Showing papers in "Boundary-Layer Meteorology in 1986"

Frequency response corrections for eddy correlation systems

Abstract: Simplified expressions describing the frequency response of eddy correlation systems due to sensor response, path-length averaging, sensor separation and signal processing are presented. A routine procedure for estimating and correcting for the frequency response loss in flux and variance measurements is discussed and illustrated by application to the Institute of Hydrology's ‘Hydra’ eddy correlation system. The results show that flux loss from such a system is typically 5 to 10% for sensible and latent heat flux, but can be much larger for momentum flux and variance measurements in certain conditions. A microcomputer program is included which, with little modification, can be used for estimating flux loss from other eddy correlation systems with different or additional sensors.

A simple model of the atmospheric boundary layer; sensitivity to surface evaporation

Abstract: A simple formulation of the boundary layer is developed for use in large-scale models and other situations where simplicity is required. The formulation is suited for use in models where some resolution is possible within the boundary layer, but where the resolution is insufficient for resolving the detailed boundary-layer structure and overlying capping inversion. Surface fluxes are represented in terms of similarity theory while turbulent diffusivities above the surface layer are formulated in terms of bulk similarity considerations and matching conditions at the top of the surface layer. The boundary-layer depth is expressed in terms of a bulk Richardson number which is modified to include the influence of thermals. Attention is devoted to the interrelationship between predicted boundary-layer growth, the turbulent diffusivity profile, ‘countergradient’ heat flux and truncation errors. The model predicts growth of the convectively mixed layer reasonably well and is well-behaved in cases of weak surface heat flux and transitions between stable and unstable cases. The evolution of the modelled boundary layer is studied for different ratios of surface evaporation to potential evaporation. Typical variations of surface evaporation result in a much greater variation in boundary-layer depth than that caused by the choice of the boundary-layer depth formulation.

Experiments on scalar dispersion within a model plant canopy part I: The turbulence structure

Abstract: This is the first of a series of three papers describing experiments on the dispersion of trace heat from elevated line and plane sources within a model plant canopy in a wind tunnel. Here we consider the wind field and turbulence structure. The model canopy consisted of bluff elements 60 mm high and 10 mm wide in a diamond array with frontal area index 0.23; streamwise and vertical velocity components were measured with a special three-hot-wire anemometer designed for optimum performance in flows of high turbulence intensity. We found that: (i) The momentum flux due to spatial correlations between time-averaged streamwise and vertical velocity components (the dispersive flux) was negligible, at heights near and above the top of the canopy. (ii) In the turbulent energy budget, turbulent transport was a major loss (of about one-third of local production) near the top of the canopy, and was the principal gain mechanism lower down. Wake production was greater than shear production throughout the canopy. Pressure transport just above the canopy, inferred by difference, appeared to be a gain in approximate balance with the turbulent transport loss. (iii) In the shear stress budget, wake production was negligible. The role of turbulent transport was equivalent to that in the turbulent energy budget, though smaller. (iv) Velocity spectra above and within the canopy showed the dominance of large eddies occupying much of the boundary layer and moving downstream with a height-independent convection velocity. Within the canopy, much of the vertical but relatively little of the streamwise variance occurred at frequencies characteristic of wake turbulence. (v) Quadrant analysis of the shear stress showed only a slight excess of sweeps over ejections near the top of the canopy, in contrast with previous studies. This is a result of improved measurement techniques; it suggests some reappraisal of inferences previously drawn from quadrant analysis.

A mixed-layer model for regional evaporation

Abstract: A simple mixed-layer model is developed to describe evaporation into a convective planetary boundary layer (PBL). The model comprises volume budget equations for temperature and humidity, equations to describe transport through the surface layer which is treated as part of the lower boundary, and equations to describe entrainment at the top of the PBL. The ground surface is modelled as a canopy resistance. The model was integrated with canopy resistance, surface-layer resistance and available energy, (R n − G), input as given functions of time, and the simulated PBL was allowed to grow into an atmosphere with known temperature and humidity profiles. Two variants of the mixed-layer model were tested using data from the KNMI tower site at Cabauw in the Netherlands. These variants differed only in the formulation of entrainment: one used a formulation developed by Driedonks (1982) while the other was a simpler formulation. Simulated evaporation agreed very well with observations irrespective of which entrainment formulation was used, despite discrepancies between simulated and observed PBL height growth which were sometimes quite large for the simpler formulation. Sensitivity analysis of the model confirms that good PBL height-growth predictions are not always a prerequisite for good evaporation predictions.

Scaling the atmospheric boundary layer

Abstract: We review scaling regimes of the idealized Atmospheric Boundary Layer. The main emphasis is given on recent findings for stable conditions. We present diagrams in which the scaling regimes are illustrated as a function of the major boundary-layer parameters. A discussion is given on the different properties of the scaling regimes in unstable and stable conditions.

Eddy fluxes of CO2, water vapor, and sensible heat over a deciduous forest

Abstract: Fluxes of CO2, latent heat and sensible heat were measured above a fully-leafed deciduous forest in eastern Tennessee with the eddy correlation technique. These are among the first reported observations over such a surface. The influences of solar radiation, vapor pressure deficit and the aerodynamic and canopy resistances on these mass and energy exchanges are examined. Following a concept introduced by McNaughton and Jarvis (1983), examination of our data suggest that the water vapor exchange of a deciduous forest is not as strongly coupled with net radiation as is that of agricultural crops. The degree of decoupling is smaller than in the case of a coniferous forest. This difference may be attributable in part to the greater aerodynamic resistance to water vapor transfer in a deciduous forest. It appears that the concept of decoupling may be extended to the CO2 exchange of a deciduous forest as well.

A note on estimating the effect of a limited fetch on micrometeorological evaporation measurements

Abstract: A formula for the effective fetch of micrometeorological evaporation measurements is derived by application of diffusion theory, using Calder's approximation of a uniform wind field and neutral atmospheric stability. This simplification allows estimation of the likely sampling error, which would result from an upwind step-change in evaporation rate, without change in roughness.

Testing of a higher-order closure model for modeling airflow within and above plant canopies

Abstract: A higher-order closure model was developed to simulate airflow within and above vegetative environments. The model consists of equations for the mean wind, turbulent kinetic energy (TKE) components, tangential stress and simplified equations for the third-order transport terms that appear in the second-order equations. The model in general successfully simulated wind speed profiles within and above maize, been, soybeen, wheat, orange and spruce canopies. Profiles of % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaa0aaaeaace% WG1bGbauaadaahaaWcbeqaaiaaikdaaaaaaaaa!37EC!\[\overline {u'^2 } \] and % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaa0aaaeaace% WG3bGbauaadaahaaWcbeqaaiaaikdaaaaaaaaa!37EE!\[\overline {w'^2 } \] for the maize canopy were overestimated near the top of the canopy where both shear and wake production of TKE are high. These errors are believed to be caused by incorrect parameterizations for either the dissipation rate of TKE and/or the pressure-velocity correlations in the budget equations for the second moments.

Radiative surface temperature and energy balance of a wheat canopy. I: Comparison of radiative and aerodynamic canopy temperature

Abstract: For a stand of winter wheat, radiative canopy temperature measured with an infra-red radiometer was systematically related to a surface temperature derived from air temperature and wind speed profiles. Radiative temperature changed significantly with viewing angle and azimuth, but the influences of sun angle and ground cover were minimised by inclining the radiometer at 55 ° to the vertical and at right angles to the solar beam.

A simple model of neutrally stratified boundary-layer flow over complex terrain with surface roughness modulations (MS3DJH/3R)

Abstract: The MS3DJH series of simple models of flow over low hills and other terrain features described in earlier papers (see Taylor et al, 1983) required that the terrain was of uniform surface roughness In the present paper, we describe an approximate theory of flow above variations in surface roughness using a similar structure to that established by Jackson and Hunt (1975) for flow over hills This then allows us to include the calculation of flow perturbations due to roughness variations within a modified version of our model which we designate as MS3DJH/3R Comparisons are made with alternative calculations for simple two-dimensional flows; and sample three-dimensional calculations are presented The model retains its essential features of high spatial resolution and low computing cost

Inertial-dissipation methods and turbulent fluxes at the air-ocean interface

Abstract: The use of high frequency atmospheric turbulence properties (inertial subrange spectra, structure function parameters or dissipation rates) to infer surface fluxes of momentum, sensible heat and latent heat is more practical for most ocean going platforms than direct covariance measurement. The relationships required to deduce the fluxes from such data are examined in detail in this paper and several ambiguities and uncertainties are identified. It is noted that, over water, data on water vapor properties (the dimensionless functions for the mean profile, the structure function parameter and the variance transport term) are extremely sparse and the influence of sea spray is largely unknown. Special attention is given to flux estimation on the basis of the structure function formalism. Existing knowledge about the relevant similarity functions is summarized and discussed in light of the ambiguities identified above.

On similarity in the atmospheric boundary layer

Abstract: A similarity theory for the atmospheric boundary layer is presented. The Monin-Obukhov similarity theory for the surface layer is a particular case of this new theory, for the case of z → 0. Universal functions which are in agreement with empirical data are obtained for the stable and convective regimes.

Experiments on scalar dispersion within a model plant canopy part II: An elevated plane source

Abstract: An experiment is reported in which heat was released as a passive tracer from an elevated lateral line source within a model plant canopy, with hs = 0.85 hc (hs and hc being the source and canopy heights, respectively). A sensor assembly consisting of three coplanar hot wires and one cold wire was used to measure profiles of mean temperature % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaaiikamaana% aabaGaeqiUdehaaiaacMcaaaa!390C!\[(\overline \theta )\], temperature variance (Σθ2), vertical and streamwise turbulent heat fluxes, and third moments of wind and temperature fluctuations. Conclusions were: (i) Despite the very heterogeneous flow within the canopy, the observed dispersive heat flux (due to spatial correlation between time-averaged temperature and vertical velocity) was small. However, there is evidence from the plume centroid (which was lower than hs at the source) of systematic recirculating motions within the canopy. (ii) The ratio % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeq4Wdm3aaS% baaSqaaiabeI7aXjaab2gacaqGHbGaaeiEaaqabaGccaGGVaWaa0aa% aeaacqaH4oqCaaWaaSbaaSqaaiaab2gacaqGHbGaaeiEaaqabaaaaa!41DF!\[\sigma _{\theta {\text{max}}} /\overline \theta _{{\text{max}}} \] (of maximum values on vertical profiles) decreased from 1 near the source to an asymptotic value of 0.4 far downstream, in good agreement with previous experimental and theoretical work for concentration fluctuations in the surface layer well above the canopy. (iii) The eddy diffusivity for heat from the line source (KHL) increased, downstream of the source, to a nearly constant ‘far-field’ vertical profile. Within the canopy, the far-field KHL was an order of magnitude larger than KHP, the equivalent diffusivity for a plane source; well above the canopy, the two were equal. The time scale defined by (far-field KHL)/(vertical velocity variance) was independent of height within the canopy. (iv) Budgets for temperature variance, vertical heat flux and streamwise heat flux are remarkably similar to the equivalent budgets for an elevated line source in the surface layer well above the canopy, except in the lower part of the canopy in the far field, where vertical transport is much more important than in the surface layer. (v) A random flight simulation of the mean height and depth of the temperature plume was generally in good agreement with experiment. However, details of the temperature and streamwise turbulent heat flux profiles were not correct, suggesting that the model formulation needs to be improved.

Aerodynamic roughness over an inhomogeneous ground surface

Abstract: The aerodynamic roughness parameter z 0 over inhomogeneous ground surfaces, such as cities, rural towns and so on, is determined by analyzing the wind data at AMeDAS observatories in the Tohoku and Kanto districts of Japan, by making use of Rossby number similarity theory It is found that the aerodynamic roughness parameter is proportional to the average size of the roughness elements A practical method of estimating the aerodynamic roughness parameter over an extensive area with various inhomogeneities is developed In this method, the Digital National Land Information data bank is employed As an example, the roughness parameter distribution around Tsukuba Academic City is presented

Nocturnal drainage flow on simple slopes

Abstract: Observations of nocturnal slope flow have been made at two sites with quite different topography and vegetation. In both cases, continuous measurements of wind and temperature profiles were made from towers that extended through the depth of the katabatic flow. At the simpler site, which approximates a tilted plane, three towers were located at different distances down the slope to measure the development of slope flow with downslope distance. Slope flow depth, downslope wind speed, and temperature deficit are found to change with downslope distance at rates that are consistent with the predictions of Manins and Sawford's (1979) layer-averaged model of slope flow, while measured entrainment rates are found to be comparable to those predicted by Ellison and Turner's (1959) laboratory experiments. The depth of slope flow is found to be roughly 0.05 times the vertical drop from the top of the slope, a relationship that also follows from combining Manins and Sawford's model and Ellison and Turner's laboratory data. Analysis of the wind spectra and a simple numerical model suggest that the turbulent kinetic energy profiles in slope flow are dependent on the speed and direction of the ambient wind and can differ substantially from those found over flat terrain. At the more complex of the two measurement sites, the occurrence of slope flow was found to correlate well with a dimensionless number 5 that is a function of the ridge-top wind speed and of the strength and depth of the inversion and that is an estimate of the ratio of the buoyancy deficit to the external horizontal pressure gradient.

Suburban-rural energy balance comparisons in summer for Vancouver, B.C.

Abstract: Simultaneous energy balance observations at a rural and a suburban site in Vancouver, B.C. during the summer of 1983 are presented. The study is a follow-up to that conducted in 1980. Many of the 1980 results were unexpected and the present study seeks to assess their representativeness. The net radiant, turbulent sensible, and rural soil heat flux densities were measured directly. The suburban heat storage was parameterized and the turbulent latent heat flux densities were resolved as residuals in the energy balances. The 1983 average diurnal energy partitioning for both sites was typical of those quoted in the literature, suggesting that the 1980 results represent an extreme case. Suburban-rural differences showed the suburban area to have a 4% increase in net radiation, a 51% increase in turbulent sensible heat, and a 46% decrease in turbulent latent heat flux density. The values of the average daytime Bowen ratio were 0.46 and 1.28 for the rural and suburban areas, respectively. The sensible heat flux density exhibited relatively large values in the late afternoon and remained directed upward on many summer evenings. Large day-to-day variability in the relative magnitude of the suburban turbulent fluxes may have been due to synoptic influences. In this environment, the turbulent surface and mixed layers are closely coupled because of the low aerodynamic resistance over the rough surface.

The wind-induced shaping and migration of an isolated dune: a numerical experiment

Abstract: Numerical experiments are carried out to simulate the development and migration of a barchan dune starting with a conical pile of sand. Such an experiment is done in three steps: (1) computation of the steady-state wind field over and around a barchan using the numerical meso-scale simulation model FITNAH, whereby the horizontal variation of the friction velocity is also calculated; (2) computation of the sand transport using the friction velocity in the transport formula by Lettau and Lettau (1978); (3) computation of the erosion and deposition rates as the divergence of the sand transport, where a special treatment is used for the slip-face of the barchan dune. Adding these rates to the height field leads to a different shape of the dune after a time step δt h . Then this procedure has to be repeated for the next time step δt h . The results are in good agreement with observations: the initial pile of sand develops wings (‘horns’) and a slip-face between them. In addition, flow separation over the lee-side can be simulated. Finally, the tendency to form a barchan in equilibrium is considered.

Observations of turbulence downwind of a forest-heath interface

Abstract: The growth of the equilibrium layer downwind of a forest — heath interface has been observed using eddy correlation measurements made in real time. The atmosphere adjusts more quickly to the transition from heath to forest than to the transition from forest to heath.

Transport rate of drifting snow and the mean wind speed profile

Abstract: Transport rates, measured by weighing snow blown into a filter fabric trap, were greater over hard snow or ice than for the same wind speed over soft, fresh snow surfaces. Analysis of wind speed profiles from nine blizzards showed that friction between moving particles and the surface was less, and particle speeds were greater over hard surfaces. Transport rates at a given wind speed increased rapidly as aerodynamic roughness decreased in the rough-smooth transition region. Bagnold's theory for bed load transport provided a useful framework for the analysis.

Lidar measurements of the atmospheric entrainment zone and the potential temperature jump across the top of the mixed layer

Abstract: Lidar measurements of the thickness of the atmospheric entrainment zone are presented. The measurements were obtained in central Illinois during 6 days of clear-air convection.

Characteristics of the sea breeze in Attica, Greece

Abstract: The characteristics of the sea breeze in the Attica region of Greece, in which Athens is located, have been studied for occasions of weak synoptic-scale pressure gradient. The analysis is based on synoptic observations from six meteorological stations, three on the coast and three inland. The three inland stations and one of the coastal stations lie almost in a straight line at different distances from the coast. For each meteorological station, the basic characteristics of the sea breeze were determined, i.e., (1) The mean number of sea-breeze days for each calendar month. (2) The monthly mean wind speed for each synoptic hour. (3) The times of onset and cessation of the sea breeze. (4) The monthly vector mean wind, and its constancy1 for each synoptic hour. (5) For days on which there was a sea breeze at Helliniko (the coastal reference station), the percentage number of days on which there was also a sea breeze at the given station.

Bulk transfer coefficient over a snow surface

Abstract: The drag coefficient C D and the bulk transfer coefficient for sensible heat C H over a flat snow surface were determined experimentally. Theoretical considerations reveal that C D depends on the friction velocity u * as well as on the geometrical roughness h of the snow surface. It is found that C D increases with increasing u * and/or h. The dependency of C H on u * and h is so small that it is possible to consider C H as a constant for practical purposes: C H, 1 = 2.0 × 10−3 for a reference height of 1 m. The bulk transfer coefficient for water vapor is estimated at C E, 1 = 2.1 × 10−3 for a reference height of 1 m.

Wind profile constants in a neutral atmospheric boundary layer over complex terrain

Abstract: The roughness height z 0 and the zero-plane displacement height d 0 were determined for a region of complex terrain in the Pre-Alps of Switzerland. This region is characterized by hills of the order of 100 m above the valley elevations, and by distances between ridges of the order of 1 km; it lies about 20 to 30 km north from the Alps. The experimental data were obtained from radiosonde observations under near neutral conditions. The analysis was based on the assumption of a logarithmic profile for the mean horizontal wind existing over one half of the boundary layer. The resulting (z 0/h) and (d 0/h) (where h is the mean height of the obstacles) were found to be in reasonable agreement with available relationships in terms of placement density and shape factor of the obstacles, which were obtained in previous experiments with h-scales 2 to 4 orders of magnitude smaller than the present ones.

Flow over the summit of an isolated hill

Abstract: Observations of the mean flow and turbulence statistics over the summit of an isolated, roughly circular hill, Nyland hill, are presented, Nyland hill rises 70 m above the surrounding terrain and has a base diameter of about 500 m. The summit of the hill is very smooth and allows representative measurements to be made close to the surface. The flow speed 8 m above the summit is increased by a factor of 2 over the upstream speed 8 m above level terrain, and flow separation occurs in the lee of the hill. The mean velocity profile over the summit shows an increase in velocity with height up to about 2 m and then a near constant velocity between 2 and 16 m. The flow perturbation relative to the upstream profile is thus a maximum at about 2 m. The measurements of turbulence structure show how the influence of the hill depends on the length scale of the turbulent eddies involved. Scales greater than the scale of the hill are modified through the flow speed-up whilst scales shorter than the hill suffer complex changes. The short-scale turbulence over the summit is only in local equilibrium in the lowest fraction of a metre. Above this equilibrium region, there is a complex adjustment towards the rapid distortion dynamics which appear to dominate at heights above about 8 m. The detailed results are compared with previous studies and available theories.

Theoretical values of daily atmospheric and soil thermal admittances

Abstract: Effective daily atmospheric and soil thermal admittances were calculated using harmonic solutions to the one-dimensional heat transfer equation for a semi-infinite medium. The thermal properties of both media were assumed to be power functions of depth only and it was shown how to calculate the parameters in these functions. The admittances critically depended on the manner in which the thermal properties varied near the surface.

An experimental study of sulfur and NOx fluxes over grassland

Abstract: Three independent sulfur sensors were used in a study of sulfur eddy fluxes to a field of wheat stubble and mixed grasses, conducted in Southern Ohio in September, 1979. Two of these sensors were modified commercial instruments; one operated with a prefilter to measure gaseous sulfur compounds and the other with a denuder system to provide submicron particulate sulfur data. The third sensor was a prototype system, used to measure total sulfur fluxes. The data obtained indicated that the deposition velocity for gaseous sulfur almost always exceeded that for particulate sulfur; average surface conductances were about 1.0 cm s - ' for gaseous sulfur in the daytime and about 0.4 cm s - ' for particulate sulfur. The data indicate that nighttime values were probably much lower. The total sulfur sensor provided support for these conclusions. The boundary-layer quantity ln(z,/z,) was found to be 2.75 f 0.55, in close agreement with expectations and thus providing some assurance that the site was adequate for eddy flux studies. However, fluxes derived using a prototype NO, sensor were widely scattered, partially as a consequence of sensor noise but also possibly because of the effects of nearby sources of natural nitrogen compounds.

Interactions of nocturnal slope flows with ambient winds

Abstract: A quasi-one-dimensional numerical model containing a prognostic turbulent kinetic energy parameterization and simplified approximations to horizontal gradients is used to study interactions of thermally induced nocturnal slope flows with following and opposing ambient winds. It is found that a following ambient wind causes the peak perturbation wind to be weaker and to be realized at a greater height, while an opposing ambient wind leads to a stronger perturbation wind at a lower height. The reason for this response lies in the interactions of the shears of the thermal and ambient components through the mechanical production of turbulent kinetic energy.

Radiative surface temperature and energy balance of a wheat canopy. II: Estimating fluxes of sensible and latent heat

Abstract: Suite a la premiere partie ibid, p. 1-17. Calcul de la perte de chaleur sensible par un champ de ble d'hiver a partir de la t radiometrique, de la t de l'air et de la resistance au transfert de quantite de mouvement; calcul du flux de chaleur latente a l'aide de l'equation de balance d'energie. Confrontation de ces deux flux de chaleur avec ceux obtenus par la methode du rapport de Bowen. Resultats: lorsque la t radiative utilise un pouvoir emissif du couvert egal a 0,98, le flux de chaleur sensible est systematiquement de 50-100 Wm −2 inferieur a celui calcule par le rapport de Bowen; les deux methodes s'accordent meme lorsqu'une emissivite apparente de 0,96 est employee avec une reflectivite apparente de 0,03; la difference entre les deux calculs de flux de chaleur latente est alors de −16±32 Wm −2 ; la methode de la t de surface manifeste moins d'erreur systematique en comparaison avec celle du rapport de Bowen, que ne le fait la methode aerodynamique

On the effective roughness length for use in numerical three-dimensional models

Abstract: We present analytical and numerical calculations of the effective roughness length (ERL) over a flat surface with varying roughness elements, for use in large-scale models. It is shown that ERL is mostly determined by the roughest elements present inside the averaging domain and that, more surprisingly, the ERL increases as the first level of the numerical model gets closer to the surface and its altitude approaches the value of the largest local roughness length. This effect further increases the drag coefficient, in addition to the well-known increase due to the lowering of the first model level.

Local similarity of spectral and cospectral characteristics in the stable-continuous boundary layer

Abstract: The local similarity theory, presented in the recent papers of Sorbjan (1986a, b), is extended by taking into consideration spectral (u, v, w, θ) and cospectral (uw, wθ, uθ) densities in the stable-continuous boundary layer The resulting universal expressions for spectra, cospectra and the reduced frequencies of their peaks are in agreement with empirical data from the Kansas 1968 surface-layer and Minnesota 1973 boundary-layer experiments In addition, the universal functions for the structure parameters and the dissipation rates are also derived and shown to fit the empirical data well