AREA-AVERAGED SENSIBLE HEAT FLUX AND A NEW METHOD TO DETERMINE ZERO-PLANE DISPLACEMENT LENGTH OVER AN URBAN SURFACE USING SCINTILLOMETRY

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1 AREA-AVERAGED SENSIBLE HEAT FLUX AND A NEW METHOD TO DETERMINE ZERO-PLANE DISPLACEMENT LENGTH OVER AN URBAN SURFACE USING SCINTILLOMETRY MANABU KANDA and RYO MORIWAKI Department of International Development Engineering, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152, Japan MATTHIAS ROTH Department of Geography, National University of Singapore, Singapore TIM OKE Department of Geography, University of British Columbia, Vancouver, Canada (Received in final form 30 October 2001) Abstract. Field observations of area-averaged turbulence characteristics were conducted in a densely built-up residential neighbourhood in Tokyo, Japan. In addition to eddy-correlation (EC) sensors a scintillometer was used for the first time in a city. Significant results include: (1) Scintillometerderived sensible heat fluxes, Q H, obtained at a height 3.5 times the building height agree well with those using the EC technique; (2) source areas for the scintillometer fluxes are larger than for the EC sensors, so that at low heights over inhomogeneous terrain scintillometry offers advantages; (3) new similarity relationships for dissipation rates are proposed for urban areas; (4) a new technique that uses simultaneous scintillation measurements at two heights to directly estimate area-averaged zeroplane displacement height, z d, is proposed. z d estimated in this way depends slightly on atmospheric stability (lower z d under more unstable conditions). Keywords: Area-averaged turbulence, Scintillometer, Urban field observation, Urban similarity function, Zero-plane displacement length. 1. Introduction Evaluation of turbulence quantities such as heat, mass and momentum fluxes using micrometeorological techniques in urban areas requires considerable care as first noted by Oke et al. (1989). Methodological problems arise because the atmosphere close to the urban surface is spatially and temporally highly inhomogeneous due to the complex three-dimensional (3-D) source/sink distribution of surface characteristics. Although it is possible to obtain reliable results using eddy-correlation (EC) approaches, the fluxes and other statistics still vary in space within the roughness sublayer (RSL). An alternative is offered by scintillometers. Their long pathlength enables them to measure area-averaged fluxes thereby potentially overcoming the inherent lack kanda@fluid.cv.titech.ac.jp Boundary-Layer Meteorology 105: , Kluwer Academic Publishers. Printed in the Netherlands.

2 178 MANABU KANDA ET AL. of spatial representativeness of observations over heterogeneous terrain. A number of studies have used this method to measure the sensible heat flux, Q H, over homogeneous, low-roughness surfaces (e.g., Wesely, 1976; Champagne et al., 1977; Kohsiek, 1982; Hill et al., 1992; Thiermann and Grassl, 1992; Nieveen and Green, 1999). Others have used large-aperture infrared scintillometers over vegetated surfaces (dry vineyard: De Bruin et al., 1995; rice paddy: Green and Hayashi, 1998), which required a priori determination of friction velocity, u, and zero-plane displacement length, z d, in order to derive Q H. Similar data from urban surfaces are not available. The theory behind scintillometry is well understood (see below), however, its application in urban areas needs careful consideration. Firstly, it assumes Monin Obukhov similarity (MOS) relationships that are only valid in the homogeneous surface layer and therefore probably need modification in the urban case (Roth, 2000). Secondly, a z d that accounts for the existence of the urban canopy layer, needs to be known in advance. Grimmond and Oke (1999a; hereinafter GO99) discuss the ability to assign values of z d to an urban surface and clearly demonstrate the difficulties inherent in all currently used approaches. The objective of this paper is twofold. First, to compare area-averaged turbulent sensible heat fluxes to those from single-point EC measurements using urban-specific non-dimensional dissipation functions. Second, to introduce a new technique to determine the area-averaged displacement height using two heightdisplaced scintillometers. 2. Theory 2.1. DETERMINATION OF FLUXES FROM DISPLACED-BEAM SCINTILLATION Several methods exist to derive Q H from scintillometer measurements (see review by Hill, 1992). Here we follow the theory developed for the displaced-beam scintillometer (Thiermann, 1992). With this technique it is possible to simultaneously estimate the momentum flux, τ, andq H without a priori knowledge of the friction velocity, u. We also include a simplification for relatively dry conditions (large Bowen ratios) (Green and Hayashi, 1998), which should apply in sparsely vegetated urban areas. The non-dimensional forms for dissipation rate, ɛ, and structure parameter of temperature, C T,aregivenby φ ε = εk(z z d), (1) u 3 φ CT = C2 T [k(z z d)] 2/3, (2) T 2

3 NEW METHOD TO DETERMINE ZERO-PLANE DISPLACEMENT LENGTH 179 where k (= 0.4) is von Karman s constant; T (= Q H /ρc p u ) is the surface-layer temperature scale; z is height above ground; ρ is density of air; and c p is specific heat of air. Equations (1) and (2) follow Monin Obukhov similarity (MOS) scaling and the non- dimensional functions φ ε and φ CT depend only on ζ = (z z d )/L, where L = Tu 2 /kgt (3) with g the acceleration due to gravity (= 9.81 m s 2 )andt is temperature. Inverting (1) and (2), and applying a relationship between ε and the inner scale of turbulence, l 0, (Hill and Clifford, 1978), u and T are given by ( ) 7.4 8/3 u 2 = ν2 [k(z z d )] 2/3 φε 2/3 (ζ ), (4) l 0 = C2 T [k(z z d)] 2/3 φ 1 (ζ ). (5) T 2 CT Using optical measurements of CT 2 and l 0, (4) and (5) can be solved for the homogeneous surface layer over low-roughness terrain where z d is small or negligible, and φ ε and φ CT are known empirical functions that follow MOS theory (e.g., Wyngaard and Côté, 1971; Thiermann and Grassl, 1992). Modified urban relationships are introduced below. The turbulent fluxes τ and Q H are then obtained as: τ = ρu 2, (6) Q H = ρc p u T. (7) 2.2. SCINTILLOMETER HEAT FLUX METHOD (SHM) TO ESTIMATE z d It is very difficult to explicitly measure or even estimate z d over complex surfaces such as cities. Grimmond et al. (1998) and GO99 discuss applications and limitations of micrometeorological measurements and morphometric (based on the description of surface form) approaches, respectively. Here we propose a new technique that uses simultaneous scintillation measurements of sensible heat fluxes at two different heights. It employs the continuity equation for the turbulent sensible heat flux between two measurement levels, which, knowing the temperature gradient, can be solved by minimizing the difference between the measured fluxes using z d as the free parameter: Q H = Q H 1 ( z2 ) T Q H2 + ρc p z 1 t z, (8)

4 180 MANABU KANDA ET AL. where subscripts 1 and 2 denote the two observation heights. The SHM procedure is summarized in Figure 1. Successful convergence to a stable z d value has been achieved with a threshold value of 0.1 W m 2 in the present case. It is important to note that the method described above is different from the profile method used to estimate Q H based on CT 2 measurements at two levels (Hill, 1992). Unlike the dual-beam scintillometers employed in the present study, the scintillometers used in previous work were single-beam sensors and therefore unable to simultaneously estimate both T and u. In this case additional information is needed to determine Q H. The profile method further assumes that T is constant between the two levels which severely restricts its applicability and requires high sensitivity and accuracy of the scintillometers involved (e.g., Andreas, 1988; Lagouarde et al., 2000). In the present study, the profile information is used only for the estimation of z d (but feeds back into the determination of Q H according to Figure 1) and the accuracy of the scintillometers is estimated to calculate the standard deviations of z d values (see below). 3. Experimental The observations were conducted in Tokyo, Japan, which is located in the Kanto plain facing the Pacific Ocean (Tokyo Bay). The plain extends to more than 100 km at its widest point and is surrounded by mountains that reach to about 1500 m a.s.l.. The urban area is extensive and heavily developed and as a result there are marked urban effects on climate. The observation site for the present study is located in Setagaya ( N, E), a district of Tokyo about 10 km west of the Emperor s palace and 20 km from the coastline to the southeast. The elevation of the site is 35 m a.s.l., terrain is generally flat and land-use predominantly residential. Typical of many Japanese residential neighbourhoods, the houses are 2 3 stories high and densely arranged leaving little space for roads or alleys (Figure 2). Characteristics of the surface morphology within a radius of 200 m centered on the main site were determined by field surveys (Table I). Both the plan area (0.61) and frontal area index (0.55) are much larger than observed at most previously studied urban sites ( and , respectively for single houses with gardens densely-built single houses or apartment blocks; e.g., GO99, Table III). Only a site in the centre of Zürich, Switzerland reports a similar frontal area index (0.61). The greenspace fraction (10 15%) is also much lower than in North American residential neighbourhoods (more typically 20 50%; Grimmond and Oke, 1999b). There is no open water or irrigation. The physical nature of cities often imposes limitations on experimental design. For example, potential observation sites may be restricted or pre-determined by the availability of towers or other tall structures able to serve as instrument platforms. In the present study we used two mobile industrial cranes, which could be moved to the site that offered the best fetch conditions. Each crane had a small basket (2.5

5 NEW METHOD TO DETERMINE ZERO-PLANE DISPLACEMENT LENGTH 181 Figure 1. Flow chart detailing the SHM procedure used to estimate the area-averaged zero-plane displacement length (see text for full definition of symbols and further explanation). initial estimate.

6 182 MANABU KANDA ET AL. TABLE I Morphological characteristics within main source area (Schmid, 1994) of Setagaya, Tokyo site. UTC urban terrain zone classification (Ellefsen, ); zh mean height of buildings; λp plan area index (ratio of plan area of roughness elements to total lot area); λf frontal area index (ratio of frontal area of roughness elements to total lot area); Lx mean dimension of buildings in alongwind direction, Ly mean in crosswind direction; Wx mean building spacing in alongwind direction, Wy mean in crosswind direction; Dx mean inter-building spacing (between building centroids) in alongwind direction, Dy mean in crosswind direction (surface dimensions as described in Figure 2 of GO99). UTC zh (m) z/zh λp λf Lx (m) Ly (m) Wx (m) Wy (m) Dx (m) Dy (m) Dc2 8.5 upper: lower: 1.9

7 NEW METHOD TO DETERMINE ZERO-PLANE DISPLACEMENT LENGTH 183 Figure 2. View from platform at 32 m above the surface towards the south-west. Transmitters were installed on the 4th and 10th floors of the building visible near the top-center of the photograph. m 2 and 1 m high) attached to the top that served as instrument platforms and to hold the data-logging equipment and up to two observers. The site and set-up were chosen to ensure high-quality data by applying criteria similar to those outlined by GO99: flat terrain; slender, open tower construction; use of fast-response sensors; sufficient measurement height to be above the roughness sublayer; sufficient fetch over similar roughness with no anomalous structures nearby. The turbulent fluxes Q H and τ were measured directly at two levels using 3-D sonic anemometers (Kaijo Denki; transducer spacing = 0.05 m; sampling frequency = 10 Hz). Unfortunately the sensor at the lower level frequently malfunctioned. In order to ensure unobstructed approach flow and minimal flow interference from the baskets and cranes, the turbulence sensors were mounted at the end of booms extending about 1.6 m into the flow. The entire boom-basket-sensor array was rotated as necessary into the mean wind between individual runs (averaging time, 30-min). In addition to the EC sensors, two dual-beam scintillometers (Scintec, SLS20) were used to obtain area-averaged fluxes. The transmitters were placed on the balconies of a slender building about 250 m south-west from the main site (Figure 2) at 16 and 32 m above ground, respectively. When the scintillometers were in operation the height of the top of the cranes, which supported the receivers, were adjusted to

8 184 MANABU KANDA ET AL. the same levels. The temperature difference between the two levels was measured using unshielded thermocouples (diameter = 0.08 mm). Ideally, sensors for turbulent fluxes should be located in the constant flux layer, if one exists, above the RSL. According to Roth (2000) the height of the RSL over a range of urban surfaces is times the height of the buildings. At Setagaya the upper measurement level (z/z H = 3.8) is therefore within the surface layer, whereas the lower level (z/z H = 1.9) is probably located near the transition between the RSL and the surface layer. The research-grade 30-min runs were obtained during October 9 and 10, 1998 when the weather was sunny with cloudy periods and winds from the south. The relatively few data obtained are due to logistical, financial and experimental restrictions of this study. Further, to satisfy the fetch requirements, only flow from a restricted wind direction sector (SE SW) could be used and operation of the crane was not permitted at night. The number of useful observations is also low because of difficulties in obtaining consistent, adequate signal strength for two scintillometers simultaneously. 4. Area-Averaged Sensible Heat Fluxes and Non-Dimensional Dissipation Rates Because these are some of the first reported scintillometer measurements over a city it is necessary initially to establish the usefulness of the technique. This is done by comparison with the otherwise best available estimates obtained using the EC approach. The 30-min averaged EC and scintillometer sensible heat fluxes obtained at the top level for a few hours on two days are plotted in Figure 3. From this limited sample it appears that there is good agreement between the two (< 15 W m 2 for any value, and for midday data < 2% in the mean with the scintillometer usually being larger and < 10% for most values). These results broadly agree with those obtained during an exploratory study at another site in Tokyo in the densely built-up Ginza shopping district (Kanda et al., 1997). This is encouraging, especially given that the sensors use completely different properties of the atmosphere to determine the fluxes. While the scintillometer provides an integrated value of the flux over its observational path, which in the present study is 250 m in the horizontal, EC is essentially a point measurement. Because the present study is concerned with the determination of regionally representative fluxes over patchy urban terrain, it is important to address the representativeness of the observations. The point-to-area representativeness of components involved in flux measurements can be assessed using well-known models for source area analysis. The model used in the present study (FSAM; Schmid 1994, 1997) is based on a reverse-plume diffusion approach to estimate the source area influencing a measurement at a certain point. The dimensions and shape of the elliptical shape of the upwind source area depend on the flow characteristics (wind speed, level of turbulence and atmospheric stability).

9 NEW METHOD TO DETERMINE ZERO-PLANE DISPLACEMENT LENGTH 185 Figure 3. Diurnal variation of 30-min averaged Q H from scintillation (at z = 16 and 32 m) and EC (at z =32m)systems. Present results (for the 90% source weight) for three cases are summarized in Table II. The lengths of the source areas agree well with each other, whereas the crosswind dimensions are larger for the scintillometer. The source areas for the latter are about 3.5 times larger under unstable, and times larger in stable conditions, compared to those for the EC sensor. Hence, as instability increases, the scintillation method represents an increasingly larger surface area compared to the EC technique. The good agreement between the scintillometer and EC observations implies that the sensors for both techniques were mounted at a height sufficient for their source areas to include a representative sample of the surface characteristics. Assuming a long enough path length at low heights, scintillometer measurements will be more useful than those from EC sensors over inhomogeneous terrain. This is because the relatively large size of the scintillometer s source area will include a more representative range of surface types than the EC sensors can see. To be able to solve (4) and (5) for the turbulent fluxes the non-dimensional dissipation rates for TKE (φ ε ), temperature variance (φ N ) and the non-dimensional structure parameter for temperature (φ CT ) need to be determined first. They are defined as (e.g., Wyngaard and Coté, 1971; Thiermann and Grassl, 1992): φ ε (ζ ) = kε(z z d )/u 3, (9) φ N (ζ ) = kn(z z d )/u T 2, (10) φ CT (ζ ) = 4β 1 φ N φ 1/3 ε, (11) where β 1 is the Obukhov Corrsin constant (= 0.86) and N is the dissipation rate for temperature. In practice ε and N are determined from the spectral densities within

10 186 MANABU KANDA ET AL. TABLE II Summary of source area dimensions for EC and scintillometer systems. U mean wind speed; σ v /u normalized standard deviation of transverse velocity; a along-wind dimension, b cross-wind dimension, A area of elliptically-shaped source region. ζ z (m) U (m s 1 ) σ v /u Eddy correlation Scintillometer a (m) b (m) A (m 2 ) a (m) b (m) A (m 2 ) the 2/3 region of the corresponding spectra (e.g., Roth, 1993). The computations have been made using z d = 6.6 m (see below). It is well known that the normalized turbulence characteristics in the urban atmosphere are different from those in the homogeneous surface layer (e.g., Roth, 2000). In the following the present data are compared with those from a residential site in Vancouver, Canada (Roth and Oke, 1993) and empirical relations from the homogeneous surface layer over rural terrain. Urban values of φ ε and φ N are lower, in particular for φ ε under moderate instabilities, compared to the reference data for the homogeneous surface layer above rural terrain (Figure 4) given by Thiermann and Grassl (1992): φ ε (ζ ) = (1 3ζ) 1 ζ ζ < 0 (12) and Wyngaard and Coté (1971): φ N (ζ ) = 0.74(1 9ζ) 1/2 2 <ζ< (13) Because of form drag, which affects momentum transfer only, the transfer of TKE is expected to be affected more strongly than that of heat. At larger instabilities urban-rural differences may disappear in this buoyancy-dominated regime. The functions fitted to V89 and the present data, and used in the computation of the turbulent heat and momentum fluxes (Equations (4) and (5)), herein are, for 3 <ζ <0, φ ε (ζ ) = (1 10.5ζ) 1, (14) φ N (ζ ) = 0.68(1 9.69ζ) 1/2, (15) φ CT (ζ ) = 4β 1 [0.68(1 9.69ζ) 1/2 ][(1 10.5ζ) 1 ζ ] 1/3. (16)

11 NEW METHOD TO DETERMINE ZERO-PLANE DISPLACEMENT LENGTH 187 Figure 4. Non-dimensional dissipation rate for (a) turbulent kinetic energy, and (b) temperature variance, versus stability. Dotted lines are empirical fits to data from the homogeneous surface layer over rural terrain (Equations (12) and (13)); solid lines are fits to urban data from present study and V89, a residential site in Vancouver, Canada (Roth and Oke, 1993) (Equations (14) and (15)). 5. Estimation of Zero-Plane Displacement Height Having established the suitability of the scintillometer as a tool to measure sensible heat flux over an urban surface, z d is estimated using the new SHM method outlined above. The two scintillometers used in the present study were carefully calibrated

12 188 MANABU KANDA ET AL. Figure 5. Estimated z d versus stability. Error bars represent 2.3 W m 2 measurement error. Symbols are present results (SHM method); filled area indicates range of values from morphometric methods (see text and Table III). and compared to each other before the observations using data obtained at a height of 2 m above an extensive, flat surface covered with short grass. As a result the mean values from min calibration runs are identical with a standard deviation of the differences between the two sensors of 2.3 W m 2 or 8% of absolute values. This is still not negligible compared to the heat storage term in (8). For example, using a layer 16 m thick this could result in an error of about 0.47 K h 1 for the average heating/cooling of the layer. SHM z d estimates are plotted as a function of ζ in Figure 5. The error bars reflect the ±2.3 W m 2 measurement error. The largest errors occur during neutral or near-neutral conditions when heat fluxes are small, however, during unstable stratification the method is robust. The average value of 6.6 m compares well with estimates from a range of morphometric methods (Table III). Although the present data set is small and generalizations are not warranted, it is interesting to note a possible dependency of z d on atmospheric stability (lower z d under more unstable conditions). A similar trend for z d was observed over grass by Moriwaki et al. (1999) using the SHM. Other techniques to determine z d do not demonstrate such a relationship. Morphometric methods are solely based on a surface description of the roughness elements and the coefficients often rely on

13 NEW METHOD TO DETERMINE ZERO-PLANE DISPLACEMENT LENGTH 189 TABLE III Summary of aerodynamic properties zd and z0 of the present study site (Setagaya, Tokyo) estimated using morphometric (Mc, Ra, Bo) and micrometeorological (TVM, SHM, Es) methods. See GO99 for details of morphometric methods. Rt rule-of-thumb; Es eddy stress. Method Reference Length (m) Comments zd z0 Rt 5.95 Equation (1) in GO99; fd =0.7 Mc Macdonald et al. (1998) 7.16 Staggered (Equation (13) in GO99; α = 4.43) Ra Raupach (1994, 1995) 5.71 Equation (15) in GO99 Bo Bottema (1995, 1997) 7.26 Dense, staggered (Equation (18), Table 1 in GO99) TVM Rotach (1994) 13 Minimum rms value corresponds to zd which is > zh SHM Present study 6.6 Using height-displaced scintillometers Rt 0.85 Equation (1) in GO99; f0 =0.1 Mc Macdonald et al. (1998) 0.23 Staggered (Equation (14) in GO99; β =1) Ra Raupach (1994, 1995) 0.89 Equation (16) in GO99 Bo Bottema (1995, 1997) 0.51 Dense, staggered (Equation (18), Table 1 in GO99) Es Present study 0.94 From 5 EC runs using logarithmic wind profile at near neutral ( 0.06 < ζ < 0.06)

14 190 MANABU KANDA ET AL. wind-tunnel data taken during neutral stratification. According to Thom (1971) z d is defined as the effective height of momentum absorption within a canopy and can therefore theoretically depend on the stability within the canopy. It seems reasonable that under more stable conditions the air within the canopy is relatively stagnant, which places the level of z d nearer the top of the canopy. Under neutral conditions the turbulence created by the roughness results in a vigorous exchange of air and individual eddies are able to penetrate more effectively into the canopy, thereby decreasing the mean height of momentum absorption. This situation is possibly more marked under unstable stratification when thermal plumes are emitted from the bottom of the canyon and off the walls. However, it is also possible that a potential dependence on stability is in part caused by differences in surface areas contributing to a measurement. The source areas decrease with increasing instability and although care has been taken to ensure homogeneous surface characteristics within the fetch, it is still conceivable that differences in the surface morphology contribute to the present result. Another approach available to estimate z d based on fast response micrometeorological observations has been developed by Rotach (1994). His TVM method uses the scaled temperature variance, σ T /T (where σ T is the standard deviation for temperature), which is a well-known function of ζ in the homogeneous surface layer. Since ζ includes z d, the height dependence of σ T /T via the variation of stability can be used to determine zero-plane displacement through an iterative procedure. Application of this technique to the present data set results in a value that is too large (Table III). The success of the TVM method is based on the premise that even over dynamically rough surfaces the thermal regime can be considered homogeneous and the normalized temperature variance follows similarity relationships. The present results follow the shape of standard relationships, however, the values are systematically larger (not shown). The same has been observed in other urban observations (Roth, 2000). Albeit small, the differences are enough for the method to result in values that are physically inplausible. Grimmond et al. (1998) list other urban studies where the TVM method has failed to produce realistic values. The morphometric methods used to predict z d also yield estimates of z 0.The values of z 0 here were obtained from direct observations of u under neutral conditions and applying the logarithmic wind profile (Table III). The morphometric values are smaller than directly observed here, however, very few neutral data points were available and the range of estimates was large (from 0.43 to 1.2 m). 6. Summary Field observations of turbulence characteristics were conducted in a densely built-up residential area of Tokyo, Japan. EC sensors (point measurement) and scintillometers (area-averaged measurement) were mounted at the top of mobile

15 NEW METHOD TO DETERMINE ZERO-PLANE DISPLACEMENT LENGTH 191 industrial cranes and for the scintillometer also on a nearby building. Use of a mobile instrument platform offers the advantage of being able to choose the best site for the application of interest. A limited sample of simultaneous scintillometer and EC sensible heat flux observations shows good agreement between the two methods. It is concluded that the scintillometer is a potentially useful tool to obtain area-averaged data in an urban setting. This is in particular true at low heights where the source area of the scintillometer is large enough to include a representative range of surface types. A new comparison shows that normalized urban TKE and, to a lesser degree, temperature dissipation rates are smaller compared to rural observations under near-neutral and slightly unstable conditions. At larger instabilities urban values are similar to homogeneous surface layer data. A new method (SHM) to estimate z d is developed, which uses scintillometer measurements at two heights. The area-average z d obtained using this technique corresponds well with estimates based on morphological methods. z d is found to decrease with increasing instability, possibly due to thermal plumes and turbulence eddies generated within the canopy that lower the mean height of momentum absorption. Acknowledgements This research was partially supported by CREST (Core Research for Evolution Science and Technology) of the Japan Science and Technology Corporation (JST). We wish to express our appreciation to Profs. H. Ueda, Y. Nakamura, I. Uno and J. Voogt for additional funding, advice and general support. Funds to support the field project were also provided by Grant-in-Aid for Developmental Scientific Research from the Ministry of Education, Science and Culture of Japan and by the Natural Sciences and Engineering Council of Canada. The authors thank A. Soux and J. Suzuki for their help in the field and with data processing. References Andreas, E. L.: 1987, On the Kolmogorov Constants for the Temperature-Humidity Cospectrum and the Refractive Index Spectrum, J. Atmos. Sci. 44, Andreas, E. L.: 1988, Atmospheric Stability from Scintillation Measurements, Appl. Optics 27, Bottema, M.: 1995, Parameterisation of Aerodynamic Roughness Parameters in Relation to Air Pollutant Removal Efficiency of Streets, in H. Power et al. (eds.), Air Pollution Engineering and Management, Computational Mechanics, pp Bottema, M.: 1997, Urban Roughness Modelling in Relation to Pollutant Dispersion, Atmos. Environ. 32, Champagne, F. H., Friehe, C. A., Larue, J. C., and Wyngaard, J. C.: 1977, Flux Measurements, Flux Estimation Techniques and Fine-Scale Turbulence Measurements in the Unstable Surface Layer over Land, J. Atmos. Sci. 34,

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17 NEW METHOD TO DETERMINE ZERO-PLANE DISPLACEMENT LENGTH 193 Schmid, H. P.: 1994, Source Areas for Scalars and Scalar Fluxes, Boundary-Layer Meteorol. 67, Schmid, H. P.: 1997, Experimental Design for Flux Measurements: Matching Scales of Observations and Fluxes, Agric. For. Meteorol. 87, Thiermann, V.: 1992, A Displaced Beam Scintillometer for Line-Averaged Measurements of Surface Layer Turbulence, in 10th Symposium on Turbulence and Diffusion, Portland, OR, American Meteorological Society, Boston, MA, pp Thiermann, V. and Grassl, H.: 1992, The Measurement of Turbulent Surface-Layer Fluxes by Use of Bichromatic Scintillation, Boundary-Layer Meteorol. 58, Thom, A. S.: 1971, Momentum Absorption by Vegetation, Quart. J. Roy. Meteorol. Soc. 97, Wesely, M. L.: 1976, A Comparison of Two Optical Methods for Measuring Line Averages of Thermal Exchanges above Warm Surfaces, J. Appl. Meteorol. 15, Wesely, M. L. and Derzko, Z. I.: 1975, Atmospheric Turbulence Parameters from Visual Resolution, Appl. Optics 14, Wyngaard, J. C. and Côté, O. R.: 1971, The Budgets of Turbulent Kinetic Energy and Temperature Variance in the Atmospheric Surface Layer, J. Atmos. Sci. 28,

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