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1.
Within the canopy sub-layer (CSL), variability in scalar sources and sinks are known to affect flux–variance (FV) similarity relationships for water vapour (q) and carbon dioxide (C) concentrations, yet large-scale processes may continue to play a significant role. High frequency time series data for temperature (T), q and C, collected within the CSL of an uneven-aged mixed coniferous forest in Lavarone, Italy, are used to investigate these processes within the context of FV similarity. This dataset suggests that MOST scaling describes the FV similarity function of T even though the observations are collected in the CSL, consistent with other studies. However, the measured FV similarity functions for q and C appear to have higher values than their temperature counterpart. Two hypotheses are proposed to explain the measured anomalous behaviour in the FV similarity functions for q and C when referenced to T. Respired CO2 from the forest floor leads to large positive excursions in the C time series at the canopy top thereby contributing significantly to both C variance increase and C vertical flux decrease—both leading to an anomalous increase in the FV similarity function. For q, transport of dry air from the outer-layer significantly increases both the variance and the water vapour flux. However, the expected flux increase is much smaller than the variance increase so that the net effect remains an increase in the measured FV similarity function for water vapour above its T counterpart. The hypothesis here is that identifying these events in the temporal and/or in the frequency domain and filtering them from the C and q time series partially recovers a scalar flow field that appears to follow FV similarity theory scaling. Methods for identifying both types of events in the time and frequency domains and their subsequent effects on the FV similarity functions and corollary flow variables, such as the relative transport efficiencies, are also explored.  相似文献   

2.
An important parameterization in large-eddy simulations (LESs) of high- Reynolds-number boundary layers, such as the atmospheric boundary layer, is the specification of the surface boundary condition. Typical boundary conditions compute the fluctuating surface shear stress as a function of the resolved (filtered) velocity at the lowest grid points based on similarity theory. However, these approaches are questionable because they use instantaneous (filtered) variables, while similarity theory is only valid for mean quantities. Three of these formulations are implemented in simulations of a neutral atmospheric boundary layer with different aerodynamic surface roughness. Our results show unrealistic influence of surface roughness on the mean profile, variance and spectra of the resolved velocity near the ground, in contradiction of similarity theory. In addition to similarity-based surface boundary conditions, a recent model developed from an a priori experimental study is tested and it is shown to yield more realistic independence of the results to changes in surface roughness. The optimum value of the model parameter found in our simulations matches well the value reported in the a priori wind-tunnel study.  相似文献   

3.
Profiles of wind velocity and temperature in the outer region of the atmospheric boundary layer (ABL) were used together with surface temperature measurements, to determine regional shear stress and sensible heat flux by means of transfer parameterizations on the basis of bulk similarity. The profiles were measured by means of radiosondes and the surface temperatures by infrared radiation thermometry over hilly prairie terrain in northeastern Kansas during the First ISLSCP Field Experiment (FIFE). In the analysis, the needed similarity functions were determined and tested; the main scaling variables used for the ABL were h i , the height of the convectively mixed layer, and V a and a, the wind speed and potential temperature averaged over the mixed layer. Good agreement (r = 0.80) was obtained between values of friction velocity u * determined by this ABL bulk similarity approach and those obtained by Monin-Obukhov similarity in the surface sublayer. Similarly, values of surface flux of sensible heat H determined by this method compared well (r = 0.90) with the regional means measured at six ground stations. The corresponding regional evaporation values, determined with the energy budget equation, also compared favorably (r = 0.94).  相似文献   

4.
Aerodynamic Variables in the Bulk Formulation of Turbulent Fluxes   总被引：1，自引：1，他引：0  
Aerodynamic variables are required to apply Monin–Obukhov similarity theory in the bulk formulation of surface fluxes. In the literature, these aerodynamic variables are commonly misinterpreted. In this paper, we review the concept of the aerodynamic variable, its connection to surface-layer similarity theory and how and why the aerodynamic variable is replaced with other variables.Observed mean variables below the surface layer, such as the surface radiation temperature, or the air temperature at canopy height, are often used in place of the extrapolated aerodynamic variables in the bulk formula, requiring empirical relationships between aerodynamic and observed variables, or requiring empirical adjustments of bulk resistances. The present study examines the validity of these relationshi Experiment (CODE). The results indicate that using a measured substitute for an aerodynamic variable can lead to significant errors in estimates of turbulent surface fluxes.  相似文献   

5.
The Height Correction of Similarity Functions in the Stable Boundary Layer   总被引：1，自引：1，他引：0  
Empirical similarity functions of the Richardson number, obtained from bin-averaged data in the lower part of the stable boundary layer, show an undesired dependence on height at which the observations are collected. A correction of this flaw is proposed and tested by employing the neutral mixing length l o as a similarity scale for height. The function of height describing l o is assumed to be linear in the surface layer, and approaching a specified value with increasing height. The modification does not alter the dependence of similarity functions on the Richardson number, and is shown to be supported by the Cooperative Atmospheric-Surface Exchange Study-1999 (CASES-99) data.  相似文献   

6.
An operational Planetary Boundary Layer model which employs similarity principles and two-layer patching to provide state-of-the-art parameterization for the PBL flow is used to study the popularly used similarity functions, A and B. The expected trends with stratification are shown. The effects of baroclinicity, secondary flow, humidity, latitude, surface roughness variation and choice of characteristic height scale are discussed.  相似文献   

7.
A one-dimensional numerical model based on the equations of mean motion and turbulent kinetic energy (TKE), with Delage's (1974) mixing-length parameterization has been used to simulate the mean and turbulent structure of the evolving stably stratified nocturnal boundary layer (NBL). The model also includes a predictive equation for the surface temperature and longwave radiational cooling effects.In the absence of advective and gravity wave effects, it is found that the model-simulated structure, after a few hours of evolution, could be ordered fairly well by a similarity scaling (u *0, *0, L 0, and h) based on surface fluxes and the NBL height. Simple expressions are suggested to describe the normalized profiles of momentum and heat fluxes, TKE, eddy-viscosity and energy dissipation. A good ordering of the same variables is also achieved by a local scaling (u *0, * and L) based on the height-dependent local fluxes. The normalized TKE, eddy viscosity and energy dissipation are unique functions of z/L and approach constant values as z/L , where L is the local Monin-Obukhov length. These constants are close to the values predicted for the surface layer as z/L , thus suggesting that the Monin-Obukhov similarity theory can be extended to the whole NBL, by using the local (height-dependent) scales in place of surface-layer scales. The observed NBL structure has been shown to follow local similarity (Nieuwstadt, 1984).  相似文献   

8.
Estimates of the geostrophic drag coefficient and the Rossby similarity functions, A and B obtained from data collected by an instrumented aircraft over the sea are presented. The average value of the geostrophic drag coefficient is 0.027 and is independent of the geostrophic windspeed. The dependence of the similarity functions A and B on boundary-layer parameters is investigated. The function A is found to depend on baroclinicity parameters, while B depends on the parameter u */fh (where u * is the surface friction velocity, f is the Coriolis parameter, and h is the boundary-layer depth). Using the geostrophic drag coefficient found here and the results of surface drag coefficient studies, a relationship between geostrophic windspeed and surface windspeed is obtained which shows good agreement with empirical data.  相似文献   

9.
Local similarity in the convective boundary layer (CBL)   总被引：1，自引：0，他引：1  
The paper presents similarity hypotheses that in the CBL the structure of turbulence is described by two different sets of local scales, defined by local (z-dependent) values of governing parameters. Arguments for local scaling are presented and the form of the similarity functions is derived.  相似文献   

10.
Similarity between temperature and water vapour was investigated in the Fourier domain by using their coherency spectra, defined as the correlation coefficient between their Fourier modes, and the relative efficiency of their vertical transport. The class-averaged values of these indices were computed from the turbulence measurements over sparse grasslands on the Tibetan plateau during the intensive observations of GEWEX (Global Energy and Water Experiment) Asian Monsoon Experiment (GAME). It was found that the energy-containing eddies at scales up to 100z (z being height) are characterised by well-maintained similarity between these scalars. The scalars are highly correlated with each other, and their transport efficiencies are almost equal within this scale range. In contrast, similarity was not always maintained at scales larger than 1000z. Detailed analyses showed that this breakdown of similarity occurs occasionally or sporadically, suggesting that it is caused by events whose average return period is not much smaller than the current averaging time, i.e. 30 min. We speculate that entrainment of drier and warmer air at the top of the atmospheric boundary layer caused the scalar dissimilarity at this scale range.  相似文献   

11.
A two-layer, first-order closure model for the Planetary Boundary Layer (PBL) is developed with the objective of parameterizing the surface stress with respect to the synoptic scale. The model includes stability effects by considering stratification-dependent secondary flow in the outer layer and empirical corrections to the surface layer flow. It shows the compatibility of simple eddy viscosity closure solutions with similarity theory by producing the now well-known Rossby similarity equations. It allows further insight into the Rossby similarity parameters by relating them to a single similarity parameter which is the ratio of the characteristic scales of the PBL and the surface layer.The measured and derived values of the similarity parameters A and B are compared with AIDJEX data and other published values. The variation in these values in stably stratified conditions is predicted and two alternate similarity parameters are calculated, one a constant and the other with a small variation and decreasing influence on the drag coefficient in stable stratification. The result is an empirical resistance law for a geostrophic drag coefficient variation which parameterizes an observed order-of-magnitude change in surface stress with changes in roughness or PBL stratification. This variation is related to similarity parameters characteristic of the region and to measurable changes in the geostrophic departure angle.  相似文献   

12.
The relaxed eddy accumulation (REA) method allows the measurement of trace gas fluxes when no fast sensors are available for eddy covariance measurements. The flux parameterisation used in REA is based on the assumption of scalar similarity, i.e., similarity of the turbulent exchange of two scalar quantities. In this study changes in scalar similarity between carbon dioxide, sonic temperature and water vapour were assessed using scalar correlation coefficients and spectral analysis. The influence on REA measurements was assessed by simulation. The evaluation is based on observations over grassland, irrigated cotton plantation and spruce forest.Scalar similarity between carbon dioxide, sonic temperature and water vapour showed a distinct diurnal pattern and change within the day. Poor scalar similarity was found to be linked to dissimilarities in the energy contained in the low frequency part of the turbulent spectra ( < 0.01 Hz).The simulations of REA showed significant change in b-factors throughout the diurnal course. The b-factor is part of the REA parameterisation scheme and describes a relation between the concentration difference and the vertical flux of a trace gas. The diurnal course of b-factors for carbon dioxide, sonic temperature and water vapour matched well. Relative flux errors induced in REA by varying scalar similarity were generally below ± 10%. Systematic underestimation of the flux of up to − 40% was found for the use of REA applying a hyperbolic deadband (HREA). This underestimation was related to poor scalar similarity between the scalar of interest and the scalar used as proxy for the deadband definition.  相似文献   

13.
The statistics of turbulence, such as the standard deviation of fluctuating velocities, in an unstable atmospheric boundary layer are assumed to be characterized by the combination of three specific lengths, Monin-Obukhov length L, observation height z and the height of mixing layer h. Unlike Monin-Obukhov similarity, even near the ground the effect of h is taken into account. According to observation, the length scale of the vertical velocity is proportional to z at least near the ground, but the lateral component depends mostly on h alone. The length scale of the longitudinal component depends on z and h.  相似文献   

14.
On similarity in the atmospheric boundary layer   总被引：2，自引：2，他引：2  
A similarity theory for the atmospheric boundary layer is presented. The Monin-Obukhov similarity theory for the surface layer is a particular case of this new theory, for the case of z 0. Universal functions which are in agreement with empirical data are obtained for the stable and convective regimes.On leave from Institute of Environmental Engineering, Warsaw Technical University, 00653 Warsaw, Poland. Present address, Department of Geological and Geophysical Sciences, University of Wisconsin, Milwaukee, WI 53201 U.S.A.  相似文献   

15.
A large-eddy simulation of the atmospheric boundary layer, large enough to contain both an urban surface layer and a convective mixed layer, was performed to investigate inner-layer and outer-layer scale motions. The objective was to determine the applicability of Monin–Obukhov similarity theory to inner-layer motions, to investigate the influence of outer-layer motions on surface-layer structure, as well as to assess the interaction of the two scales of motion. The urban surface roughness consisted of square-patterned cubic buildings of dimension H (40 m). A spatial filter was used to decompose the two scales in the inertial sublayer. The horizontal square filter of size 10H was effective in separating the inner-layer (surface-layer height ≈ 2 H) and outer-layer scales (boundary-layer height δ ≈ 30H), where the Reynolds stress contribution of the inner layer dominates in the logarithmic layer (depth 2H). Similarity coefficients for velocity fluctuations were successfully determined for inner-layer motions in the surface layer, proving the robustness of Monin–Obukhov similarity for surface-layer turbulence. The inner-layer structures exhibit streaky structures that have similar streamwise length but narrower spanwise width relative to the streamwise velocity fluctuation field, consistent with observations from an outdoor scale model. The outer-layer motions to some extent influence the location of ejections and sweeps through updraft and downdraft motions, respectively, thus, disturbing the homogeneity and similarity of inner-layer motions. Although the horizontal averages of the variances and covariance of motions reveal that the Reynolds stresses are dominated by inner-layer structures, the localized influence of the interaction of outer-layer horizontal and inner-layer vertical motions on the Reynolds stress is not insignificant.  相似文献   

16.
Nine profiles of the temperature structure parameter C T 2 and the standard deviation of vertical velocity fluctuations ( w) in the convective boundary layer (CBL) were obtained with a monostatic Doppler sodar during the second intensive field campaign of the First ISLSCP Field Experiment in 1987. The results were analyzed by using local similarity theory. Local similarity curves depend on four parameters: the height of the mixed layer (z i ), the depth of the interfacial layer (), and the temperature fluxes at the top of the mixed layer (Q i ) and the surface (Q o). Values of these parameters were inferred from sodar data by using the similarity curve for C T 2 and observations at three points in its profile. The effects of entrainment processes on the profiles of C T 2 and wnear the top of the CBL appeared to be described well by local similarity theory. Inferred estimates of surface temperature flux, however, were underestimated in comparison to fluxes measured by eddy correlation. The measured values of wappeared to be slightly smaller than estimates based on available parmeterizations. These discrepancies might have been caused by experimental error or, more likely, by the distortion of turbulence structure above the site by flow over the nonuniform terrain at the observation site.  相似文献   

17.
Fluctuations of the horizontal wind under unstable conditions   总被引：1，自引：0，他引：1  
The similarity relations for u/u* proposed by Panofskyet al. (1977) and Højstrup (1982) have been verified using eddy-correlation data collected during the EFEDA-experiment, conducted over the extensive plain of La Mancha (Spain), where vine plants form a primary crop. Also, the standard Monin-Obukhov relation is considered. It is found that the expressions by Panofskyet al. and Højstrup both yield almost identical results, and are better than the Monin-Obukhov expression. Also, u measured with a cup anemometer obeys the similarity expressions well, provided that the effect of the variation of wind direction on propellor wind speed is accounted for. The relationship of Panofskyet al. works rather well even when the boundary-layer height scaleh is replaced by a fixed height,h c. Best results were obtained forh c=1800 m. This height scale is possibly associated with the horizontal variability in the surface sensible heat flux pattern.  相似文献   

18.
The linear functions for non-dimensional wind and temperature profiles are commonly used to describe the surface layer fluxes in atmospheric models. However, their applicability is limited to smaller values of the stability parameter z/L (where z is the height above ground and L is the Obukhov length) i.e. z/L < 1.0. These linear functions have been modified (Webb 1970, Quart. J. Roy. Meteor. Soc. 96, 67–90; Clarke 1970, Quart. J. Roy. Meteor. Soc. 96, 91–114; Hicks 1976, Quart. J. Roy. Meteor. Soc. 102, 535–551; Beljaars and Holtslag 1991, J. Appl. Meteorol. 30, 327–341; Cheng and Brutsaert 2005, Boundary-Layer Meteorol. 114, 519–538) over the years for calculating fluxes when z/L > 1.0 under strongly stable conditions. In view of this, the objective of the present study is to analyze the performance of these similarity functions to compute surface fluxes in stable conditions.The meteorological observations from the Cooperative Atmosphere-Surface Exchange Study (CASES-99) experiment are utilized for computing the surface fluxes in stable conditions. The computed fluxes are found to be reasonably close to those observed. The ratio of observed to computed fluxes reveals that the computed fluxes are close to the observations for all the similarity functions for z/L < 1.0 whereas the computed values show relatively a large scatter from observations for z/L > 1.0. The computed values of u and heat flux do not show significant differences from those observed at 99% confidence limit. The performance of all the similarity functions considered here is found to be comparable to each other in strongly stable conditions.  相似文献   

19.
Local similarity theory, an analogy to the Monin-Obukhov similarity theory, is successfully applied to airborne observations in a coastal area of South Australia. The boundary layer over this highly non-uniform surface is characterized by extensive variations in its thermal stratification and turbulence characteristics. However, the behaviour of some statistical parameters of second- and higher moments seems to be determined mainly by local forcing, while horizontal advection plays a less important role. For these parameters, local scaling is effective. It is shown that the dimensionless variances of vertical velocity and potential temperature are functions of z/ only, where is the local Monin-Obukhov length and z is the height above ground. The dimensionless variance of horizontal velocity components is found to depend on h/, where h is the height of the oundary layer. Similarity relationships for some triple correlations are also discussed. The empirically determined local similarity relationships are found to agree with those obtained from surface-layer similarity. Finally, to illustrate the complexity of the local forcing, distributions of vertical energy and momentum fluxes, from which the local scaling parameters are derived, are shown.  相似文献   

20.
In analogy with two-dimensional turbulent layers, the surface layer (where wall similarity is fulfilled) and the region near the outer edge of the boundary layer (where the flow described by the velocity defect belongs to then-parameter family) may be postulated to exist for the atmospheric boundary layer over a wavy surface. The matching of the two regions yields a resistance law.  相似文献   

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