Showing papers in "Boundary-Layer Meteorology in 1994"

Source areas for scalars and scalar fluxes

Abstract: The spatial resolution of meteorological observations of scalars (such as concentrations or temperature) and scalar fluxes (e.g., water-vapour flux, sensible heat flux) above inhomogeneous surfaces is in general not known. It is determined by the surface area of influence orsource area of the sensor, which for sensors of quantities that are subject to turbulent diffusion, depends on the flow and turbulence conditions.

Simplified expressions for vegetation roughness length and zero-plane displacement as functions of canopy height and area index

Abstract: Using a previous treatment of drag and drag partition on rough surfaces, simple analytic expressions are derived for the roughness length (z 0) and zero-plane displacement (d) of vegetated surfaces, as functions of canopy height (h) and area index (Λ). The resulting expressions provide a good fit to numerous field and wind tunnel data, and are suitable for applications such as surface parameterisations in atmospheric models.

A subgrid-scale model for large-eddy simulation of planetary boundary-layer flows

Abstract: A long-standing problem in large-eddy simulations (LES) of the planetary boundary layer (PBL) is that the mean wind and temperature profiles differ from the Monin-Obukhov similarity forms in the surface layer. This shortcoming of LES has been attributed to poor grid resolution and inadequate sub-grid-scale (SGS) modeling. We study this deficiency in PBL LES solutions calculated over a range of shear and buoyancy forcing conditions. The discrepancy from similarity forms becomes larger with increasing shear and smaller buoyancy forcing, and persists even with substantial horizontal grid refinement. With strong buoyancy forcing, however, the error is negligible. In order to achieve better agreement between LES and similarity forms in the surface layer, a two-part SGS eddy-viscosity model is proposed. The model preserves the usual SGS turbulent kinetic energy formulation for the SGS eddy viscosity, but it explicitly includes a contribution from the mean flow and a reduction of the contributions from the turbulent fluctuations near the surface. Solutions with the new model yield increased fluctuation amplitudes near the surface and better correspondence with similarity forms out to a distance of 0.1-0.2 times the PBL depth, i.e., a typical surface-layer depth. These results are also found to be independent of grid anisotropy. The new model is simple to implement and computationally inexpensive.

A wind tunnel study of air flow in waving wheat: Two-point velocity statistics

Abstract: We analyse single-point velocity statistics obtained in a wind tunnel within and above a model of a waving wheat crop, consisting of nylon stalks 47 mm high and 0.25 mm wide in a square array with frontal area index 0.47. The variability of turbulence measurements in the wind tunnel is illustrated by using a set of 71 vertical traverses made in different locations, all in the horizontally-homogeneous (above-canopy) part of the boundary layer. Ensemble-averaged profiles of the statistical moments up to the fourth order and profiles of Eulerian length scales are presented and discussed. They are consistent with other similar experiments and reveal the existence of large-scale turbulent coherent structures in the flow. The drag coefficient in this canopy as well as in other reported experiments is shown to exhibit a characteristic height-dependency, for which we propose an interpretation. The velocity spectra are analysed in detail; within and just above the canopy, a scaling based on fixed length and velocity scales (canopy height and mean horizontal wind speed at canopy top) is proposed. Examination of the turbulent kinetic energy and shear stress budgets confirms the role of turbulent transport in the region around the canopy top, and indicates that pressure transport may be significant in both cases. The results obtained here show that near the top of the canopy, the turbulence properties are more reminiscent of a plane mixing layer than a wall boundary layer.

Boundary-layer depth and entrainment zone characterization with a boundary-layer profiler

Abstract: A technique for determining the height of the convective atmospheric boundary layer (CBL) with a 915 MHz boundary-layer profiler is discussed. The results are compared with CBL heights determined from radiosonde measurements. The profiler provides continuous CBL height measurements with very good time resolution (30 minutes or less), allowing for detailed understanding of the growth and fluctuations of the CBL. In addition, the profiler provides information about the degree of definition of the CBL top and the thickness of the entrainment zone. The measurements discussed were taken during the Rural Oxidants in the Southern Environment II (ROSE II) experiment.

Short-term prediction of local wind conditions

Abstract: Using Numerical Weather Prediction (NWP) models it has been shown that they, combined with models (either physical or statistical) taking local effects into account, can be used to predict the wind locally better than the models commonly used today (as eg persistence). By “local” is meant at one distinct spot, as eg the location of a meteorological mast. The physical model of local effects takes the following into account: shelter from near-by obstacles, the effect of roughness changes and the effect of the local orography. The large-scale flow is linked to the surface flow by the geostrophic drag law, and the logarithmic wind profile. The predictions are made up to 36 hours ahead. The model is tested on data from 50 meteorological stations scattered all over Europe.

Wind and wind forces in a plantation spruce forest

Abstract: Observations have been made of the tubulent structure within and above a dense (LAI=10.2) plantation spruce forest along with measurements of the movement of individual trees. The mean statistics of the turbulence and the turbulence spectra are compared with observations in other crops and complementary wind-tunnel studies using 1∶75 scale plastic trees. The measurements show that momentum transport and the subsequent motion of the trees is dominated by intermittent sweep/ejection events associated with ‘honami’ waves moving across the forest. The trees themselves act as forced damped harmonic oscillators and appear to short circuit the normal turbulent energy dissipation process by efficiently absorbing energy at their resonant frequencies. It is argued that understanding the nature and formation of ‘honami’ waves over forests and crops is a crucial problem in agricultural and forest meteorology because of their important role both in turbulent transport and in causing wind damage.

On the relation between ozone storage in the residual layer and daily variation in near-surface ozone concentration: a case study

Abstract: Numerous tethered balloon soundings which measured meteorological parameters and ozone concentration during the POLLUMET field experiments are investigated. They give indications of the influence of ozone conservation in the nocturnal residual layer (RL) on the development of the near surface ozone concentration of the next day. Mixing down from the RL to the surface is simulated by a simple model based on transilient turbulence theory, using measured profiles as initial values or model verification. The cases investigated show that the ozone mixed down from the RL contributes 50–70% to the maximum concentration near the surface on the following day, the rest coming from chemical production and possibly advection. Various features of exchange characteristics between the RL and the nocturnal boundary layer, (NBL) are described by the analysis of the soundings. It can be shown that the vertical exchange situation during the night can have a considerable influence on the ozone concentration of the following day.

Heat and water vapour fluxes and scalar roughness lengths over an Antarctic ice shelf

Abstract: We present eddy-correlation measurements of heat and water vapour fluxes made during the Antarctic winter The surface layer was stably stratified throughout the period of observation and sensible heat fluxes were always directed downwards However, both upward and downward water vapour fluxes were observed Their magnitude was generally small and the latent heat flux was not a significant fraction of the surface energy budget The variation of heat and water vapour fluxes with stability is well described by Monin-Obukhov similarity theory but the scalar roughness lengths for heat and water vapour appear to be much larger than the momentum roughness length Possible explanations of this effect are discussed

Organized structures in developing turbulent flow within and above a plant canopy, using a large eddy simulation

Abstract: A Large Eddy Simulation (LES) model representing the air flow within and above a plant canopy layer has been completed. Using this model, the organized structures of turbulent flow in the early developmental stages of a crop are simulated and discussed in detail.

Wind flows and forces in a model spruce forest

Abstract: Wind tunnel tests have been conducted on a 1:75 scale model of a Sitka spruce forest in a correctly scaled turbulent boundary-layer flow. 12000 tree models were manufactured with mass, flexibility and aerodynamic drag characteristics chosen to give dynamical similarity with typical 15 m trees in a 30ms−1 gale. To measure the dynamic response of a sample tree, set within this model forest, a miniature, fast response strain-gauge balance was designed and built. Linked to a computer for on-line data sampling, this balance provided measurements of the fluctuating along-wind and acrosswind components of the overturning moment at ground level, leading to values of mean and extreme moments and the frequency spectrum of the sway motion. Associated measurements of local wind flow characteristics were made with hot-wire anemometers and a laser anemometer. The response of the tree has the characteristics of classical lightly damped vibration and there is evidence that resonant sway motion increases the extreme overturning moments significantly above the values produced by wind gust forces alone.

Momentum extraction with saltation: implications for experimental evaluation of wind profile parameters

Abstract: In wind tunnel studies of aeolian transport, the number and position of pitot tubes are decided by the researcher, so that there are important variations in the computation ofU* between studies. Velocity measurements seldom are made very close to mobile surfaces because the tubes become blocked by drifting sand grains. This practice is fortuitous as demonstrated by recent selfregulatory models of saltation which indicate that fluid and grain-borne shear stress vary substantially within the lowest 0.01 m and application of the logarithmic law is therefore unsound. This study reports detailed velocity measurements which further suggest that no single logarithmic expression, based on fixed values of κ and τ, adequately represents the full wind profile which includes the inner saltation cloud above 0.01 m and the outer grain-free region of the boundary layer. A much improved fit over the logarithmic wind profile model is achieved with a square root relation, although there is no known physical basis for this specific form of power model. Relatively shallow boundary-layer development in wind tunnels forces the velocity gradient above the region of momentum extraction to attain exceptionally large values, uncommon in natural settings.

Variation in soil parameters: implications for modeling surface fluxes and atmospheric boundary-layer development

Abstract: Soil texture can be heterogeneous; however for land surface-atmospheric modeling purposes, it is often considered homogeneous at a particular point and described by empirical equations which have been formulated to describe ‘average’ hydraulic and thermodynamic processes in the soil. Large deviations in the variables and coefficients used in these empirical equations have been previously documented. One of the coefficients is varied by plus-and-minus one standard deviation about its mean, and tested in a coupled atmospheric-plant-soil model. Results of model simulations show that the effects on surface fluxes and boundary-layer development are larges for dry to moderate values of soil moisture, particularly for bare soil conditions.

A comparison of bowen ratio and eddy correlation sensible and latent heat flux measurements above deciduous forest

Abstract: Sensible and latent heat flux densities (H and ΛE) were measured above a mature, 18 m deciduous forest during July and August, 1988, using the Bowen ratio-energy balance (BREB) and eddy correlation (EC) methods. EC estimates ofH and ΛE underestimated day-time surface available energy by 11%. EC also partitioned available energy differently than BREB. forź/L<0.0, EC favouredH and BREB favoured ΛE.

Seasonal and diurnal variations of coherent structures over a deciduous forest

Abstract: Coherent structures in turbulent flow above a midlatitude deciduous forest are identified using a wavelet analysis technique Coupling between motions above the canopy (z/h=15, whereh is canopy height) and within the canopy (z/h=06) are studied using composite velocity and temperature fields constructed from 85 hours of data Data are classified into winter and summer cases, for both convective and stable conditions Vertical velocity fluctuations are in phase at both observation levels Horizontal motions associated with the structures within the canopy lead those above the canopy, and linear analysis indicates that the horizontal motions deep in the canopy should lead the vertical motions by 90° On average, coherent structures are responsible for only about 40% of overall turbulent heat and momentum fluxes, much less than previously reported However, our large data set reveals that this flux fraction comes from a wide distribution that includes much higher fractions in its upper extremes The separation distanceL s between adjacent coherent structures, 6–10h, is comparable to that obtained in previous observations over short canopies and in the laboratory Changes in separation between the summer and winter (leafless) conditions are consistent withL s being determined by a local horizontal wind shear scale

An applied model for the height of the daytime mixed layer and the entrainment zone

Abstract: A model is presented for the height of the mixed layer and the depth of the entrainment zone under near-neutral and unstable atmospheric conditions. It is based on the zero-order mixed-layer height model of Batchvarova and Gryning (1991) and the parameterization of the entrainment zone depth proposed by Gryning and Batchvarova (1994). However, most zero-order slab type models of mixed-layer height may be applied. The use of the model requires only information on those meteorological parameters that are needed in operational applications of ordinary zero-order slab type models of mixed-layer height: friction velocity, kinematic heat flux near the ground and potential temperature gradient in the free atmosphere above the entrainment zone. When information is available on the horizontal divergence of the large-scale flow field, the model also takes into account the effect of subsidence, although this is usually neglected in operational models of mixed-layer height owing to lack of data. Model performance is tested using data from the CIRCE experiment.

The sensitivity of winter evaporation to the formulation of aerodynamic resistance in the ECMWF model

Abstract: In atmospheric models, the roughness length for momentum, heat and moisture are often taken equal, and tuned to the momentum budget problem In this paper, it is shown that the roughness lengths have considerable impact on the evaporation in winter One-column simulations of the land-surface scheme are driven with a long time series of observations for Cabauw in The Netherlands It is shown that with the operational roughness lengths for this location (as in use at ECMWF in May 1993), evaporation in January, February and March is overestimated by more than a factor 2 More realistic parameters, as documented for this site, virtually eliminate the error This study shows the importance of the surface roughness lengths in determining evaporation from wet surfaces It also illustrates the strength of long observational time series in identifying model deficiencies

Observations of fluxes over heterogeneous surfaces

Abstract: This study analyzes data collected from repeated aircraft runs 30 m over alternating regions of irrigated and dry nonirrigated surfaces, each region on the order of 10 km across, during the California Ozone Deposition Experiment (CODE). After studying the scale dependence of the flow, the variables and their fluxes are decomposed into means for sublegs defined in terms of irrigated and nonirrigated regions and deviations from such subleg means. Since the repeated runs were flown over the same track, compositing the eight flight legs for each of the two days allows partial isolation of the influences of surface heterogeneity and transient mesoscale motions.

Determination of the zero plane displacement in an urban environment

Abstract: A method to estimate the zero plane displacement is presented that is based onin situ turbulence measurements. As it does not depend on flux-gradient relations. It can be applied over non-homogeneous surfaces if they are dynamically but not thermally distorted. The zero plane displacement at an urban site is found to vary considerably with wind direction. The results compare well with estimates from conventional methods using the heights and distribution of the upwind roughness elements.

Effective parameters of surface energy balance in heterogeneous landscape

Abstract: This paper addresses the problem of estimating surface fluxes at large scale over heterogeneous terrain, and the corresponding determination of effective surface parameters. Two kinds of formulation are used to calculate the fluxes of sensible and latent heat: the basic diffusion equations (Ohm's law type) and the Penman-Monteith equations. The strategy explored is based upon the principle of flux conservation, which stipulates that the average flux over a large area is simply the area-weighted mean of the contributions from the different patches making up the area. We show that the application of this strategy leads to different averaging schemes for the surface parameters, depending on the type of flux (latent heat, sensible heat) and on the type of formulation used to express the flux. It appears that the effective value of a given parameter must be appraised for each individual application, because it is not unique, but differs according to the magnitude being conserved and the equation used to express this magnitude. Numerical simulations are carried out to test over contrasted areas the aggregation procedures obtained. The areal fluxes estimated from these effective parameters, together with the areal fluxes calculated by means of a simple areal averaging of the parameters, are compared to the “true’ average fluxes, calculated as area-weighted means of the elementary fluxes. The aggregation procedures obtained prove to be much more accurate for estimating areal fluxes and for closing the energy balance equation than those based upon simple areal averaging of the parameters.

A comparison of ABL heights inferred routinely from lidar and radiosondes at noontime

Abstract: The height of the atmospheric boundary layer (ABL) obtained with lidar and radiosondes is compared for a data set of 43 noon (12.00 GMT) cases in 1984. The data were selected to represent the synoptic circulation types appropriately. Lidar vertical profiles at 1064 nm were used to obtain three estimates for the ABL height (h lid), based on the first gradient in the back-scatter profile, namely, at the beginning, middle and top of the gradient. The boundary-layer height obtained with the radiosondes (h s) was determined with the dry-parcel-intersection method in unstable conditions. As a first guess for near-neutral and stable conditions, the height of the first significant level in the potential temperature profile was taken. Overall, the boundary-layer thickness estimates agree surprisingly well (regression lineh lidb=hs:cc.=0.93 and the standard error=121 m). However, in 10% of the cases, the lidar estimate was significantly lower (difference>400 m) than the routinely inferredh s. These outliers are discussed separately. For stable conditions, an estimate of ABL height (h N) is also made based on the friction velocity and the Brunt-Vaisala frequency. The agreement betweenh Nandh lidbis good. Discrepancies between the two methods are caused by:

The conductance of a maize crop and the underlying soil to ozone under various environmental conditions

Abstract: Flux measurements of ozone and water vapour employing the eddy correlation technique were used to determine the surface conductance and canopy conductance to ozone. In the surface conductance to ozone, all surfaces at which ozone is destroyed and the transport process to these surfaces are included. The canopy conductance to ozone represents the ozone uptake of transpiring plant parts. The surface conductance to ozone of the maize crop and the underlying soil was generally larger than the canopy conductance to ozone. This means that beside the uptake by stomata, there was another important ozone sink. Under wet soil surface conditions, the surface conductance and the canopy conductance to ozone coincided. This indicates that the resistance of wet soil and the remaining plant parts (cuticle) to ozone was much larger than the stomatal or soil resistance. On the other hand, under dry soil conditions the conductances differ, largely caused by a variation in the transport process to the soil. The transport of ozone to soil increased with increasing friction velocity (u *) and decreased with increasing atmospheric stability, leaf area index (LAI) or crop height (h). These effects for midday (unstable) conditions were parameterized with an “in-crop” aerodynamic resistance,r inc in a very straightforward way;r inc=13.9 LAIh/u *+67 (cc.=0.77). If the ozone flux in air pollution models is described with a simple resistance model (Big Leaf model), the extra destruction at the soil should be modelled using an “in-crop” aerodynamic resistance. For these measurements the ozone flux to the soil was 0–65% of the total ozone flux measured above the crop. Under wet soil conditions, this was less than 20%; under dry soil conditions, this was 30–65%.

Modelling evaporation from wetland tundra

Abstract: Evapotranspiration is a major component of both the energy and water balances of wetland tundra environments during the thaw season. Reliable estimates of evapotranspiration are required in the analysis of climatological and hydrological processes occurring within a wetland and in interfacing the surface climate with atmospheric processes. Where direct measurements are unavailable, models designed to accurately predict evapotranspiration for a particular wetland are used. This paper evaluates the performance, sensitivity and limitations of three physically-based, one-dimensional models in the simulation of evaporation from a wetland sedge tundra in the Hudson Bay Lowland near Churchill, Manitoba. The surface of the study site consists of near-saturated peat soil with a sparse sedge canopy and a constantly varying coverage of standing water. Measured evaporation used the Bowen ratio energy balance approach, to which the model results were compared. The comparisons were conducted with hourly and daily simulations. The three models are the Penman-Monteith model, the Shuttleworth-Wallace sparse canopy model and a modified Penman-Monteith model which is weighted for surface area of the evaporation sources. Results from the study suggest that the weighted Penman-Monteith model has the highest potential for use as a predictive tool. In all three cases, the importance of accurately measuring the surface area of each evaporation source is recognized. The difficulty in determining a representative surface resistance for each source and the associated problems in modelling without it are discussed.

Atmospheric deposition in complex forest landscapes

Abstract: Atmospheric dry deposition in eight forest edges was estimated by means of throughfall and bulk precipitation measurements. Dry deposition was found to be enhanced in the first 5 edge heights. Deposition enhancement was strongly dependent on forest density, edge aspect and on the gas or particle under consideration. Incorporating edge effects in present-day deposition models will offer prospects to estimate deposition to individual forest stands as well as regional deposition amounts more accurately. From information available in the ‘Dutch Forest Statistics’, it became clear that at least 50% of the total forested area in the Netherlands is influenced by edge effects. By neglecting edge effects, dry deposition of acidifying compounds to Dutch forests is underestimated by current deposition models by approximately 10%.

Bulk parameterization for a vegetated surface and its application to a simulation of nocturnal drainage flow

Abstract: Two simple models are presented for describing the surface energy budget above vegetated surfaces. One is the traditional single-source model that includes only one energy budget equation for the entire canopy-soil system, and the other is the double-source model that includes separate energy budget equations for the vegetation canopy and the underlying soil surface. In both models, the bulk transfer coefficients needed to solve the energy budget equations are parameterized as functions of leaf area index, leaf transfer coefficients, and soil surface roughnesses to obtain the best fit to values calculated by a standard multilayer-canopy model. The validity of these models was tested by comparing their performance with that of the multilayer-canopy model for simulation of the surface energy balance and nocturnal drainage flow above vegetation. Results show that the double-source model gives reliable estimations for all cases ranging from sparse to dense vegetation covers; the single-source model is only applicable to dense, fully-covered vegetation. It is also shown that sparse vegetation weakens nocturnal drainage flow, since it isolates the cool underlying soil surface from the atmosphere above the canopy. This phenomenon cannot be described by a traditional single-source model incorporated commonly in many atmospheric models; however, the double-source model adequately describes this process.

The sea surface is aerodynamically rough even under light winds

Abstract: The sea surface is generally considered to be aerodynamically rough at high winds (U>7 m/s), where the roughness length increases with wind velocity; below this velocity, the atmospheric surface layer enters a transition region and then becomes aerodynamically smooth as the wind velocity further decreases. The sea surface is shown, however, to reach its smoothest condition at a wind velocity of about 5 m/s, and then become rough again at lower velocities. In the latter case, the roughness length increases as the wind velocity decreases in accordance with the surface-tension relation governing wind-wave interactions.

Radiometrically determined skin temperature and scalar roughness to estimate surface heat flux. Part I : Parameterization of radiometric scalar roughness

Abstract: A series of experiments carried out in a pasture field during a growing season, allowed a radiometric determination of the scalar roughness for sensible heatz oh,r . The values ofz oh,r are shown to vary over the range of 10−1–10−7m both diurnally and seasonally, and an existing theoretical model for the estimation of scalar roughness for sensible heat is found to be inappropriate for the precise estimation ofz oh,r . To parameterizez oh,r better, a multiple regression analysis was performed, with predictor candidates such as solar elevation, solar radiationR s , leaf area index LAI, canopy height, the ratio of the solar radiation and the extraterrestrial radiationR s /R e , the ratio of the direct and the total solar radiationR d /R s , and the roughness Reynolds number among others. The best regression equation which usesR s , LAI,R s /R e , andR d /R s is derived withr=0.75; with smaller numbers of predictors, values ofr tend to deteriorate gradually down tor=0.52 when only one predictor, LAI, was incorporated into the equation.

Nighttime free convection characteristics within a plant canopy

Abstract: An intensive measurement campaign within and above a maize row canopy was carried out to investigate flow characteristics within this vegetation. Attention was given to finding adequate scaling parameters of the within-canopy windspeed and air temperature profiles under above-canopy stable stratification. During clear and calm nights the within-canopy condition differs considerably from the abovecanopy state. In contrast to the daytime, the windspeed and temperature profiles do not scale with the above-canopy friction velocity,u * , and the scaling temperature,T * , respectively. A free convection flow regime is generated, forced by the soil heat flux at the canopy floor and by cooling at the top of the canopy. However, the windspeed and temperature profiles appear to scale well with the free convective velocity scale,w * , and the free convective temperature scale,T f , respectively. The free convective state within the canopy agrees well with the free convection criterion Gr>16Re2(u * ), where Gr is the Grashof number and Re(u * ) the Reynolds number, a criterion often used in technical flow problems. Also it is shown that under within-canopy free convection, there is a unique relation between the Grashof number, Gr, and the Reynolds number if the latter is based on the free convective velocity scale. Under within-canopy free convective conditions, it appears that within the canopy the fluxes of heat and water vapour can be estimated well with the relatively simple variance technique. Under these conditions, the Grashof, or Rayleigh number, represents a measure for the kinetic energy of the turbulence within the canopy.

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

Abstract: Measurements of mean wind flow and turbulence parameters have been made over Cooper's 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 model's 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.

Study of the moist Convective Boundary Layer structure by backscattering lidar

Abstract: Measurements of the structure of the Convective Boundary Layer (CBL) were made from the ground and from the ARAT aircraft with a backscattering lidar during the preliminary phase of the French project LEANDRE. A numerical model of Mie theory diffusion was used to determine a theoretical profile of the extinction coefficient, as a function of relative humidity. Comparisons between theoretical and measured profiles were made. Good agreement was found, which confirms that the variation of the extinction coefficient in the CBL is primarily controlled by the relative humidity. This paper also presents a detailed analysis of the small-scale structure of the CBL. The normalized length and number of thermals are estimated. Results are in agreement with previousin-situ measurements. An analysis of the bidimensional cross-section of the extinction coefficient shows that this coefficient permits one to retrieve more quantitative information than the range-corrected signal, as the fractions of entrained air and surface-layer air at a given altitude.