensional nonlinear numerical model was applied to predict the flow over an isolated hill, Askervein, in order to predict wind speeds approaching the hill from 210° and 180° clockwise from north.
Abstract: A three-dimensional nonlinear numerical model, that has been extensively used previously to predict environmental water flows, was applied to predict the flow over an isolated hill, Askervein. Predictions are reported for winds approaching the hill from 210 ° and 180 ° clockwise from north, both under almost neutral atmospheric conditions.
102 citations
TL;DR: In this paper, a non-hydrostatic numerical model was used to investigate the air flow and turbulence around a single tree in a 3D non-rigid environment, and the results showed that the model was accurate.
Abstract: A three-dimensional non-hydrostatic numerical model has been used to investigate the air flow and turbulence around a single tree.
97 citations
TL;DR: The fine structure of laboratory wind-wave surfaces was investigated using an optical method as discussed by the authors, and several characteristic structures, closely associated with wind speed and the stage of development of the waves, were described.
Abstract: The fine structure of laboratory wind-wave surfaces was investigated using an optical method Several characteristic structures, closely associated with wind speed and the stage of development of the waves, are described A parallel light, incident on the wind-wave surface, was backscattered by ‘specular facets’ perpendicular to the incident angle These specular facets, which represented the fine structures, were photographed and quantitatively analyzed by image processing techniques
89 citations
TL;DR: In this paper, three-dimensional wind velocity components were measured above and within a uniform almond orchard and the authors examined the turbulent statistics associated with the turbulent flow inside the canopy.
Abstract: Three-dimensional wind velocity components were measured above and within a uniform almond orchard. Turbulent statistics associated with the turbulent flow inside the canopy are examined in detail. Turbulence in an almond orchard is characterized by relatively high turbulent intensities and large skewness and kurtosis values. These results indicate that the frequency distribution of wind velocity components is non-Gaussian. Conditional sampling of the turbulent measurements show that large, infrequent sweeps provide the predominant mechanism for tangential momentum stress in the canopy crown. Deep inside the canopy, a secondary wind maximum and small, but positive, tangential momentum stresses are observed.
TL;DR: In this paper, a two-dimensional prognostic numerical model has been used to study a lake breeze event reported by Keen and Lyons (1978), and model predictions showed fair to good agreement with the observations.
Abstract: A two-dimensional prognostic numerical model has been used to study a lake breeze event reported by Keen and Lyons (1978). Model predictions showed fair to good agreement with the observations. For the mature lake breeze, the model predicted inflow at the coast within about 1.5 m s−1 of the observed value, lake breeze depth within 50–90 m of the observed, and inland penetration within about 6 km of the observed. The top of the thermal internal boundary layer (TIBL) was associated with a minimum in the predicted turbulent kinetic energy profile. This may be of consequence for attempts to evaluate pollutant dispersion using numerical models.
TL;DR: In this article, mean windspeed and turbulence measurements were carried out behind two sections of 50% porous fence differing only in the vertical distribution of their porosity, one section of the fence was uniformly porous, the other relatively dense near the ground and open aloft.
Abstract: Measurements of mean windspeed and turbulence were carried out behind two sections of 50% porous fence differing only in the vertical distribution of their porosity. One section of the fence was uniformly porous, the other relatively dense near the ground and open aloft. Slightly greater mean speed reduction was observed near the ground in the near lee of the section which was dense at ground-level without any detrimental increase in turbulence. However, this advantage was offset by less effective protection in the far lee. These findings are consistent with earlier work by Gandemer (1979) and with the predictions of a numerical model of windbreak flow.
TL;DR: In this paper, a random-walk model is presented for calculating the dispersion of heavy particles in a turbulent air flow when only air turbulence statistics and the drag characteristics of the particle are known.
Abstract: A random-walk model is presented for calculating the dispersion of heavy particles in a turbulent air flow when only air turbulence statistics and the drag characteristics of the particle are known. Algebraic expressions for the modification of air velocity variance Σ2 and Lagrangian autocorrelation tune-scale T
L,due to particle inertia effects, are derived. These expressions introduce only a very small computational overhead on the random-walk models for inertia-less particles of Wilson et al. (1983). Measurements of T
Land Σ by Snyder and Lumley (1971) for four different particles are used to determine constants in the heavy-particle model. It is shown that the agreement between the model, for a single set of constants, and the dispersion measurements is good for the 47 Μm hollow glass, 87 Μm glass, and 47 Μm copper particles. The predictions for the 87 Μm corn pollen particles show less satisfactory agreement by underestimating dispersion measurements by 15% after 0.4s. Finally, some aspects of the model's application to spray dispersion in and above a crop canopy are considered.
TL;DR: In this article, a two-dimensional numerical mesoscale model is used to investigate the internal structure and growth of the stably stratified internal boundary layer (IBL) under warm, continental air flowing over a cooler sea.
Abstract: A two-dimensional numerical mesoscale model is used to investigate the internal structure and growth of the stably stratified internal boundary layer (IBL) beneath warm, continental air flowing over a cooler sea. Two situations are studied — steady-state and diurnally varying offshore flow. In the steady-state case, vertical profiles of mean quantities and eddy diffusion coefficients (K) within the IBL show small, but significant, changes with increasing distance from the coast. The top of the IBL is well defined, with large vertical gradients within the layer and a maximum in the coast-normal wind component near the top. Well away from the coast, turbulence, identified by non-zero K, decreases to insignificant levels near the top of the IBL; the IBL itself is characterised by a critical value of the layer-flux Richardson number equal to 0.18. The overall behaviour of the mean profiles is similar to that found in the horizontally homogeneous stable boundary layer over land.
TL;DR: In this paper, wind-tunnel simulations of neutrally stable atmospheric boundary-layer flow over an isolated, low hill (Askervein) have been carried out at three different length scales in two windtunnel facilities and the results are compared with each other and with full-scale data and are shown in general to model the prototype flow very well.
Abstract: Wind-tunnel simulations of neutrally-stable atmospheric boundary-layer flow over an isolated, low hill (Askervein) have been carried out at three different length scales in two wind-tunnel facilities. The objectives of these simulations were to assess the reliability with which changes in mean wind and turbulence structure induced by the prototype hill on boundary-layer flow can be reproduced in the wind tunnel, and to determine the relative impact of certain modelling approaches (surface roughness, model scale, measurement techniques, etc.) on the quality of the simulations. The wind-tunnel results are compared with each other and with full-scale data and are shown in general to model the prototype flow very well. The effects of relaxing the criterion of aerodynamic roughness of the model surface were limited to certain regions in the lee of the hill and were linked to separation phenomena.
TL;DR: In this article, an improved first-order closure approximation is developed for the transilient turbulence parameterization, which uses non-local approximations to the shear, buoyancy, storage, and dissipation terms of the turbulence kinetic energy equation to parameterize the turbulent mixing potential between every combination of grid points.
Abstract: An improved first-order closure approximation is developed for the non-local ‘transilient turbulence’ parameterization. Instead of using Richardson numbers, this improved approach uses non-local approximations to the shear, buoyancy, storage, and dissipation terms of the turbulence kinetic energy equation to parameterize the turbulent mixing potential between every combination of grid points in a 1-D model of the atmosphere. The original (n2 − n) degrees of freedom associated with the independent transilient matrix coefficients for a model of n grid points is thus reduced to four degrees of freedom associated with the four free parameters.
TL;DR: In this article, a semi-analytical method for describing the mean wind profile and shear stress within plant canopies and for estimating the roughness length and the displacement height is presented.
Abstract: A semi-analytical method for describing the mean wind profile and shear stress within plant canopies and for estimating the roughness length and the displacement height is presented. This method incorporates density and vertical structure of the canopy and includes simple parameterizations of the roughness sublayer and shelter factor. Some of the wind profiles examined are consistent with first-order closure techniques while others are consistent with second-order closure techniques. Some profiles show a shearless region near the base of the canopy; however, none displays a secondary maximum there. Comparing several different analytical expressions for the canopy wind profile against observations suggests that one particular type of profile (an Airy function which is associated with the triangular foliage surface area density distribution) is superior to the others. Because of the numerical simplicity of the methods outlined, it is suggested that they may be profitably used in large-scale models of plant-atmosphere exchanges.
TL;DR: In this paper, a set of concentration time series from ground-level plumes in the atmosphere has been used to generate conditionally sampled (zeros ignored) plume concentration statistics These have been compared and contrasted with corresponding unconditionally sampled statistics.
Abstract: A set of concentration time series from ground-level plumes in the atmosphere has been used to generate conditionally sampled (zeros ignored) plume concentration statistics These have been compared and contrasted with corresponding unconditionally sampled statistics It is found that conditional statistics are much less sensitive to the location of the receptor (relative to the mean plume) and to averaging time Indeed, most of the variation apparent in unconditionally sampled statistics (both explained and unexplained) resides in the intermittency, the fraction of non-zero readings
TL;DR: In this paper, an observational study of sea-breeze fronts as they cross a shoreline is presented, where the vertical wind component, w, is found to be of the order of 1.0-1.5 ms−1 in the front zone of the sharp front but only 5 as large in the weaker front.
Abstract: Results from an observational study of sea-breeze fronts as they cross a shoreline are presented. Two kinds of fronts are analyzed, one with an offshore regional wind and one without. Their structure is found to be substantially different, the former being steeper and having stronger gradients. Measurements of the profiles of the vertical component of the wind speed, its standard deviation and the structure parameter for temperature are presented along with time series of the structure parameters for water vapor pressure and wind speed. The vertical wind component, w, is found to be of the order of 1.0–1.5 ms−1 in the front zone of the sharp front but only 5 as large in the weaker front. The usual height variation laws under convective conditions are found to apply for both the vertical velocity variance and the temperature structure parameter, which in conjunction with the appropriate spectra indicate that local equilibrium is re-established fairly quickly after the passage of the front. Substantial differences have also been noted in the values of the structure parameters before and after the front, especially in the water vapor pressure and wind speed, differences which are of dissimilar magnitude and sign for the two kinds of fronts.
TL;DR: In this article, a properly formulated Markov displacement model which includes a time-dependent settling velocity, deposition and a method to simulate boundary conditions in which the flux is proportional to the concentration is presented.
Abstract: Stochastic models of turbulent atmospheric dispersion treat either the particle displacement or particle velocity as a continuous time Markov process. An analysis of these processes using stochastic differential equation theory shows that previous particle displacement models have not correctly simulated cases in which the diffusivity is a function of vertical position. A properly formulated Markov displacement model which includes a time-dependent settling velocity, deposition and a method to simulate boundary conditions in which the flux is proportional to the concentration is presented. An estimator to calculate the mean concentration from the particle positions is also introduced. In addition, we demonstrate that for constant coefficients both the velocity and displacement models describe the same random process, but on two different time scales. The stochastic model was verified by comparison with analytical solutions of the atmospheric dispersion problem. The Monte Carlo results are in close agreement with these solutions.
TL;DR: In this article, the governing Eulerian equations for the velocity products of wind velocities within a plant canopy are examined using data from a wind tunnel study with an artificial canopy consisting of an array of 5 cm lengths of monofilament fishing line, and from measurements in corn.
Abstract: Wind velocities within a plant canopy are much more strongly skewed than those of the air flow above. We have examined the governing Eulerian equations for the velocity products u′i, u′j u′k using data from a wind tunnel study with an artificial canopy consisting of an array of 5 cm lengths of monofilament fishing line, and from measurements in corn (Zea mays L).
TL;DR: In this article, meteorological techniques were used to monitor evapotranspiration (ET) at two sites in a boreal forest drainage basin located in southeastern Manitoba, Canada, where an energy balance method was used in which net radiation (RN) and ground heat flux (G) were measured directly.
Abstract: Meteorological techniques were used to monitor evapotranspiration (ET) at two sites in a boreal forest drainage basin located in southeastern Manitoba, Canada. An energy balance method was used in which net radiation (RN) and ground heat flux (G) were measured directly. Sensible heat flux (H) was measured by the eddy correlation technique using a propeller anemometer and a fine-wire thermocouple. The energy components were calculated hourly on-line, and data were collected reliably over a five-month period.
TL;DR: In this article, a k-e turbulence model was applied to a numerical simulation of sea breeze and the dynamical behaviors of eddy diffusivity, turbulent kinetic energy and its dissipation rate associated with moving sea breeze front were predicted and analyzed.
Abstract: A k-e turbulence model was applied to a numerical simulation of sea breeze. The dynamical behaviors of eddy diffusivity, turbulent kinetic energy and its dissipation rate, associated with moving sea breeze front, were predicted and analyzed. Results demonstrated, for example, difference of the turbulence structure between thermal internal boundary layer and inland mixed layer, and the double maxima structures of turbulence-related quantities in their vertical profiles just behind sea breeze front. The properties of the computed sea breeze front agreed qualitatively with those of a gravity current in unstable environment, observed by Simpson et al. (1977). Furthermore, the possibility that air pollutants released in the sea breeze layer might be trapped within small circulating flow at the sea breeze front, and move with it was shown in an advection simulation of hypothetical fluid particles using flows obtained.
TL;DR: In this article, the three-dimensional mesoscale model FITNAH has been modified to simulate effects of a tall tree canopy on airflow in complex terrain, showing that surface wind speed will increase and the atmosphere near the ground will become colder after complete deforestation.
Abstract: The three-dimensional mesoscale model FITNAH has been modified to simulate effects of a tall tree canopy on airflow in complex terrain. Numerical experiments show the general features of meteorological variables inside a plant stand with low wind speeds and a nearly neutral thermal stratification during night. Available observations from the Finkenbach valley and the simulated temperatures near the ground show good agreeement. Comparison of model results for a nighttime situation for cases with and without a canopy (after complete deforestation) leads to the main results, viz., that surface wind speed will increase and the atmosphere near the ground will become colder after deforestation. However, the production rate of cooled air (expressed in m3m−2h−1) decreases; that means that forested slopes are more effective in ventilating a city than slopes covered with short vegetation.
TL;DR: In this article, a non-hydrostatic mesoscale model that achieves full vectorization on computers like the CYBER 205 was used to simulate the steady-state inviscid flow over a single mountain.
Abstract: This paper deals with a non-hydrostatic mesoscale model that achieves full vectorization on computers like the CYBER 205. The model formulation ensures the conservation of all fluxes and takes into account the terrain inhomogeneities by the aid of suitable transformations. The diagnostic equation for the pressure change is solved using a very efficient vectorized elliptic solver. By imposing appropriate boundary conditions no additional precautions at the boundaries are necessary to achieve meaningful results. As an application, the steady-state inviscid flow over a single mountain is simulated.
TL;DR: The vortex structure formed in front of and behind a hemispheric surface hump has been studied in a water recirculating flume.
Abstract: The vortex structure formed in front of and behind a hemispheric surface hump has been studied in a water recirculating flume. In the recirculation zone behind the hemisphere, arched vortex tubes were formed. Up to a certain critical Reynolds number, each tube was shed individually from the recirculation zone. However, when the Reynolds number exceeded a critical value, several of the tubes coalesced in the recirculation zone before shedding. In front of the hemisphere, some parabolic vortex tubes formed. Their number increased with Reynolds number below its critical value and decreased above this value. The dimensionless height of the parabolic vortex tube decreased with Reynolds number.
TL;DR: In this article, the authors present observations of internal gravity waves in the stably-stratified atmospheric boundary layer at Halley, Antarctica, made on 1 February, 1986 and take the form of temperature measurements from a 30 m mast and a Sodar record.
Abstract: Observations of internal gravity waves in the stably-stratified atmospheric boundary layer at Halley, Antarctica are presented. These were made on 1 February, 1986 and take the form of temperature measurements from a 30 m mast and a Sodar record. The temperature record shows a clearly defined, dominant wave period of around 11 min. A high-resolution radiosonde ascent made during the period of wave activity exhibits thin layers of low Richardson number and it is suggested that these are regions of dynamic instability where the waves are generated. A linear stability analysis of the radiosonde data supports this idea. It is argued from simple theoretical ideas and by means of a numerical model that only waves with a wavelength greater than a certain critical value are likely to be observed at the surface. The observations are shown to be consistent with this hypothesis.
TL;DR: In this article, a new parameterization of turbulent exchanges is proposed, which results in a thickening of the turbulent and inversion layers in a model of the nocturnal boundary layer.
Abstract: Exchange coefficients and mixing lengths under stable stratification have been studied through measurements of mean wind velocity and temperature in the nocturnal boundary layer. For values of the gradient Richardson number lower than 0.15, our measurements fit well the relation of Delage (1974). Beyond Ri = 0.15, the decrease of mixing length is much slower. So a new parameterization of turbulent exchanges is suggested. When introduced in a model of the nocturnal boundary layer, it results in a thickening of the turbulent and inversion layers.
TL;DR: Doppler acoustic sounders have been used extensively in a series of noctural drainage flow experiments carried out by the US Department of Energy's Atmospheric Studies in Complex Terrain (ASCOT) program.
Abstract: Acoustic sounders have now been used extensively in a series of noctural drainage flow experiments carried out by the US Department of Energy's Atmospheric Studies in Complex Terrain (ASCOT) program Doppler acoustic sounders, located in three different valleys during the sequence of experiments, reveal drainage-wind profiles that depend strongly on the ambient meteorological conditions and the elevation of each observing site relative to surrounding terrain In elevated sites that drain easily, Doppler-sounder derived wind profiles show a simply-structured flow; in lower lying areas, subject to topographic constriction and cold-air pooling, and where Archimedean forces are comparable to those due to synoptic and mesoscale pressure gradients, the wind profiles show considerable vertical and temporal variation In particular, in the Geysers area of northern California, the seabreeze and the depth of the Pacific Coast marine inversion affect not only the initiation of drainage winds but also their subsequent evolution