Showing papers in "Boundary-Layer Meteorology in 1989"

Observation of organized structure in turbulent flow within and above a forest canopy

Abstract: Ramp patterns of temperature and humidity occur coherently at several levels within and above a deciduous forest as shown by data gathered with up to seven triaxial sonic anemometer/thermometers and three Lyman-alpha hygrometers at an experimental site in Ontario, Canada. The ramps appear most clearly in the middle and upper portion of the forest. Time/height cross-sections of scalar contours and velocity vectors, developed from both single events and ensemble averages of several events, portray details of the flow structures associated with the scalar ramps. Near the top of the forest they are composed of a weak ejecting motion transporting warm and/or moist air out of the forest followed by strong sweeps of cool and/or dry air penetrating into the canopy. The sweep is separated from the ejecting air by a sharp scalar microfront. At approximately twice the height of the forest, ejections and sweeps are of about equal strength.

Turbulent exchange above a pine forest. II: Organized structures

Abstract: Data from two 100-min runs on the turbulent atmospheric wind, temperature and humidity fields above a pine forest have been analysed using conditional sampling techniques. With the aid of the temperature time series, ramp events were identified and all fields were averaged in order to remove smaller scale turbulence and random low-frequency turbulence, and to map the organized structures revealed in this way. It is shown that the turbulent fluxes of momentum, heat and humidity, determined from these ramp-events, in fact constitute a large part of the total fluxes, about 90% during the actual events.

An evaluation of aircraft flux measurements of CO2, water vapor and sensible heat

Abstract: Ground-based flux measurements of carbon dioxide and water vapor integrate physiological processes taking place on a field scale. Aircraft flux measurements have recently been undertaken to attempt to widen the scope of applicability of such measurements. However, because of the intermittency of turbulent transfer, flux measurements must be averaged over long periods of time or long distances to give reproducible results. This requirement makes it difficult to relate aircraft flux measurements to local surface processes. Flux measurements of CO2, latent and sensible heat obtained from repeated passes in four directions and at three elevations over a homogeneous wheat-growing area are compared with ground-based measurements. Averages based on four runs of 4 km in length gave results consistent with ground-based measurements. The largest percentage differences were in the sensible heat flux. Cospectral analyses showed no significant high frequency losses for the data from flight levels of 25 and 50 m, but an underestimation of approximately 10% resulted at 10 m. Flight direction with respect to wind direction was relatively unimportant at 10 and 25 m but some effects were observed at 50 m. It was also shown that at 25 m, over a relatively smooth and homogeneous surface, the means of either three or four runs 4 km in length were similar to the means of 12–16 km runs. This confirms that at this altitude, most of the flux contribution is contained at wavelengths less than 4 km and that the mean of 3 to 4 passes accounts for most of the intermittency of turbulent transfer.

A wind tunnel study of turbulent flow over model hills

Abstract: Detailed wind tunnel measurements have been made of mean flow and turbulence over a two-dimensional ridge and a circular hill, both having cosine-squared cross-section and maximum slope about 15 °. The measurements were made in an artificially thickened neutrally stratified boundary layer, and have been compared with results from linear models and rapid distortion theory as appropriate.

The spectral velocity tensor for homogeneous boundary layer turbulence

Abstract: We have postulated a simple model for the spectral tensor Φ ij (k) of an anisotropic, but homogeneous turbulent velocity field. It is a simple generalization of the spectral tensor Φ ij iso (k) for isotropic turbulence and we show how in the limit of isotropy, Φij(k) becomes equal to Φ ij iso (k). Whereas Φ ij iso (k) is determined entirely by one scalar function of k = |k|, namely the energy spectrum, we need three independent scalar functions of k to specify Φ ij (k). We show how it is possible by means of the three stream-wise velocity component spectra to determine the three scalar functions in Φ ij (k) by solving two uncoupled, ordinary linear differential equations of first and second order. The analytic form of the component spectra each has a set of three parameters: the variance and the integral length scale of the velocity component and a dimensionless parameter, which governs the curvature of the spectrum in the transition domain from the inertial subrange towards lower wave numbers. When the three sets of parameters are the same, the three spectra correspond to isotropic turbulence and they are all interrelated and related to the energy spectrum. We show how it is possible to obtain these spectral forms in the neutral surface layer and in the convective boundary layer from data reported in the literature. The spectral tensor is used to predict the lateral coherences for all three velocity components and these predictions are compared with coherences obtained in two experiments, one using three masts at a horizontally homogeneous site in Denmark and one employing two aircraft flying in formation over eastern Colorado. Comparison shows reasonable agreement although with considerable experimental scatter.

Velocity profiles, the resistance law and the dissipation rate of mean flow kinetic energy in a neutrally and stably stratified planetary boundary layer

Abstract: The logarithmic + polynomial approximation is suggested for vertical profiles of velocity components in a planetary boundary layer (PBL) at neutral and stable stratification. The resistance law functions A and B are determined on the basis of this approximation, using integral relations derived from the momentum equations, the Monin-Obukhov asymptotic formula for the wind profile in a stably stratified near-surface layer and the known expressions for the PBL depth. This result gives a realistic and convenient method for calculating the surface friction velocity and direction and the total dissipation rate of mean flow kinetic energy in terms of geostrophic velocity, buoyancy flux at the surface, the roughness parameter and the Coriolis parameter. In the course of these derivations a review is given of current views on the main problems of the neutral and stable PBL.

Turbulent exchange above a pine forest i. fluxes and gradients

Abstract: Measurements of gradients of wind, temperature and humidity and of the corresponding turbulent fluxes have been carried out over a sparse pine forest at Jadra»s in Sweden. In order to ascertain that correct gradient estimates were obtained, two independent measuring systems were employed: one system with sensors at 10 fixed levels on a 51 m tower and another with reversing sensors for temperature and humidity, covering the height interval 23 to 32 m. Turbulent fluxes were measured at three levels simultaneously. Data from three field campaigns: in June 1985, June 1987 and September 1987 have been analyzed. The momentum flux is found on the average to be virtually constant from tree top level, at 20 to 50 m. The average fluxes of sensible and latent heat are not so well behaved. The ratio of the non-dimensional gradients of wind and temperature to their corresponding values under ‘ideal conditions’ (low vegetation) are both found to be small immediately above the canopy (about 0.3 for temperature and 0.4 for wind). With increasing height, the ratios increase, but the values vary substantially with wind direction. The ratios are not found to vary systematically with stability (unstable stratification only studied). The ratio of the non-dimensional humidity gradient to the corresponding non-dimensional potential temperature gradient (equivalent to k h /k w ) is found to be unity for (z − d)/L v less than about −0.1 and about 1.4 for near neutral stratification, but the scatter of the data is very large.

HNO3 deposition to a deciduous forest

Abstract: The deposition velocity (Vd) of nitric acid vapor over a fully leafed deciduous forest was estimated using flux/gradient theory. HNO3 deposition velocities ranged from 2.2 to 6.0cm/s with a mean Vdon the order of 4.0cms-1. Estimates of Vdfrom a detailed canopy turbulence model gave deposition velocities of similar magnitude. The model was used to investigate the sensitivity of Vdto the leaf boundary-layer resistance and leaf area index (LAI). Although modeled deposition velocities were found to be sensitive to the parameterization of the leaf boundary-layer resistance, they were less sensitive to the LAI. Modeled Vd's were found to peak at LAI = 7.

Water Vapor Flux at the Sea Surface

Abstract: Methods and instrumentation for determining the rate of evaporation at the sea surface are reviewed At experimental sites free of local influences, there is a consensus that the evaporation coefficient in neutral conditions C EN = 12 × 10−3 at low and moderate wind speeds Combining this with Businger-Dyer flux-gradient formulas, a parameterization scheme is proposed Evaporation of spray droplets from breaking waves is expected to cause C EN to increase at high wind speeds, but no direct observations of this are found Recently it has become possible to estimate water vapor flux in tropical regions from satellite data, opening the possiblity of studying large-scale evaporative events as a function of both time and space

Carbon dioxide, water vapor and sensible heat fluxes over a tallgrass prairie

Abstract: Fluxes of CO2, water vapor and sensible heat were measured in a grassland ecosystem near Manhattan, Kansas, employing the eddy correlation technique. The vegetation at this site is dominated by big bluestem (Andropogon gerardii), switchgrass (Panicum virgatum), and indiangrass (Sorghastrum nutans). Diurnal patterns of the energy budget components and CO2 fluxes are evaluated on a few selected days. Influence of high atmospheric evaporative demand and low availability of soil water are examined on (a) energy partitioning, and (b) the magnitudes and patterns of atmospheric carbon dioxide exchange.

Numerical modeling of the nocturnal urban boundary layer

Abstract: A numerical case study with a second-order turbulence closure model is proposed to study the role of urban canopy layer (UCL) for the formation of the nocturnal urban boundary layer (UBL). The turbulent diffusion coefficient was determined from an algebraic stress model. The concept of urban building surface area density is proposed to represent the UCL. Calculated results were also compared with field observation data. The height of the elevated inversion above an urban center was simulated and found to be approximately twice the average building height. The turbulent kinetic energy k, energy dissipation rate e, and turbulence intensities 〈u2〉 and 〈w2〉 increase rapidly at the upwind edge of the urban area. The Reynolds stress 〈uw〉 displayed a nearly uniform profile inside the UBL, and the vertical sensible heat flux 〈wθ〉 had a negative value at the inversion base height. This indicates that the downward transport of sensible heat from the inversion base may play an important role in the formation of the nocturnal UBL.

Inland penetration of the sea breeze over the suburban area of Tokyo

Abstract: Observational results of the structure of the sea breeze over the urban and suburban areas of Tokyo for four summer days are presented. On two of these days, the inland penetration of the sea breeze front could be clearly traced. In one case, the sea breeze was first observed along the shores of Tokyo Bay around 0900 JST, and propagated in three hours through the Tokyo City area, the width of which is about 20 km. It then advanced inland at a rate of 16 km h−1. Prior to the arrival of the sea breeze at the suburban site, the mixing height had remained at about 600 m for four hours. With the arrival of the sea breeze front, accompanied by an abrupt change in wind speed and direction, the mixing height increased sharply to 1700 m. It is suggested that this behavior and the structure of the front are intensified due to the urban effect, or the difference in the thermal characteristics between the urban and rural areas. On the days without a sea breeze front, the land breeze system during the early morning was less intense, allowing the sea breeze to develop simultaneously with the inland valley wind and easily form a large-scale local wind system during the morning hours. In both cases, the vertical motion accompanying the local wind system works as a feedback mechanism to control the local winds by modifying the thermal and pressure fields.

The structure of the stably stratified internal boundary layer in offshore flow over the sea

Abstract: Observations obtained mainly from a research aircraft are presented of the mean and turbulent structure of the stably stratified internal boundary layer (IBL) over the sea formed by warm air advection from land to sea. The potential temperature and humidity fields reveal the vertical extent of the IBL, for fetches out to several hundred of kilometres, geostrophic winds of 20–25 m s−1, and potential temperature differences between undisturbed continental air and the sea surface of 7 to 17 K. The dependence of IBL depth on these external parameters is discussed in the context of the numerical results of Garratt (1987), and some discrepancies are noted.

Current problems in the stratocumulus-topped atmospheric boundary layer

Abstract: Extended sheets of stratocumulus (Sc) in the upper part of the atmospheric boundary layer (ABL) often occur under appropriate meteorological conditions. These cloud decks are important both in climate studies and in weather forecasting. We review the current knowledge of the turbulent structure of the ABL capped by a cloud deck, in the light of recent observations and model studies. The most important physical processes determining this structure are longwave radiative cooling at cloud top, shortwave radiative wanning by absorption in the cloud, surface buoyancy flux, and wind shear in the ABL. As a result, turbulence can cause entrainment against the buoyancy jump at cloud top. In cases where only longwave radiative fluxes and surface buoyancy fluxes are important, the turbulent structure is relatively well understood. When shortwave radiative fluxes and/or wind shear are also important, the resulting turbulent structure may change considerably. A decoupling of the cloud from the sub-cloud layer or of the top of the cloud from the rest of the ABL is then regularly observed. In no cases are the details of the entrainment at cloud top understood well enough to derive a relatively simple formulation that is consistent with observations. Cloud-top entrainment instability may lead to the break-up of a cloud deck (but also to cloud deepening). The role of mesoscale circulations in determining fractional cloudiness is not yet well understood.

Regional roughness of the landes forest and surface shear stress under neutral conditions

Abstract: Note: 48: 69-81 Reference EFLUM-ARTICLE-1989-001doi:10.1007/BF00121783 Record created on 2005-09-08, modified on 2017-02-23

On the parameterization of drag over small-scale topography in neutrally-stratified boundary-layer flow

Abstract: Predictions of the surface drag in turbulent boundary-layer flow over two-dimensional sinusoidal topography from various numerical models are compared. For simple 2D terrain, the model results show that the drag increases associated with topography are essentially proportional to (slope)2 up to the steepness at which the flow separates. For the purposes of boundary-layer parameterisation within larger-scale models, we propose a representation of the effects of simple 2D topography via an effective roughness length, z 0 eff. The form of the varation of z 0 eff with terrain slope and topographic wavelength is established for small slopes from the model results and a semi-empirical formula is proposed.

A Micrometeorological Investigation of Surface Exchange of O3, SO2 and NO2: A Case Study

Abstract: Data obtained in an intensive field study of the dry deposition of sulfur dioxide, ozone, and nitrogen dioxide, conducted in 1985 in central Pennsylvania, are used to illustrate the factors that must be considered to assure that high quality results are derived. In particular, the quality of the site must be such that flux measurements made above the surface are representative of surface values. For this purpose, tests involving momentum transfer and the surface energy budget are especially useful. In addition, conditions must not be changing rapidly, and the statistical uncertainty associated with flux measurement must be low. For the set of data presented here, conservative quality-assurance guidelines are used to reject potentially erroneous flux data. For ozone, most of the measured fluxes are of use in deriving surface resistances. For SO2, far fewer data points are available. For NO2, fluxes appear to lack the order of the O3 and SO2 fluxes, and do not enable surface resistances to be computed. The highest-quality SO2 and O3 data yield surface resistances in fair average agreement with model predictions for SO2, but substantially higher than predictions for O3.

Shapes of annual frequency distributions of wind speed observed on high meteorological masts

Abstract: Annual wind distributions from six masts are used to investigate the annual variability of hourly-average wind. A variability extremum occurs at the height where the average diurnal cycle of wind reverses its phase from a nocturnal minimum to a nocturnal maximum. A simple profile model shows that in non-complex terrain, this reversal height varies approximately between 50 m at coasts and 90 m inland. The Weibull distribution shape factor k has a maximum at the reversal height, and with decreasing height, k decreases approximately as a linear function of height. Therefore estimation of wind distribution shape from surface data is possible in the surface layer. In the upper PBL, however, such estimation is not very feasible, because no simple methods are available to estimate the reversal height from routine surface observations.

The atmospheric heat source over the Bolivian plateau for a mean January

Abstract: The atmospheric heat sources of large plateaus strongly influence the general circulation particularly in the summer season. The Bolivian plateau and the adjacent areas affect the upper tropospheric flow in a typical summer month by developing an anticyclone and deflecting the prevailing westerlies. The plateau initially warms the atmosphere through sensible heating and then through latent heating as thunderstorms develop.

Dispersion of spores released from an elevated line source within a wheat canopy

Abstract: Turbulent dispersion of spores was studied near a source located inside a wheat canopy. Two colors of Lycopodium spores were released simultaneously at a steady rate from line sources at two heights (0.4–0.5 m and 0.7–0.8 m) in a 0.8 to 1.0 m tall crop. The number of spores of each color released was estimated by weighing the sources before and after each release. Aerial spore concentrations were measured at 2 and 4 m downwind of the sources using rotorods placed at four heights above the canopy and small suction traps at two heights inside the canopy. Concentrations near the ground were estimated from deposits on sticky glass microscope slides placed on the ground. Experiments were conducted on six different days. Friction velocities ranged from about 0.3 to 0.5 m s−1. The average horizontal fluxes of spores were calculated as the product of the observed concentrations and average wind speeds. At a distance of 2 m downwind from the sources, more than 16 to 44% of the flux of spores released from the lower source and more than 41 to 50% of the flux of spores released from the upper source were estimated to be above the canopy. These fluxes were compared with fluxes calculated using both a K-theory model and a random-flight-fluid-parcel-trajectory simulation model. The fluxes predicted by the models were generally considerably less than the values determined experimentally.

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

Abstract: 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.

Energy balance and transpiration from tussock grassland in New Zealand

Abstract: The energy balance was measured for the dry canopy of narrow-leaved snow tussock (Chionochloa rigida), and measurements of transpiration were obtained from a large weighing lysimeter.

Time Series Analyses of Concentration and Wind Fluctuations

Abstract: Analyses of concentration fluctuation (C’) spectra from boundary-layer smoke plume experiments at six separate locations show that the spectra from these experiments generally exhibit an inertial subrange at high frequencies with a slope of −5/3 and indicate peak energy at a time period of about 50 to 100s. These periods of peak energy are a factor of two to five less than those for the peak of the wind speed fluctuation (u’ or v’) spectra. A general spectral formula fits normalized spectra from the U.S. and Australia, where the frequency, n, is made dimensionless by multiplying by the plume dispersion parameter, σy, and dividing by the wind speed, u. Peak energy occurs at a dimensionless frequency of nσy/u equal to about 0.15. The Kolmogorov constant in the inertial subrange is estimated from a set of averaged spectra. Cross-spectra indicate little relation between concentration and wind fluctuations. However, most of the correlation that exists is due to periods larger than about 10 or 20 s.

Effect of finite sampling on atmospheric spectra

Abstract: The effect of a finite averaging time on variances is well known, but its effect on power spectra is less clearly understood. We present numerical solutions for the spectral distortion arising from sampling over a finite time interval T and show that the commonly used filter function (1-sinc2πfT), valid for variances, is a reasonable approximation for power spectra only when T≥ 10τm, where / is the cyclic frequency, and τm is the dominant time scale of the process. Our results exhibit an increasingly steeper low-frequency roll-off as T decreases relative to τm, indicating that the measured spectrum is subject to a greater suppression of the lower frequencies (f < 1/T) than predicted by (1-sinc2πfT). This suppression is, in a sense, compensated by an overestimation of spectral estimates in the frequency range f ≥ 1/T.

Internal Boundary-Layer Height Formulae — A Comparison with Atmospheric Data

Abstract: The height of the internal boundary layer (IBL) downwind of a step change in surface roughness is computed using formulae of Elliott (1958), Jackson (1976) and Panofsky and Dutton (1984). The results are compared with neutral-stratification atmospheric data extracted from the set of wind-tunnel and atmospheric data summarized by Jackson (1976) as well as neutral-stratification data presented by Peterson et al. (1979) and new data measured at Cherrywood, Ontario. It is found that the Panofsky-Dutton formulation gives the least root-mean-square (RMS) absolute errors for atmospheric applications.

An analysis scheme for determination of true surface winds at sea from ship synoptic wind and pressure observations

Abstract: For oceanographie studies, a high resolution description of the wind field at the sea surface is required. In order to estimate the air-sea interaction fluxes correctly, it is important that the derived wind field has the characteristics of actual surface winds. Ship synoptic observations are at irregular positions, while applications require boundary conditions at a regular grid. The problem is solved by locally fitting a second-order pressure surface to both wind and pressure observations, with the aid of a boundary-layer formulation. The results show that fitting wind and pressure data together provides better spatial resolution than using wind or pressure data alone, because more information is available. Wind data provide at least the same accuracy and less bias than pressure data, tested against independent wind observations. Also a stability and wind-dependent boundary-layer formulation results in less bias than a constant one. For the ship synoptic data available via GTS for the North Atlantic Ocean, spatial resolution, as expressed by an equivalent filter half-width, is 220 km, considerably improved compared to typical data assimilation schemes used in numerical forecast models.

Sensitivity studies on the calculation of the radiation balance of urban surfaces: I. Shortwave radiation

Abstract: The model developed in Verseghy and Munro (1989) is extended to the calculation of the longwave radiation incident on building surfaces. When compared with field measurements, the average magnitude of error associated with model predictions is found to be 10 W m−2. The effects of six simplifying assumptions are investigated. The neglect of horizon obstructions is found to lead to errors of up to 60 W m−2; the assumption of wall temperatures equal to air temperatures results in errors of up to 35 W m−2. The neglect of absorption and emission by air between pairs of walls causes errors of the same magnitude as those associated with the predictions of the rigorous model itself. Of the three remaining simplifying assumptions tested (the assumption of isotrophic sky radiation, the use of published values of emissivities instead of measured values, and the blackbody surface assumption), none results om errors >5 W m−2. As in the shortware case, the errors are site-specific, but nevertheless indicate the care with which the use of simplifying assumptions must be approached.

Wave Drag in the Planetary Boundary Layer Over Complex Terrain

Abstract: The concepts of mountain-induced wave drag are applied to the smaller scale problem of the boundary layer over complex terrain. It is found that the Reynolds stress and surface drag caused by surface-generated waves can be at least as large as those conventionally associated with turbulence. Conditions in which wave effects are important are identified.

Dry deposition of particles to a pine plantation

Abstract: There has been some controversy concerning the rate of deposition of particles having diameters near 1.0 μm to vegetated surfaces. In this size range, the processes of Brownian diffusion and inertial impaction are not effective and deposition to smooth surfaces reaches a minimum. However, most measurements of deposition of micrometer diameter particles to vegetated surfaces indicate a greater deposition than extrapolation of the results from less rough surfaces would suggest. In this study, the aerodynamic profile method was used to estimate deposition to a pine plantation. The deposition velocities were found to be sensitive to the displacement height and the form of the profile stability correction used in the calculations. An analysis of a limited set of Bowen ratio data, collected over the same forest, suggests that the data are reasonably described by using a displacement height of 7.9 m and the stability correction proposed by Raupach (1979). The average deposition velocities, measured over a 9-month period were 0.0043, 0.0078, and 0.0092 m/s for the three diameter classes 0.5\2-1.0, 1.0\2-2.0 and 2.0\2-5.0 \gmm. These deposition velocities are lower than the corresponding aerodynamic conductance for the same periods, indicating that the deposition rate is limited by surface phenomena. Average surface conductances calculated for the three size classes of particles were 0.0060, 0.0141, and 0.0276 m/s, respectively. A multiple regression analysis showed high correlation between deposition velocity and wind speed. No other measured environmental factor or linear combination of factors was significantly correlated with deposition velocity.

Velocity and temperature spectra and cospectra in an unstable suburban atmosphere

Abstract: Boundary-layer flow over very rough surfaces is poorly understood so the applicability of standard micrometeorological theory is uncertain. This study presents observations of the turbulent fluctuations of meteorological parameters over a suburban area. Even though the height of measurement is considered to be close to the junction between the inertial and roughness sub-layers, the wind and temperature spectra and the momentum and sensible heat flux cospectra are in good agreement with reference data from smoother surfaces. Recommendations are made concerning site requirements, height of measurement and averaging times for the study of turbulence and turbulent fluxes over suburban terrain.