The application of a two-dimensional numerical model of the planetary boundary layer to the Nanticoke region on the north shore of Lake Erie

A two-dimensional atmospheric boundary-layer model is applied to the Nanticoke region on the northern shore of Lake Erie (80 ° 03W and 42 ° 50N) to simulate numerically the observed wind and temperature profiles. In general, the profiles predicted by the model agree reasonably well with the observed profiles.     

Calculation of velocity skewness in real and artificial plant canopies

Wind velocities within a plant canopy are much more strongly skewed than those of the air flow above. We have examined the governing Eulerian equations for the velocity products u _i, u _j u_k using data from a wind tunnel study with an artificial canopy consisting of an array of 5 cm lengths of monofilament fishing line, and from measurements in corn (Zea mays L).Simple parameterizations for pressure-velocity correlations, and for the quadruple velocity products allowed reasonably accurate calculations of the third moments using measured profiles of the mean velocity, variance and covariance fields. Comparisons of individual terms in the rate equations for ovu _i, u _j u _krevealed that diffusion (from above) and mean shear were most important in creating large skewness in the canopy. A drag term also contributed but was of lesser importance. These terms were balanced by return-to-isotropy and a turbulence interaction term. A quasi-Gaussian approximation considerably underestimated the magnitude of the fourth moments within the canopy.     

Wind characteristics in the lowest 340 m of the atmospheric boundary layer at Pune,India

Wind characteristics in the lowest 340 m (agl) of the atmospheric boundary layer at Pune (18°32N, 73°51E, 559 m ASL) have been investigated using the pilot balloon wind observations obtained during the summer monsoon seasons of 1976, 1979 and 1980. Variations in the zonal and meridional components of wind at the surface, 40, 150 and 340 m (agl) have been described. Frequency distribution of the zonal component showed an unimodal character at the surface, which gradually approached a multimodal character at 340 m. The distribution pattern of the wind components was, by and large, normal. Spectral analysis of the wind components showed that the spectral energy was predominantly shared by 5–7 and 2–3 day periods.     

Nitrate in the atmospheric boundary layer of the tropical South Pacific: Implications regarding sources and transport

Weekly bulk aerosol samples collected at Funafuti, Tuvalu (8°30S, 179°12E), American Samoa (14°15S, 170°35W), and Rarotonga (21°15S, 159°45W), from 1983 through most of 1987 have been analyzed for nitrate and other constituents. The mean nitrate concentration is about 0.11 g m^–3 at each of these stations: 0.107±0.011 g m^–3 at Funafuti; 0.116±0.008 at American Samoa; and 0.117±0.010 at Rarotonga. Previous measurements of mineral aerosol and trace metal concentrations at American Samoa are among the lowest ever recorded for the near-surface troposphere and indicate that this region is minimally affected by transport of soil material and pollutants from the continents. Consequently, the nitrate concentration of 0.11 g m^–3 can be regarded as the natural level for the remote marine boundary layer of the tropical South Pacific Ocean. In contrast, over the tropical North Pacific which is significantly impacted by the transport of material from Asia and North America, the mean nitrate concentrations are about three times higher, 0.29 and 0.36 g m^–3 at Midway and Oahu, respectively. The major sources of the nitrate over the tropical South Pacific are still very uncertain. A very significant correlation between the nitrate concentrations at American Samoa and the concentrations of ²¹⁰Pb suggests that transport from continental sources might be important. This continental source could be lightning, which occurs most frequently over the tropical continents. A near-zero correlation with ⁷Be indicates that the stratosphere and upper troposphere are probably not the major sources. A significant biogenic source would be consistent with the higher mean nitrate concentrations, 0.16 to 0.17 g m^–3, found over the equatorial Pacific at Fanning Island (3°55N, 159°20W) and Nauru (0°32S, 166°57E). The lack of correlation between nitrate and nss sulfate at American Samoa does not necessarily preclude an important role for marine biogenic sources.     

Free convection scaling over the ocean derived from buoy measurements during gate

The structure of atmospheric turbulence in the surface layer over the open ocean is examined under conditions of local free convection. The raw data consist of profile and fluctuation measurements of wind and temperature as obtained from a meteorological buoy. For near neutral conditions and for waves running approximately along the wind direction, wave-induced wind fluctuations can be described by a simplified linear theory based on Miles (1957). In this case, the spectrum of wind velocity is given as the sum of two parts; for the turbulent part, the parameterization as obtained by Kaimal et al. (1972) applies, while the wave-induced part is parameterized using a simplification of Miles' linear theory. For cases of local free convection, the measurements of the vertical component of the wind velocity are well described by similarity theory; as expected, _w /(-uw)^1/2 is proportional to (- z/L)^1/3. In order to scale the longitudinal wind velocity component, it seems to be reasonable to extend the list of relevant parameters by the height of the mixed layer z _i. We obtain _u /(- uw)^1/2 (z/z _i)^1/3(- z/L)^1/3 with only a poor correlation coefficient of r = 0.6. Overall, the results of local free convection scaling obtained from direct measurements show good agreement with those obtained from profile measurements. A comparison between direct and indirect determination of turbulent fluxes of momentum shows an unexplained difference of about 20%. This discrepancy is mainly due to a gap in the uw-cospectrum at the swell frequency.     

Effects of heat and water vapor transport on eddy covariance measurement of CO2 fluxes

Flux densities of carbon dioxide were measured over an arid, vegetation-free surface by eddy covariance techniques and by a heat budget-profile method, in which CO₂ concentration gradients were specified in terms of mixing ratios. This method showed negligible fluxes of CO₂, consistent with the bareness of the experimental site, whereas the eddy covariance measurements indicated large downward fluxes of CO₂. These apparently conflicting observations are in quantitative agreement with the results of a recent theory which predicts that whenever there are vertical fluxes of sensible or latent heat, a mean vertical velocity is developed. This velocity causes a mean vertical convective mass flux (= _cw for CO₂, in standard notation). The eddy covariance technique neglects this mean convective flux and measures only the turbulent flux _c w. Thus, when the net flux of CO₂ is zero, the eddy covariance method indicates an apparent flux which is equal and opposite to the mean convective flux, i.e., _c w = – _c w. Corrections for the mean convective flux are particularly significant for CO₂ because _cw and _c w are often of similar magnitude. The correct measurement of the net CO₂ flux by eddy covariance techniques requires that the fluxes of sensible and latent heat be measured as well.     

A numerical study of boundary-layer dynamics in a mountain valley

A higher order closure model is applied to simulate the dynamics in an area with a deep valley characterized by complex terrain in the southwestern US. The simulation results show generally good agreement with measured profiles at two locations within the valley. Both the measurements and the simulations indicate that the flow dynamics in the area are highly influenced by the topography and meandering of the valley, and can be resolved only by the full three-dimensional model code. The wind veering simulated over the range of the topographic elevations is often larger than 100 deg and in some cases as large as 180 deg, as a consequence of topographic forcing. In the case of an infinitely long valley, as is assumed in two-dimensional test simulations, a strong low-level jet occurs within the valley during stable conditions. The jet is mainly a consequence of the Coriolis effect. However, the jet development is significantly reduced due to asymmetric effects of the actual topography treated in the three-dimensional simulations. Tests with the two-dimensional nonhydrostatic version of the model show significant wave responses for a stable stratified flow over the valley. The structure resembles nonlinear mesoscale lee waves, which are intrinsically nonhydrostatic. However, considering the three-dimensional nature of this valley system, a better understanding and verification of the nonhydrostatic effects requires both a three-dimensional nonhydrostatic numerical model and an observational data set which is fully representative in all three dimensions.List of symbols (unless otherwise defined in the text) B ₁ closure constant - f Coriolis parameter - g acceleration of gravity - K _M ,K _H ,K _R turbulent exchange coefficients for momentum, heat and moisture - k von Karman constant - L Monin-Obukhov length - q ² twice the turbulent kinetic energy - R specific humidity - s height of the model top - T _g ground surface temperature - t time - U, V horizontal components of wind - U _g ,V _g geostrophic wind components - u, w perturbation components ofU andW wind components - u _* friction velocity - W vertical wind component in the terrain-following coordinates - x, y horizontal coordinates - Z actual height above sea level - z actual height above ground - z ₀ roughness length - z _g terrain height - z _i depth of the convective boundary layer - ₁ closure constant - coefficient of thermal expansion - height in the terrain-following coordinate - master length scale in the turbulent parameterization - scaled pressure (Exner function) - potential temperature - _m normalized vertical wind shear     

Three-dimensional modeling of synthetic cold fronts interacting with northern Alpine foehn

Summary The effect of the Alpine orography on prototype cold fronts approaching from the west is investigated by three-dimensional numerical model simulations. The numerical experiments cover a range of parameter constellations which govern the prefrontal environment of the front. Especially, the appearance and intensity of prefrontal northern Alpine foehn varies from case to case.The behaviour of a cold front north of the Alps depends much on the prefrontal condition it encounters. It is found that prefrontal foehn can either accelerate or retard the approaching front.An important feature is the pressure depression along the northern Alpine rim that results from the southerly foehn flow. In cases where this depression compensates the eastward directed pressure gradient associated with the largescale flow, the front tends to accelerate and the foehn breaks down as soon as the front passes. In contrast, the foehn prevents the front from a rapid eastward propagation if it is connected with a strong southerly wind component.No-foehn experiments are performed for comparison, where either the mountains are removed, or the static stability is set to neutral. Also shown are effects of different crossfrontal temperature contrasts.List of Symbols c _F propagation speed of a front - x, y horizontal grid spacing (cartesian system) - , horizontal grid spacing (geographic system) - t time step - z vertical grid spacing (cartesian system) - cross-frontal potential temperature difference - _i potential temperature step at an inversion - E turbulent kinetic energy - f Coriolis parameter - FGP frontogenesis parameter (see section 2.2) - g gravity acceleration (g=9.81 m s^–2) - vertical gradient of potential temperature - h terrain elevation (above MSL) - h _i height of an inversion (h _i =1000 m MSL) - H height of model lid (H=9000 m MSL) - K _M exchange coefficient of momentum - K _H exchange coefficient of heat and moisture - longitude - N Brunt-Väisäla-frequency - p pressure - Exner function (=T/) - latitude - q _v specific humidity - R _d gas constant of dry air (R _d =287.06 J kg^–1 K^–1) - density of dry air - t time - T temperature - potential temperature - TFP thermal front parameter (see section 2.2) - u, v, w cartesian wind components - u _g ,v _g geostrophic wind components - horizontal wind vector - x, y, z cartesian coordinates Abbreviations GND (above) ground level - MSL (above) mean sea level - UTC universal time coordinated With 20 Figures     

Flux-gradient profiles for momentum and heat over an urban surface

Summary In this paper, we evaluate the applicability of flux-gradient relationships for momentum and heat for urban boundary layers within the Monin-Obukhov similarity (MOS) theory framework. Although the theory is widely used for smooth wall boundary layers, it is not known how well the theory works for urban layers. To address this problem, we measured the vertical profiles of wind velocity, air temperature, and fluxes of heat and momentum over a residential area and compared the results to theory. The measurements were done above an urban canopy whose mean height z_h is 7.3 m. 3-D sonic anemometers and fine wire thermocouples were installed at 4 heights in the region 1.5z_h < z < 4z_h. We found the following: (1) The non-dimensional horizontal wind speed has good agreement with the stratified logarithmic profile predicted using the semi-empirical Monin-Obukov similarity (MOS) function, when it was scaled by the surface friction velocity that is derived from the shear stress extrapolated to the roof-top level. (2) The scaled gradient of horizontal wind speed followed a conventional semi-empirical function for a flat surface at a level (z/z_h = 2.9), whereas, in the vicinity of the canopy height was larger than the commonly-used empirical relationship. (3) The potential temperature profile above the canopy shows dependency on the atmospheric stability and the scaled gradient of temperature is in good agreement with a conventional shear function for heat. In the case of heat, the dependency on height was not found. (4) The flux-gradient relationship for momentum and heat in the region 1.5z_h < z < 4z_h was rather similar to that for flat surfaces than that for vegetated canopies.     

Occurrence of roll circulations in a shallow boundary layer

Meteorological measurements taken at the Näsudden wind turbine site during slightly unstable conditions have been analyzed. The height of the convective boundary layer (CBL) was rather low, varying between 60 and 300 m. Turbulence statistics near the ground followed Monin-Obukhov similarity, whereas the remaining part of the boundary layer can be regarded as a near neutral upper layer. In 55% of the runs, horizontal roll vortices were found. Those were the most unstable runs, with -z _i/L > 5. Spectra and co-spectra are used to identify the structures. Three roll indicators were identified: (i) a low frequency peak in the spectrum of the lateral component at low level; (ii) a corresponding increase in the vertical component at mid-CBL; (iii) a positive covariance {ovvw} together with positive wind shear in the lateral direction (V/z) in the CBL. By applying these indicators, it is possible to show that horizontal roll circulations are likely to be a common phenomenon over the Baltic during late summer and early winter.     

Study of the influence of katabatic flows on the Antarctic circulation using GCM simulations

Summary A one dimensional analytical model of katabatic wind over the Antarctica has been developed. This parametric model is derived from the bulk two-layer model of Ball including the surface friction and taking into account the Earth's rotation and the geostrophic wind in the upper layer.This model is validated using the data set (70 soundings) collected during IAGO experiment at D47 (67°24S, 138°43E, altitude 1 564m), 110 km inland from the coast of Adélie Land.The parameteric model is then introduced into a GCM which is a spectral global version of the operational numerical weather prediction model used by the French weather service. The most significant effect of the parameterization is a 50 m increase of the geopotential height over the South Pole. The surface temperature at the South Pole increases (2°C) reducing the pole-midlatitude thermal gradient. The westerly circulation at 50° S is slowed down (4m/s at 850 hPa), and the surface pressure at the South Pole increases (4hPa). These results, consistent with an increase of katabatic winds, would however be improved by a better coupling between the parameterization and the GCM boundary layer.With 8 Figures     

Estimates of diabatic wind speed profiles from near-surface weather observations

In this paper we analyse diabatic wind profiles observed at the 213 m meteorological tower at Cabauw, the Netherlands. It is shown that the wind speed profiles agree with the well-known similarity functions of the atmospheric surface layer, when we substitute an effective roughness length. For very unstable conditions, the agreement is good up to at least 200 m or z/L–7(z is height, L is Obukhov length scale). For stable conditions, the agreement is good up to z/L1. For stronger stability, a semi-empirical extension is given of the log-linear profile, which gives acceptable estimates up to ~ 100 m. A scheme is used for the derivation of the Obukhov length scale from single wind speed, total cloud cover and air temperature. With the latter scheme and the similarity functions, wind speed profiles can be estimated from near-surface weather data only. The results for wind speed depend on height and stability. Up to 80 m, the rms difference with observations is on average 1.1 m s^–1. At 200 m, 0.8 m s^–1 for very unstable conditions increasing to 2.1 m s^–1 for very stable conditions. The proposed methods simulate the diurnal variation of the 80 m wind speed very well. Also the simulated frequency distribution of the 80 m wind speed agrees well with the observed one. It is concluded that the proposed methods are applicable up to at least 100 m in generally level terrain.     

Some aspects of determining the stable boundary layer depth from sodar data

The question of estimating the height of the stable boundary layer (SBL) based on digitalized vertical profiles of sodar signal intensity has been re-examined. A simple one-dimensional numerical boundary-layer model is used to compute vertical profiles of the temperature structure parameterC _T ² . It is shown that especially at the beginning of the night (when stratification is weak) one can not expect any significant profile structure in the upper part of the SBL if its depth is determined in terms of common turbulent height scales. From this it is concluded that the SBL-height will be underestimated early in the night when derived from the maximum gradient in the signal intensity profiles. Later in the night in contrast, the computations often show elevated maxima or even zones with reduced, and above them enhanced, vertical gradients ofC _T ² , from which a SBL-height can be deduced that compares well with other common height scales. The computed profiles ofC _T ² are shown to be in qualitative agreement with observed profiles of sodar signal intensity for several analysed cases from the HAPEX-MOBILHY experiment.Comparing different SBL-depth scales with sodar observations, it is demonstrated that most of them are often closely related to a sodar-derived SBL-height only during certain phases of the night. Thus the sodar-SBL-height can, after a transition period, be perhaps associated with the lower turbulent layer of the growing surface inversion during the first part and with the height of the low-level wind maximum during the second part of the night.     

Atmospheric trace gas measurements at Palmer Station,Antarctica: 1982–83

In early 1982 a station capable of sampling atmospheric trace gas constituents on a continuous basis was established at Palmer Station, Anvers Island, adjacent to the Antarctic Peninsula (64° 46S 64° 04W). Sampling operations began about 1 February 1982. This is an initial report on this station, its location, equipment and general research objectives along with some initial sampling results. The constituents being measured and recorded were: ozone, methane, carbon dioxide, carbon monoxide, CCl₃F (fluorocarbon-11), CCl₂F₂ (fluorocarbon-12), carbontetrachloride, methylchloroform, nitrous oxide, and Aitken nuclei (CN). Data storage, data processing, and sampling system control is handled by a Hewlett-Packard 85 system. Preliminary analyses of about the first 20–22 months of data are presented and show not only the expected long-term trends but also shorter period concentration cycles that seem to be related to synoptic meteorology.     

On the stable stratification of the nocturnal lower troposphere inferred from lidar observations over Pune,India

The nocturnal structure of the lower troposphere is studied using aerosol profile data (50–2800 m AGL) obtained with a bistatic, continuous wave, Argon ion lidar system during October 1986–September 1989 at Pune (18°32 N, 73°51 E, 559m AMSL), India. The top of the nocturnal groundbased inversion is taken as the height above ground where the negative vertical gradient in aerosol concentration first reaches a maximum. During the post-sunset period over this station, this height is as low as 160m and frequently lies around 550m. Greater heights are observed in pre-monsoon months and smaller ones during the southwest monsoon season. Positive vertical gradients in aerosol concentration, indicative of stable/elevated layers, appear frequently around 750m. Temporal variations of aerosol concentration gradients in two adjacent air layers, 920–1000m and 100–1100m, provide evidence that stability increases downward in the early night hours.     

An observational study of the vertical profile of the high frequency fluctuations of the wind in the atmospheric boundary layer

This paper summarizes some measurements of high-frequency turbulence made at Cardington during the years 1968, 1969 and 1970 at heights up to 900 m. It discusses the statistical distribution of the data which appears to be closely log-normal. Also it is shown how the mean profiles of the derived dissipation of turbulent kinetic energy () can be rationalized to some extent in terms of atmospheric stability and low-level wind speed. A close correlation between and the mean wind and temperature profiles up to 900 m is illustrated and some discussion of the turbulent energy budget throughout the boundary layer is presented. The use and limitations of the constant flux layer relations in calculatingz ₀ andL from the estimates of, at the lower heights, is brought out.     

Quantitative estimates of full glacial temperatures in equatorial Africa from palynological data*

This paper presents a new reconstruction of the mean annual temperature obtained from a high altitude pollen sequence in equatorial Africa (3°28S, 29°34E, 2240 m). It has been achieved with an extended modern African reference data set by adding spectra from 228 new sites and using another selection for pollen taxa. The purpose of this paper is to test how the obtained temperature value depends upon the availability of modern analogues. The results are in good agreement with those previously published, reinforcing the validity of the method. The mean standard error is reduced by 0.3°C. The mean temperature for the Holocene appears + 1.4°C warmer than the present and the last glacial maximum (25-18 kyrs BP) cooling is better specified at – 3 ± 1.9° C, a conservative value, more consistent with reconstructed sea surface temperature in the equatorial ocean.Contribution to Clima Locarno - Past and Present Climate Dynamics; Conference September 1990, Swiss Academy of Sciences — National Climate Program     

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

The formation mechanism of the nocturnal urban boundary layer (UBL), especially in the winter nighttime, was investigated based on the extensive field observations conducted during November 1984 in Sapporo, Japan. A strong, elevated inversion formed over the Sapporo urban area and the inversion base height was approximately twice the average building height. Velocity fluctuations _u, _w and Reynolds stress % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaa0aaaeaaca% WG1bWaaWbaaSqabeaacaaIXaaaaGGaaOGae8hiaaIaam4DamaaCaaa% leqabaGaaGymaaaaaaaaaa!3A9C!\[\overline {u^1 w^1 } \] had nearly uniform profiles within the nocturnal UBL and decreased with height above the UBL. On the other hand, temperature fluctuations _t , and heat fluxes % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaa0aaaeaaca% WG1bWaaWbaaSqabeaacaaIXaaaaGGaaOGae8hiaaIaeqiUde3aaWba% aSqabeaacaaIXaaaaaaaaaa!3B56!\[\overline {u^1 \theta ^1 } \] and % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaa0aaaeaaca% WG3bWaaWbaaSqabeaacaaIXaaaaGGaaOGae8hiaaIaeqiUde3aaWba% aSqabeaacaaIXaaaaaaaaaa!3B58!\[\overline {w^1 \theta ^1 } \] had peaks at the inversion base and small values within the nocturnal UBL. The turbulent kinetic energy budget showed that the turbulent transport term and shear generation from urban canopy elements are important in the nocturnal UBL development; the role of the buoyancy term is small. The turbulence data analysis and application of a simple advective model showed that the mechanism of UBL formation may be controlled by the downward transport of sensible heat from the elevated inversion caused by mechanically-generated turbulence.Nomenclature g accelaration due to gravity, m s^-2 - k turbulent kinetic energy, m² s^-1 - K _m eddy viscosity, m² s^-1 - L Monin-Obukhov lenght, m - p pressure, Kg m^-2 - U, V, W mean wind speed in the downwind, crosswind, and vertical directions, respectively, m s^-1 - u ¹, w ¹ wind speed fluctuation in the downwind and vertical direction, respectively, m s^-1 - u ₁ friction velocity, m s^-1 - % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaa0aaaeaaca% WG1bWaaWbaaSqabeaacaaIXaaaaGGaaOGae8hiaaIaam4DamaaCaaa% leqabaGaaGymaaaaaaaaaa!3A9C!\[\overline {u^1 w^1 } \] momentum flux, m²s^-2 - % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaa0aaaeaaca% WG1bWaaWbaaSqabeaacaaIXaaaaGGaaOGae8hiaaIaam4DamaaCaaa% leqabaGaaGymaaaaaaaaaa!3A9C!\[\overline {u^1 \theta^1 } \] sensible heat flux, m²s^-1°C - WD wind direction, deg - WS wind speed, m s^-1 - z altitude, m - Z _h inversion base height, m - Z _j wind maximum height, m - Z _t inversion top height, m - T _u-r heat island intensity, °C - temperature lapse rate at rural site, °C m^-1 - energy dissipation rate, m²s^-3 - ¹ Potential temperature fluctuation, °C - _* scaling temperature, (=-% MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaa0aaaeaaca% WG1bWaaWbaaSqabeaacaaIXaaaaGGaaOGae8hiaaIaeqiUde3aaWba% aSqabeaacaaIXaaaaaaaaaa!3B56!\[\overline {u^1 \theta ^1 } \]/u_*) °C - mean potential temperature fluctuation, K - density of air, Kgm^-3 - K von Kármán constant (=0.4) - _u, _v, _w standard deviation of wind speed in the downwind, crosswind, and vertical directions, respectively, m s^-1 - standard diviation of temperature, °C     

Determination of zero-plane displacement and roughness length of a forest canopy using profiles of limited height

Flux parameters, zero-plane displancement height and roughness length of a forest canopy are determined taking into consideration a transition layer and atmospheric diabatic influences. The present study, unlike previous studies by DeBruin and Moore (1985) and Lo (1990) that accounted for the velocity profile alone, make use of information from both wind and temperature profiles in formulating the governing equations. However, only the top level measurement is assumed to be within the logarithmic regime. In addition to the mass conservation principle (e.g., Lo, 1990; DeBruin and Moore, 1985), an analytic relationship between the Monin-Obukhov length and the bulk Richardson number is employed as the closure equation for the governing system.The present method is applied to profile measurements taken at Camp Borden (den Hartog and Neumann, 1984) in and above a forest canopy with mean crown height of about 18.5 m. Profile data under neutral or near-neutral conditions yieldedd=12.69 m andz ₀=0.97 m, which are realistic values. In general,z ₀ increases slightly with increasing wind yet remains relatively constant with respect to small variation of stabilities. On the other hand, increases of wind speed reduced values of displacement height,d, by as much as 50%. The influence, if any, of stability ond, however, is not clear from the results of the present study. The validity of using profile data of limited height is also carefully examined. At least for neutral or near-neutral stabilities, the present method can yield realistic results even though the profile heights are substantially below the transition layer height suggested by Garratt (1978).     

Full-scale measurements of the wind regime over a saddle,and correlation with wind-tunnel tests

The Gebbies Pass region consists of a saddle at a height of approximately 300 m, bounded by two ranges of hills approximately 500 m high. A survey technique which yielded mean wind speeds at a height of 10 m in this region is shown to provide excellent reproducibility for data collected 6 months apart. The survey results are also shown to correlate extremely well with those obtained from a 1:4000 scale model analysed in a Planetary Boundary Layer wind tunnel.Wind structure information obtained upstream of the saddle and at its crest is presented and compared with the wind tunnel model and other studies where possible. The velocity profile was measured up to 160 m at the crest and was found to be almost vertical; however, there was a noticeable jet at the 15 m height, which was not apparent on the model. Measurements at 10 m height showed a velocity increase of 40–60% at the crest compared to the approach terrain velocity at the same height above ground level. The longitudinal r.m.s. turbulence component _u was found to increase at the saddle but still resulted in reduced turbulence intensity values relative to the upstream situation. The longitudinal component of the energy spectrum at the upwind site was in excellent agreement with the theoretical model proposed by Kaimal et al. (1972). However, at the saddle crest the theoretical spectrum was too high and required further modification to fit the measured data. It is proposed that a terrain factor be used to make the necessary adjustment since the theoretical spectra are strictly only valid for flat homogeneous terrain.