Showing papers in "Boundary-Layer Meteorology in 1988"

Non-Dimensional Wind and Temperature Profiles in the Atmospheric Surface Layer: A Re-Evaluation

Abstract: Previous results of non-dimensional wind and temperature profiles as functions of ζ(= z/L) show systematic deviations between different experiments. These discrepancies are generally believed not to reflect real differences but rather instrumental shortcomings. In particular, it is clear that flow distortion has not been adequately treated in most previous experiments. In the present paper, results are presented from a surface-layer field experiment where great care was taken to remove any effects from this kind of error and also to minimize other measuring errors. Data from about 90 30-min runs with turbulence measurements at three levels (3, 6, and 14 m) and simultaneous profile data have been analysed to yield information on flux-gradient relationships for wind and temperature.

An eddy correlation technique with extended applicability to non-simple terrain

Abstract: A system is described which is intended to calculate vertical fluxes of heat, moisture, momentum, and certain atmospheric pollutants at sites that are less than ideal. Fluxes, along with other turbulence statistics, are computed in real-time and printed at the end of each averaging period. The main elements of the program are (1) ‘detrending’ (by use of running mean removal), (2) calculation of the entire stress tensor (which allows a three-dimensional coordinate rotation to be performed on the covariances), (3) software-adjustable timing delays for each instrument channel, and (4) real-time graphic presentation of the raw data as stripchart images. The first two of these program elements tend to relax the normal site and sensor-leveling requirements. Sample results are presented, and the sensitivities of the calculated quantities to coordinate rotation and to mean removal time are examined for both ideal and non-ideal sites.

Turbulence structure in a deciduous forest

Abstract: Three-dimensional wind velocity components were measured at two levels above and at six levels within a fully-leafed deciduous forest. Greatest shear occurs in the upper 20% of the canopy, where over 70% of the foliage is concentrated. The turbulence structure inside the canopy is characterized as non-Gaussian, intermittant and highly turbulent. This feature is supported by large turbulence intensities, skewness and kurtosis values and by the large infrequent sweeps and ejections that dominate tangential momentum transfer. Considerable day/night differences were observed in the vertical profiles of the mean streamwise wind velocity and turbulence intensities since the stability of the nocturnal boundary layer dampens turbulence above and within the canopy.

Influence of foliar density and thermal stability on profiles of Reynolds stress and turbulence intensity in a deciduous forest

Abstract: Observations were made of turbulence in an extensive deciduous forest on level terrain using a vertical array of seven three-dimensional sonic anemometer/thermometers within and above the canopy. Data were collected through the period of leaf fall and over a range of thermal stabilities. A bulk canopy drag coefficient was nearly independent of the density of the forest but decreased greatly with the onset of nocturnal stability. The depth of penetration of momentum into the forest increased with leaf fall but, again, was greatly curtailed by stable conditions. Turbulent velocities decreased with increasing depth in the forest but relative turbulence intensities increased to mid-canopy levels. Leaf density influenced turbulence levels but not as strongly as did thermal stability. Thermal effects were adequately described by the single parameter h/L, where h is the canopy height and L is the Monin-Obukhov length. The longitudinal and vertical velocity correlation coefficient was larger in magnitude than expected in the upper layers of the forest but decreased to a small value in the lowest layers where the Reynolds stress was small. The ratio Σ w /u *, where u * is the local friction velocity, reflected changes in the uw correlation, becoming smaller than usual in the upper canopy layers. It is believed that these effects result from the intermittent, spatially coherent structures that are responsible for a large fraction of the momentum flux to the forest.

A second-order closure model for flow through vegetation

Abstract: By splitting the turbulent kinetic energy into two wavebands and adopting as the turbulence timescale the ratio k/ɛ of the kinetic energy in the low-frequency band to its turnover-rate, the second-order closure scheme of Launder et al. (1975) has been adapted for flow through vegetation. Predictions of the model compare satisfactorily with observations of the mean windspeed and (somewhat less satisfactorily) with the turbulent velocity variances in two very different canopies.

Observations of a multi-level turbulence structure in a very stable atmospheric boundary layer

Abstract: Boundary-layer measurements conducted at the Marsta site in Sweden from a winter-time situation (23–25 Feb.) with stable stratification have been analysed. The data comprise wind and temperature profile measurements up to 30 m, turbulence measurements at 2, 6 and 30 m and Doppler acoustic sounder data up to about 150 m. The upwind fetch at the site is flat and free from obstacles to a distance of ca 5 km for the particular sector chosen for the experiment. During the night, a two-layer vertical structure developed. Analysis of power spectra, co-spectra and variances in a shallow and very stable turbulent boundary layer near the ground show that the turbulence is fully developed and follow the universal behaviour. Above, at a height of 30 m, another turbulent layer is produced by increased wind shear near a low-level jet. This turbulent upper layer can be regarded as a layer of free shear flow. At this height, there also exist wave-turbulence interactions at low frequencies which sometimes cause a countergradient heat flux.

A Note on the Businger-Dyer Profiles

Abstract: In the micrometeorological literature, reference is sometimes made to the ‘Businger-Dyer Profiles’ or the ‘Dyer-Businger profiles/relations’ without referring to the origin of these relations. For example, in the textbook on ‘Atmospheric Turbulence’ by Panofsky and Dutton (1984) on p. 134, reference is made to the ‘Businger-Dyer formula’. To add to the mystery, these authors refer on p. 141 to the Businger-Dyer-Pandolfo empirical result (Businger, 1966; Pandolfo, 1966) that in unstable air $$ Ri = \frac{z}{L} \equiv \zeta $$ (1) .

On the contribution of atmospheric moisture to dew formation

Abstract: The relative contributions of dewfall (a flux of water vapour from air to surface) and distillation (a flux of water vapour from soil to canopy) to dew formation on closed canopy and bare soil surfaces are assessed, and the dependence of dew amount upon wind speed, absolute temperature, atmospheric stability, relative humidity, soil characteristics and cloudiness, all of which are significant factors, is evaluated. Some of these evaluations provide refinements to similar ones given in Monteith (1961). High dewfall rates are usually ≲0.06 mm hr−1 over canopy or bare soil, though upon a canopy under soil-saturated and air-saturated conditions, rates of dew formation may reach 0.07–0.09 mm hr−1 with contributions from distillation. Various sets of observations are reanalyzed to illustrate the importance of the horizontal advection of moisture in the nocturnal boundary layer (NBL) to observed high rates of dew formation arising from the atmospheric contribution of water vapour (dewfall). These locally observed high dewfall rates must be the result of small-scale or mesoscale horizontal advection of moisture in the NBL, since the humidity changes within the typically shallow NBL required to balance the loss of water at the surface are not observed. Over extensive areas of uniform surface (horizontal scales ≫10 km), such continuously high dewfall rates could only be balanced by a local supply of atmospheric moisture since advection of moisture would necessarily be small.

The stably stratified boundary layer over the Great Plains. I: Mean and turbulence structure

Abstract: Mesures aeriennes de la couche limite stratifiee de facon stable, nocturne, obtenues au cours de SESAME en 1979. Les cas presentes ici correspondent a un terrain ondule dans l'Oklahoma central, de pente moyenne 0,003. Resultats en accord avec des modelisations et observations precedentes, en ce qui concerne la structure moyenne et celle turbulente de la couche, a l'exception de la diffusivite turbulente de chaleur et donc du nombre de Richardson de flux qui ont ete trouves plus faibles que prevus

A spectral and lag-correlation analysis of turbulence in a deciduous forest canopy

Abstract: The processes influencing turbulence in a deciduous forest and the relevant length and time scales are investigated with spectral and cross-correlation analysis. Wind velocity power spectra were computed from three-dimensional wind velocity measurements made at six levels inside the plant canopy and at one level above the canopy. Velocity spectra measured within the plant canopy differ from those measured in the surface boundary layer. Noted features associated with the within-canopy turbulence spectra are: (a) power spectra measured in the canopy crown peak at higher wavenumbers than do those measured in the subcanopy trunkspace and above the canopy; (b) peak spectral values collapse to a relatively universal value when scaled according to a non-dimensional frequency comprised of the product of the natural frequency and the Eulerian time scale for vertical velocity; (c) at wavenumbers exceeding the spectral peak, the slopes of the power spectra are more negative than those observed in the surface boundary layer; (d) Eulerian length scales decrease with depth into the canopy crown, then increase with further depth into the canopy; (e) turbulent events below crown closure are more correlated with turbulent events above the canopy than are those occurring in the canopy crown; and (f) Taylor's frozen eddy hypothesis is not valid in a plant canopy. Interactions between plant elements and the mean wind and turbulence alter the processes that produce, transport and remove turbulent kinetic energy and account for the noted observations.

The influence of surface cover and climate on energy partitioning and evaporation in a subarctic wetland

Abstract: Energy partitioning and evaporation were measured over three wetland surfaces in a subarctic coastal marsh during pre-growing and growing periods. These surfaces included an alder/willow woodland, a sedge marsh and a raised backshore sedge meadow. A combination model analysis was used to assess the relative importance of surface resistance and meteorological conditions on the magnitude of the Bowen ratio, Β, during the growing period. Overall, the three surfaces experienced important site-to-site and seasonal differences in Β and evaporation, Q E. During the non-foliated period, Q E was largest and Β was smallest for the open water marsh, while the dry backshore site experienced the smallest Q E and largest Β. The non-foliated woodland assumed intermediate values of Β and Q E. After the vegetation covers were established, the woodland assumed the smallest Β and largest Q E flux. It was also found that Β at the marsh site increased with the presence of a vegetation cover. Wind direction was always an important factor in determining Q E and Β at all sites. Β was substantially larger and Q E was smaller for onshore winds (i.e., originating from James Bay) than for offshore winds. The combination model analysis showed that canopy resistance at all sites was largest during warm offshore winds, which were associated with large saturation deficits. However, the effect of increased canopy resistance on Β during offshore winds was offset by a large climatological resistance, resulting in small Β values and large Q E. When winds originated from James Bay, canopy resistance was smaller than for offshore winds, but the climatological resistance also was much smaller, resulting in larger Β and small Q E. The results have important implications for changes in land cover and climate on the regional water balance.

Use of variance techniques to measure dry air-surface exchange rates

Abstract: The variances of fluctuations of scalar quantities can be measured and interpreted to yield indirect estimates of their vertical fluxes in the atmospheric surface layer. Strong correlations among scalar fluctuations indicate a similarity of transfer mechanisms, which is utilized in some of the variance techniques. The ratios of the standard deviations of two scalar quantities, for example, can be used to estimate the flux of one if the flux of the other is measured, without knowledge of atmospheric stability. This is akin to a modified Bowen ratio approach. Other methods such as the normalized standard-deviation technique and the correlation-coefficient technique can be utilized effectively if atmospheric stability is evaluated and certain semi-empirical functions are known. In these cases, iterative calculations involving measured variances of fluctuations of temperature and vertical wind velocity can be used in place of direct flux measurements. For a chemical sensor whose output is contaminated by non-atmospheric noise, covariances with fluctuations of scalar quantities measured with a very good signal-to-noise ratio can be used to extract the needed standard deviation. Field measurements have shown that many of these approaches are successful for gases such as ozone and sulfur dioxide, as well as for temperature and water vapor, and could be extended to other trace substances. In humid areas, it appears that water vapor fluctuations often have a higher degree of correlation to fluctuations of other trace gases than do temperature fluctuations; this makes water vapor a more reliable companion or “reference” scalar. These techniques provide some reliable research approaches but, for routine or operational measurement, they are limited by the need for fast-response sensors. Also, all variance approaches require some independent means to estimate the direction of the flux.

The Askervein Hill Project: Vertical profiles of wind and turbulence

Abstract: This is one of a series of papers on the Askervein Hill Project. It presents results from the Askervein 1982 and 1983 experiments in the form of vertical profiles of mean wind and turbulence integral statistics at upwind reference locations and at two hilltop sites. The data were obtained from a variety of sensors including sonic, Gill UVW and cup anemometers mounted on 50, 30, 17 and 10 m towers and TALA kite systems. Comparisons with numerical-model predictions are discussed.

Minimum friction velocity and heat transfer in the rough surface layer of a convective boundary layer

Abstract: A simple model is deduced for the surface layer of a convective boundary layer for zero mean wind velocity over homogeneous rough ground. The model assumes large-scale convective circulation driven by surface heat flux with a flow pattern as it would be obtained by conditional ensemble averages. The surface layer is defined here such that in this layer horizontal motions dominate relative to vertical components. The model is derived from momentum and heat balances for the surface layer together with closures based on the Monin-Obukhov theory. The motion in the surface layer is driven by horizontal gradients of hydrostatic pressure. The balances account for turbulent fluxes at the surface and fluxes by convective motions to the mixed layer. The latter are the dominant ones. The model contains effectively two empirical coefficients which are determined such that the model's predictions agree with previous experimental results for the horizontal turbulent velocity fluctuations and the temperature fluctuations. The model quantitatively predicts the decrease of the minimum friction velocity and the increase of the temperature difference between the mixed layer and the ground with increasing values of the boundary layer/roughness height ratio. The heat transfer relationship can be expressed in terms of the common Nusselt and Rayleigh numbers, Nu and Ra, as Nu ~ Ra% MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaSGbaeaaca% aIXaaabaGaaGOmaaaaaaa!3779!\[{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern- ulldelimiterspace} 2}\]. Previous results of the form Nu ~ Ra% MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaSGbaeaaca% aIXaaabaGaaG4maaaaaaa!377A!\[{1 \mathord{\left/ {\vphantom {1 3}} \right. \kern- ulldelimiterspace} 3}\] are shown to be restricted to Rayleigh-numbers less than a certain value which depends on the boundary layer/roughness height ratio.

An observational study of the structure of the nocturnal urban boundary layer

Abstract: The formation mechanism of the nocturnal urban boundary layer (UBL), especially in the winter nighttime, was investigated based on the extensive field observations conducted during November 1984 in Sapporo, Japan. A strong, elevated inversion formed over the Sapporo urban area and the inversion base height was approximately twice the average building height. Velocity fluctuations Σ u, Σw and Reynolds stress % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaa0aaaeaaca% WG1bWaaWbaaSqabeaacaaIXaaaaGGaaOGae8hiaaIaam4DamaaCaaa% leqabaGaaGymaaaaaaaaaa!3A9C!\[\overline {u^1 w^1 } \] had nearly uniform profiles within the nocturnal UBL and decreased with height above the UBL. On the other hand, temperature fluctuations Σ t , and heat fluxes % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaa0aaaeaaca% WG1bWaaWbaaSqabeaacaaIXaaaaGGaaOGae8hiaaIaeqiUde3aaWba% aSqabeaacaaIXaaaaaaaaaa!3B56!\[\overline {u^1 \theta ^1 } \] and % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaa0aaaeaaca% WG3bWaaWbaaSqabeaacaaIXaaaaGGaaOGae8hiaaIaeqiUde3aaWba% aSqabeaacaaIXaaaaaaaaaa!3B58!\[\overline {w^1 \theta ^1 } \] had peaks at the inversion base and small values within the nocturnal UBL. The turbulent kinetic energy budget showed that the turbulent transport term and shear generation from urban canopy elements are important in the nocturnal UBL development; the role of the buoyancy term is small. The turbulence data analysis and application of a simple advective model showed that the mechanism of UBL formation may be controlled by the downward transport of sensible heat from the elevated inversion caused by mechanically-generated turbulence.

The Stably Stratified Boundary Layer over the Great Plains

Abstract: Airplane measurements of the stably stratified boundary layer obtained during the Severe Environmental Storms and Mesoscale Experiment (SESAME) over rolling terrain in south-central Oklahoma indicate that considerable horizontal variability exists in the flow on scales of several kilometers. Much of this wave-like structure appears to be tied to the terrain. The criteria for existence of stationary gravity waves indicate that these waves can exist under the observed conditions. The spectrum of terrain variations also supports the existence of these waves. Observed spectra of the vertical velocity have two peaks: one at wavelengths of several kilometers, which is due to waves and the other at wavelengths of about 100 m, which is due to turbulence. The variance at several kilometers wavelength increases somewhat with height at least up to about 800 m, but the variance contributed by turbulence decreases rapidly with height.

On the micrometeorology of surface hoar growth on snow in mountainous area

Abstract: Profiles of temperature and humidity are calculated for a snow surface being cooled by outgoing radiation assuming no wind. These suggest that fog would necessarily form to block the cooling and, even in the absence of fog, the diffusion of water vapor to the surface could not account for observed rates of surface hoar growth. Therefore turbulent transfer to an interfacial sublayer with a very large temperature gradient must occur at least intermittently. In mountainous areas, gravity drainage of air over the interfacial sublayer probably accounts for the turbulence since air currents from a regional pressure gradient appear to transfer enough heat to the surface to prevent surface hoar growth. The vapor must diffuse through a 1 mm interfacial sublayer until the crystals reach a sufficient height to interact with the drainage wind.

Characterization of concentration fluctuations of a surface plume in a neutral boundary layer

Abstract: We describe a new high-resolution sampling technique which can be used to measure concentration fluctuations simultaneously at several points in space. The technique has been used to measure the probability distribution function as a function of the detector location relative to a continuous and steady source. Results are compared to previous experiments and theoretical predictions. The spectra of the concentration fluctuations are analyzed and their behaviour as a function of downwind distance from the source is described.

Flow-distortion effects on scalar flux measurements in the surface layer: Implications for sensor design

Abstract: Scalar fluxes measured through the eddy-correlation technique are prone to two types of errors caused by the sensor-induced flow distortion: those due to crosstalk from the horizontal flux, and those due to amplification or attenuation due to flow blocking. We show that the crosstalk error can be eliminated by designing the sensor array to be vertically symmetric about its horizontal midplane. In such an array, the flow-blocking effect causes the scalar flux to be overestimated, but this error can be made negligible by designing an array with minimal stagnation loss in streamwise speed at the flux-measurement point.

Nocturnal boundary-layer height: Observations by acoustic sounders and predictions in terms of surface-layer parameters

Abstract: Acoustic sounder measurements of the stable boundary-layer height taken during the EPRI Plume Model Validation and Development Project experiment are examined. Comparison of simultaneous measurements by two sodars located 15 km apart shows good agreement. Several widely used diagnostic formulas for estimation of the boundary-layer height, based on wind speed and surface-layer parameters, such as friction velocity and Monin-Obukhov length, are tested against the sodar data. Of these, best performance is found using a simple linear relationship with friction velocity or, alternatively, wind speed at 10 m height. No evidence is found to support the more often used Zilitinkevich (1972) formula. Tests using selected data from the Cabauw site in the Netherlands confirm the results found on the basis of EPRI data.

The Kettles Hill Project: Field observations, wind-tunnel simulations and numerical model predictions for flow over a low hill

Abstract: Field observations of the influence of topography on steady, neutrally-stratified boundary-layer flow were carried out in February 1981 and March 1984 on Kettles Hill near Pincher Creek, Alberta, Canada. The primary measurements were of wind speed at 3,6, and 10 m levels at stations in linear arrays along and across the major axis of this gentle, 1 km long and 100 m high, elliptical hill. Wind profile measurements up to heights of 200 m were made with TALA kites and tethersondes on the hilltop and at a reference site located about 3.7 km west of the hilltop. In addition, AIRsondes were flown and tracked from the reference site to provide additional data. The field observations provided the basic data for a comparison with wind-tunnel and numerical model simulations of the same flow. The wind-tunnel investigation was carried out in the Atmospheric Environment Service Boundary-Layer Wind Tunnel while the numerical model used was MS3DJH. For ‘horizontal’ profiles of normalized mean wind speed at given heights above the prototype terrain, model results agree reasonably well with the field data. The wind-tunnel predictions are slightly high in most cases. For vertical profiles of wind speed up to 200 m above the hilltop, the numerical and wind-tunnel values are higher than were observed. The sensitivity of the normalized wind speed at the hilltop to deviations from non-logarithmic upwind profiles is demonstrated with data from the March 1984 experiment. A comparison of prototype with numerical-model mean-wind-direction perturbations at the 10 m level shows reasonable agreement except near the summit of the hill.

Observations of longitudinal roll vortices during arctic cold air outbreaks over open water

Abstract: An evolving convective Arctic planetary boundary layer (PBL) containing longitudinal roll vortices (rolls) was observed with aircraft data during the 1983 Marginal Ice Zone Experiment and the 1984 Arctic Cyclone Experiment.

Statistics of atmospheric turbulence within a natural black spruce forest canopy

Abstract: Turbulence statistics were measured in a natural black-spruce forest canopy in southeastern Manitoba, Canada. Sonic anemometers were used to measure time series of vertical wind velocity (w), and cup anemometers to measure horizontal wind speed (s), above the canopy and at seven different heights within the canopy. Vertical profiles were measured during 25 runs on eight different days when conditions above the canopy were near-neutral. Profiles of s and of the standard deviation (Σ w ) of w show relatively little scatter and suggest that, for this canopy and these stability conditions, profiles can be predicted from simple measurements made above the canopy. Within the canopy, a negative skewness and a high kurtosis of the w-frequency distributions indicate asymmetry and the persistence of large, high-velocity eddies. The Eulerian time scale is only a weak function of height within the canopy. Although w-power spectra above the canopy are similar to those in the free atmosphere, we did not observe an extensive inertial subrange in the spectra within the canopy. Also, a second peak is present that is especially prominent near the ground. The lack of the inertial subrange is likely caused by the presence of sources and sinks for turbulent kinetic energy within our canopy. The secondary spectral peak is probably generated by wake turbulence caused by form drag on the wide, horizontal spruce branches.

The Askervein Hill Project: Mean wind variations at fixed heights above ground

Abstract: This is one of a series of papers on the Askervein Hill Project. It presents results on the variations in mean wind speed at fixed heights (δz) above the ground from linear arrays of anemometer posts and towers. Most of the data are for δz = 10 m but some are for δz = 3 m. Selected and directionally grouped data from the 55 ‘Mean Flow’ runs are presented together with mean flow data from Askervein '83 ‘Turbulence’ runs. Comparisons are made between the data and guideline estimates of fractional speed-up ratio at hilltop locations and between the data and MS3DJH/3 model predictions along the tower lines. There is good agreement in most cases.

A study of wind speed modification and internal boundary-layer heights in a coastal region

Abstract: Wind profile data within the first two kilometres of a coast have been used to study the wind field modification downstream of this surface discontinuity The land area is generally very flat, having an overall roughness length of 004 m A wind model, suitable for practical applications and inexpensive to run, has been tested against the data and was found to give satisfactory results Knowing the climatological statistics of wind and stratification, eg, at the coast, the model may thus be used to estimate, on a climatological basis, how the wind field is modified with distance inland, at least in areas with only minor topography This type of information is of great importance when locating wind turbines It is in these cases also important to know the statistics of the internal boundary-layer (IBL) height, as the turbulence intensity may be quite different in and above the IBL, which in turn may influence load and fatigue calculations Using the wind profile data, the IBL height was clearly discernible in the majority of cases Having very unstable stratification over land, the IBL height could, however, not be determined from the wind profiles, as the wind in these cases did not decrease inland This result was also obtained using the wind model A simple model of the type z IBL = a · x b, was instead tested, and was shown to give reasonable results

A simple model of drainage flow on a slope

Abstract: The concept of a cold air ‘Parcel’ is introduced for describing the bulk properties of drainage flow. By means of a model based on the momentum and sensible heat transports under calm conditions, the thickness h and velocity u of the Parcel are derived in simple forms. It is shown that h and u correspond to the inversion height and maximum velocity of actual drainage flow. The governing parameters for h and u are the length and vertical drop of the slope, potential temperature difference between the ambient atmosphere and the Parcel, aerodynamic condition of the slope surface expressed by the mean bulk coefficients, and ambient stability. The mean bulk coefficients depend on the roughness lengths for the velocity and potential temperature profiles and are decreasing functions of the slope length. The Parcel Model agrees qualitatively with Manins and Sawford's (1979) model under neutral ambient stratification. But agreement is not so good under stable conditions. The thickness and velocity of drainage flow predicted by the Parcel Model agree with observations on slopes several tens of meters to several hundred kilometers long.

On the measurement of dry deposition using imperfect sensors and in non-ideal terrain

Abstract: Important questions concerning the turbulent exchange of atmospheric pollutants between the air and natural surfaces urgently require answers, but sensors for many important species are not yet sufficiently well developed for use with standard micrometeorological methods. There is need, therefore, to develop methods by which deficient sensors can be used in micrometeorological applications. There is also need to extend micrometeorological methods to circumstances which do not satisfy the conventional perfect-site constraints. Here, methods based upon the assumption of cospectral similarity are explored. Initial tests indicate that it is possible to estimate daytime turbulent fluxes with sensors giving response times considerably greater than the values normally quoted for eddy correlation (e.g., 5 s instead of 1 s), and to compute first-order corrections for the error resulting from the lack of detection of high-frequency turbulence. It is suggested that a similar method might be used to derive flux data in terrain more complex than can be handled by conventional micrometeorology. The techniques outlined here should be applied only with caution, but appear adequate to permit the use of deficient sensors in some circumstances, and good sensors over some micrometeorologically deficient terrain.

Tests of three urban energy balance models

Abstract: Observations of surface characteristics, meteorological conditions and energy balance components from Vancouver, B.C. are used to test the validity of the output from three one-dimensional surface energy balance models. The results show that whereas all of the models provide good simulations of net radiation, none can consistently predict the turbulent fluxes of sensible and latent heat using easily available input data. Inability to handle the role of water availability and its impact on evapotranspiration is identified as the principal problem.

Structure of the stably-stratified boundary layer during the SESAME-1979 experiment

Abstract: Airplane measurements in the stable boundary layer during the SESAME-1979 experiment (southcentral Oklahoma, USA) are examined in terms of the local similarity theory The obtained results are compared with data from the Minnesota (1973) experiment and with data collected previously on towers in Cabauw (Netherlands) and Boulder (Colorado, USA) The comparison indicates that the SESAME data are strongly influenced by the terrain features of the experimental site

Boundary-layer resistances of artificial leaves in turbulent air: I. Leaves parallel to the mean flow

Abstract: Boundary-layer resistance to heat transfer from plates was studied in a wind tunnel which produced turbulence with streamwise intensity in the range 3.5 to 25% and a longitudinal integral scale of the streamwise turbulence component (L u,x) in the range from 8 to 100 mm. It was found that heat transfer enhancement occurred due to the turbulence, and that at any given intensity, this enhancement was determined by the ratio of L u,x to the characteristic dimension of the plate.