E. F. Bradley

An alternative analysis of flux-gradient relationships at the 1976 ITCE

An extensive micrometeorological data set from the 1976 International Turbulence Comparison Experiment (ITCE) is analysed to determine flux-gradient relationships in an unstable atmosphere for momentum, sensible heat and water vapour transfers. The data are first analysed for internal consistency, resulting in the rejection of some data. Following a least-square fit to the remaining data in the form Φ/k = (1 − αz/L)-β/k, rounded-off values of k, α, and β are selected for each form of transfer consistent with the statistical accuracy of the measurements. The equations finally adopted are ΦM = (1 − 28z/L)-1/4 and ΦH, W = (1 − 14z/L)-1/2 with kM = kH = kW = 0.40.These expressions fit the averaged observations to within a few per cent in the stability range of the experiment (-4 < z/L < -0.004).

Boundary-layer flow over low hills

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.

Turbulent flow in a model plant canopy

An array of slender, vertical, cylindrical rods was used in a wind tunnel to simulate a plant canopy. Turbulence measurements were made with a cross hot wire, both inside and above the canopy. Measurements were also made inside the canopy when its top was covered by boards, leaving no space above the rods. This artificially confined canopy provided reference data.The results show an exponential wind profile and constant turbulence intensity, skewness and mixing length along the height of the (unconfined) canopy, the contribution of the eddies shed by the rods to the turbulence observed inside the canopy was small, but clearly apparent.

A wind tunnel study of turbulent flow over a two-dimensional ridge

We present a wind-tunnel simulation of adiabatic atmospheric flow normal to a rough, two-dimensional ridge. The data are analyzed in physical streamline coordinates, which are described in some detail. The mean velocity speed-up on the hill top is adequately predicted by existing formulae while the behaviour of the wake flow fits into a pattern that emerges from other wind-tunnel experiments. The turbulent stresses evolve in response to the extra strain rates induced by the hill, streamline curvature and acceleration: % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4baFfea0dXde9vqpa0lb9% cq0dXdb9IqFHe9FjuP0-iq0dXdbba9pe0lb9hs0dXda91qaq-xfr-x% fj-hmeGabaqaciGacaGaaeqabaWaaeaaeaaakeaacaWG1bWaaWbaaS% qabeaaceaIYaGbaebaaaaaaa!3456!\[u^{\bar 2}\]is coupled strongly to acceleration while % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4baFfea0dXde9vqpa0lb9% cq0dXdb9IqFHe9FjuP0-iq0dXdbba9pe0lb9hs0dXda91qaq-xfr-x% fj-hmeGabaqaciGacaGaaeqabaWaaeaaeaaakeaadaqdaaqaaiaadw% hacaWG3baaaaaa!3462!\[\overline {uw}\]and % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4baFfea0dXde9vqpa0lb9% cq0dXdb9IqFHe9FjuP0-iq0dXdbba9pe0lb9hs0dXda91qaq-xfr-x% fj-hmeGabaqaciGacaGaaeqabaWaaeaaeaaakeaacaWG3bWaaWbaaS% qabeaaceaIYaGbaebaaaaaaa!3458!\[w^{\bar 2}\]follow curvature. These differing responses lead to significant phase differences between the changes in the component stresses as the hill is traversed. An analogous response is seen in the components of turbulent stress divergence, which are computed as part of streamwise momentum budgets. Only very close to the surface is turbulent stress divergence comparable to the inertial and pressure terms in the momentum budget; over most of the flow regime, the mean flow response is approximately inviscid. Finally, we compare our results with data from other wind tunnel models and from real hills.

Turbulence Reynolds number and the turbulent kinetic energy balance in the atmospheric surface layer

The relation between the turbulence Reynolds numberRλ and a Reynolds numberz* based on the friction velocity and height from the ground is established using direct measurements of the r.m.s. longitudinal velocity and turbulent energy dissipation in the atmospheric surface layer. Measurements of the relative magnitude of components of the turbulent kinetic energy budget in the stability range 0 >z/L ≥ 0.4 indicate that local balance between production and dissipation is maintained. Approximate expressions, in terms of readily measured micrometeorological quantities, are proposed for the Taylor microscale λ and the Kolmogorov length scale η.

Measurements of flow over an elongated ridge and its thermal stability dependence: The mean field

Measurements of mean wind flow and turbulence parameters have been made over Coopers Ridge, a 115 m high elongated ridge with low surface roughness. This paper describes measurements of the streamwise and vertical variations in the mean field for a variety of atmospheric stability conditions. In near-neutral conditions, the normalised speedup over the ridge compares well with measurements from Askervein (Mickleet al., 1988). The near-neutral results are also compared to an analytical flow model based on that of Huntet al. (1988a). Measured streamwise variations show less deceleration at the foot of the hill and slightly more acceleration at the crest of the hill than does the model. In non-neutral conditions, the speedup over the ridge reduces slightly in unstable conditions and increases by up to a factor of two in stable conditions. The model is modified to allow boundary-layer stability to change the upwind wind profile and the depths of the inner and middle layers. Such a modification is shown to describe the observations of speedup well in unstable and weakly stable conditions but to overestimate the speedup in moderate to strongly stable conditions. This disagreement can be traced to the models overestimation of the upstream scaling velocity at the height of the middle layer through its use of a stable wind profile form which has greater shear than that of the observed profiles, in possible combination with the three-dimensionality of the ridge which would allow enhanced flow around, rather than over, the feature in more stable conditions.

Secondary flows in the lee of porous shelterbelts

The results of wind-tunnel experiments indicate that the flow close to the surface behind shelterbelts is quite sensitive to the orientation of the barrier to the mean flow direction, and that orientation is more important for short than long windbreaks. The implications of these findings are discussed in the context of field experiments to validate mathematical models of shelter effects.

Relationships between structure functions and temperature ramps in the atmospheric surface layer

The behaviour of odd-order structure functions of temperature in the atmospheric surface layer is consistent with the presence of a coherent structure characterised by temperature ramps. The use of a simple linear model for the ramp together with measurements of second-, third-, and fifth-order temperature structure functions yields estimates for the characteristic mean amplitude and repetition interval of ramps for stability conditions ranging from neutral to moderately unstable. Both amplitude and repetition interval compare favourably with values derived from observations of temperature traces obtained simultaneously at two heights in the surface layer.

Temperature structure in the atmospheric surface layer: II. The budget of mean cube fluctuations

Production, transport and dissipation terms in the mean cube temperature equation have been evaluated from measurements in the atmospheric surface layer under moderately unstable conditions. The transport term, calculated by making use of the quasi-Gaussian approximation, is, on average, comparable with the production term. The molecular destruction or dissipation term is approximately equal to the production. Since the transport and production terms are of the same sign, the magnitude of the dissipation term is only about half that of the sum of transport and production terms. It is suggested that the imbalance is caused primarily by the assumption of isotropy in the analysis for the mean dissipation rate, 293-01, and the effects of various parameterizations for this quantity are discussed. The measured value of the inertial subrange co-spectral constant for the third moment of temperature is 1.17. It is subject to the uncertainty in the estimate of 293-02, but is unlikely to be significantly different from the spectral constant for the second moment. The ratio of dissipative time values for the thermal and velocity fields is 0.5 near neutral, reducing with increasing instability.

An experimental appraisal of the terms in the heat and moisture flux equations for local advection

Terms in the heat and vapour flux equations, appropriate to the atmospheric surface layer with horizontal heterogeneity in one direction, have been evaluated empirically. The experimental site was a flooded rice field, which was bounded to windward by a semi-arid region. Local conditions over the rice were always stable, but the vertical fluxes of heat and water vapour were large.All terms in the flux equations were either measured directly, or, if sufficiently small, estimated, except the term containing fluctuating pressure, which was obtained by difference. The relative magnitudes of the major terms (production and pressure covariance) were similar to those reported previously for horizontally homogeneous flow with similar stabilities. Current parameterizations of the pressure covariance terms sometimes gave values which differed by factors of five or ten from the experimental results, and the interrelationships depended upon the stability.