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1.
An approximate method for calculating the relationship between z/L(z = reference height, L = Obukhov length) and the bulk Richardsonnumber is presented. If this relationship is known, the momentum andheat fluxes can be computed easily without any iteration. The avoidance of iteration can speed up computationsin large-scale models considerably (up to 10 times) and cases which do not converge or converge very slowly cannot occur. The proposed formulae take into account the difference between momentum (z0M) and heat roughnesslengths (z0H). Because the roughness lengths are not neglected at any step of the derivation, the resulting analytical formulae can be used not only between the surface and the reference height but also between two finite levels z1 andz2 (by replacing z0M and z0H by z1 and z by z2). Theequations remain correct even in the limit z1 z2.The formulae are based upon the (partially modified) Businger–Dyer flux–profile relationships and,consequently, they are restricted to predominantly homogeneous terrain.These new approximations are an improvement over the existing solutions because they are simpler than most of the formulae in the literature and are able to match the numerical exact solution for different parameter sets (Businger, Dyer, Högström) with an maximum error of about 2% for a wide range of z/L, z/z0M and z0M/z0H.Furthermore, in stable conditions, schemes with and without a finitecritical bulk Richardson number can be approximated. The possibleambiguity of the exact solution =f(RIB) in (moderately) stable conditions is discussed briefly. The performance of the new formulae is compared to the exact numerical solution and to different formulae proposed in the literature.  相似文献   

2.
Flux-profile relationships based on surface-layer similarity theory are used to derive relationships between the Monin-Obukhov stability parameter = z/L and the bulk Richardson number Ri b . In contrast to previous studies, the roughness length for heat, z 0h ,is assumed unequal to the roughness length for momentum, z 0m .For the stable case, an analytic expression of in terms of Ri b can be derived and in the unstable case, the solution is obtained through a simple iterative process.Errors introduced from the simplification of z 0h = z 0m are evaluated and are shown to be very significant in most cases. Thus, this error in many practical applications may invalidate the intended solution.  相似文献   

3.
For flow over natural surfaces, there exists a roughness sublayer within the atmospheric surface layer near the boundary. In this sublayer (typically 50z 0 deep in unstable conditions), the Monin-Obukhov (M-O) flux profile relations for homogeneous surfaces cannot be applied. We have incorporated a modified form of the M-O stability functions (Garratt, 1978, 1980, 1983) in a mesoscale model to take account of this roughness sublayer and examined the diurnal variation of the boundary-layer wind and temperature profiles with and without these modifications. We have also investigated the effect of the modified M-O functions on the aerodynamic and laminar-sublayer resistances associated with the transfer of trace gases to vegetation. Our results show that when an observation height or the lowest level in a model is within the roughness sublayer, neglect of the flux-profile modifications leads to an underestimate of resistances by 7% at the most.  相似文献   

4.
The Louis scheme for calculating the vertical eddy fluxes within the atmospheric surface layer is improved by broadening the original assumptions. In our approach, the momentum and heat transfer roughness lengths (z0 and zT respectively) can be different, and z0 need not be negligibly small compared with the lowest height (z) in modelling. For these conditions, we choose more consistent wind and potential temperature profile forms, then derive new algorithms for calculating fluxes. Improvement is demonstrated for a wide range of z/L (L is the Obukhov length), z/z0 and z0 zT, by comparing these fluxes with those derived from a theoretical surface-layer model. The improved algorithms can be used in atmospheric modelling systems for more varied surfaces and a wide range of atmospheric stability.  相似文献   

5.
Vertical profiles of wind speed, temperature and humidity were used to estimate the roughness lengths for momentum (z 0), heat (z H ) and moisture (z Q) over smooth ice and snow surfaces. The profile-measurements were performed in the vicinity of a blue ice field in Queen Maud Land, East Antarctica. The values ofz 0 over ice (3·10–6 m) seem to be the smallest ever obtained over permanent, natural surfaces. The settling of snow on the ice and the loss of momentum at saltating snow particles serve as momentum dissipating processes during snow-drift events, expressed as a strong dependence ofz 0 on u#.The scalar roughness lengths and surface temperature can be evaluated from the temperature and humidity profile measurements if the ratioz H /z Q is specified. This new method circumvents the difficult measurement of surface temperature. The scalar roughness lengths seem to be approximately equal toz0 for a large range of low roughness Reynolds numbers, despite the frequent occurrence of drifting snow. Possible reasons for this agreement with theory of non-saltating flow are discussed.  相似文献   

6.
The aerodynamic drag coefficient (CD) is conjectured to change (or remains almost uniform) with the horizontal wind speed (U) over a flexible (or fixed) surface element, which is represented with the surface roughness (z0). This conjecture is tested for the near neutral atmospheric turbulence (i.e. when surface stability z/L is almost equal to 0, where z is the measurement height and L is Obukhov length) of monsoon and winter season at an on-slope and a ridge-top site in the Indian Himalaya, wherein the ridge-top site is associated with a higher degree of sensitivity to the roughness element and terrain attributes. This hypothesis is successfully verified for two conditions, (i) the monsoon period observations of ridge-top site are found to have higher z0 due to vegetative growth than the winter period for flows having similar terrain signature, and (ii) the monsoon and winter period observations of on-slope site are noted to have similar z0 for flows having signature of steep terrain. Subsequently, constants (i.e. a and b) of the power-law relationships between CD and U (i.e. CD = aUb), as a function of z0, are optimized. It is noted that the relationship between CD and U has higher sensitivity towards the terrain slope than the vegetative growth.  相似文献   

7.
The function ()=(1+|z/L|2/3)1/2,where z is the height, L the Obukhov length, and a constant,is proposed for the nondimensional wind speed and temperaturegradients (flux-profile relationships) in anunstable surface layer. This function agrees quite well withboth wind speed and temperature data,has the theoretically correct behaviour in convective conditions,and leads to simple results when integrated to produce the mean profiles.  相似文献   

8.
Source/sink distributions of heat, water vapour andCO2 within a rice canopy were inferred using aninverse Lagrangian dispersion analysis and measuredmean profiles of temperature, specific humidity andCO2 mixing ratio. Monin–Obukhov similarity theorywas used to account for the effects of atmosphericstability on w(z), the standard deviation ofvertical velocity and L(z), the Lagrangian timescale of the turbulence. Classical surface layer scaling was applied in the inertial sublayer (z > zruf)using the similarity parameter = (z - d)/L, where z is height above ground, d is the zero plane displacementheight for momentum, L is the Obukhov length,and zruf 2.3hc, where hc iscanopy height. A single length scale hc, was usedfor the stability parameter 3 = hc/L in the height range 0.25 < z/hc < 2.5. This choice is justified by mixing layer theory, which shows that within the roughness sublayer there is one dominant turbulence length scaledetermined by the degree of inflection in the windprofile at the canopy top. In the absence of theoretical or experimental evidence for guidance,standard Monin–Obukhov similarity functions, with = hc/L, were used to calculate the stabilitydependence of w(z) and L(z) in the roughness sublayer. For z/hc < 0.25 the turbulence length and time scales are influenced by the presence of the lowersurface, and stability effects are minimal. With theseassumptions there was excellent agreement between eddycovariance flux measurements and deductions from theinverse Lagrangian analysis. Stability correctionswere particularly necessary for night time fluxes whenthe atmosphere was stably stratified.The inverse Lagrangian analysis provides a useful toolfor testing and refining multilayer canopy models usedto predict radiation absorption, energy partitioningand CO2 exchanges within the canopy and at thesoil surface. Comparison of model predictions withsource strengths deduced from the inverse analysisgave good results. Observed discrepancies may be dueto incorrect specification of the turbulent timescales and vertical velocity fluctuations close to theground. Further investigation of turbulencecharacteristics within plant canopies is required toresolve these issues.  相似文献   

9.
In this paper we study the effect of atmospheric stability on the growth of surface gravity waves. To that end we numerically solved the Taylor-Goldstein equation for wind profiles which deviate from a logarithmic form because stratification affects the turbulent momentum transport. Using Charnock's relation for the roughness height z 0 of the wind profile, it is argued that the growth rate of the wave depends on the dimensionless phase velocity c/u * (where u * is the friction velocity) and a measure of the effect of atmospheric stability, namely the dimensionless Obukhov length gL/u * 2, whereas it only depends weakly on gz t /u * 2 (where z t is the roughness height of the temperature profile). Remarkably for a given value of u * /c, the growth rate is larger for a stable stratification (L > 0) than for an unstable one (L < 0). We explain why this is the case. If, on the other hand, one considers the growth rate as a function of c/U 10 (where U 10 is the windspeed at 10 m), the situation reverses for c/U 10 < 1. For practical application in wave prediction models, we propose a new parameterization of the growth rate of the waves which is an improvement of the Snyder et al. (1981) proposal because the effect of stability is taken into account.  相似文献   

10.
湍流通量参数化方案的非迭代方法研究   总被引：3，自引：2，他引：1  
基于Högström (1996) 和Beljaars et al.(1991) 的研究工作, 沿用Louis et al.(1982) 和Launiainen (1995) 的思路, 本文采用多元回归分析方法, 研发了一种采用非迭代方法的湍流通量参数化方案。该方案直接用整体理查森数、 空气动力学粗糙度长度和热力学粗糙度长度对稳定度参数进行参数化, 从而避免了通过循环迭代计算Monin－Obukhov长度。该方案不仅有效地节省了CPU计算时间, 而且其计算结果与迭代方案 (BHH方案) 的计算结果非常接近。  相似文献   

11.
A non-iterative analytical scheme is developed for unstable stratification that parametrizes the Monin–Obukhov stability parameter \(\zeta \) (\({=}z{/}L\), where z is the height above the ground and L is the Obukhov length) in terms of bulk Richardson number (\(Ri_B\)) within the framework of Businger–Dyer type similarity functions. The proposed scheme is valid for a wide range of roughness lengths of heat and momentum. The absolute relative error in the transfer coefficients of heat and momentum is found to be less than 1.5% as compared to those obtained from an iterative scheme for Businger–Dyer type similarity functions. An attempt has been made to extend this scheme to incorporate the similarity functions having a theoretically consistent free convection limit. Further, the performance of the scheme is evaluated using observational data from two different sites. The proposed scheme is simple, non-iterative and relatively more accurate compared to the schemes reported in the literature and can be used as a potential alternative to iterative schemes used in numerical models of the atmosphere.  相似文献   

12.
Analysis of wind profiles at the Boulder Tower (BAO) leads to these conclusions:
  1. The variation of roughness with wind direction found earlier is confirmed. Roughness lengths measured on the tower are larger than those measured close to the surface.
  2. The profiles and measurements of Reynolds stress are consistent with a von-Karman constant of 0.35.
  3. The form φm=(1?15z/L)-1/3 fits best in the range -0.6 < z/L < 0. In the range 0 < z/L < 0.5, θ m ～ 1 + 4.7z/L provides a good fit to the observations. For z/L < 0.1, φ m also depends on h, the thickness of the PBL. For z/L < -0.6, Φ m approaches the constant 0.5, in contrast to all previous suggestions. For larger stabilities, the upper level is usually not in the surface layer, and wind ratios become independent of z/L.
  4. With snow cover, the effective roughness diminishes to about 1 cm, even for directions for which the roughness length without snow is large.
  5. Estimation of winds at 100 or 150 m from information near the surface is best for similarity theory provided that the ratio of height to Monin-Obukhov L is less than 0.1. For larger z/L, simple power laws seem more appropriate.
  相似文献   

13.
Careful micrometeorological measurements on an empty parking lot allowed determination of the surface fluxes of sensible heatH and of momentum by applying profile equations derived from Monin-Obukhov similarity theory with two sets of the stability correction function for momentum m and sensible heat h . These fluxes were compared with reference values ofH independently determined by means of an eddy correlation technique. In general, better agreement was found betweenH values derived from profiles with the stability functions of Brutsaert (1992) and referenceH values, than when the Businger-Dyer functions were used to deriveH. The disagreement in the latter comparison was especially serious under strongly unstable conditions, with the value ofy=–z/L (wherez is the height andL is the Obukhov length) larger than 10. A closer look at the procedure for calculatingH from the profiles revealed that the large differences between theH values derived with these two different versions of the stability correction functions were caused by the small differences of the h values, and not by the larger differences of the m values. This result stems from the strong sensitivity of the resultingH values on the choice of h .  相似文献   

14.
The iteration scheme of Robinson (1962) for the determination ofz 0 andd under neutral atmospheric stability has been generalized in a relatively simple manner. It now accounts for nonneutral conditions and permits simultaneous calculation ofd,z 0, and flux densities of latent and sensible heat. Using profile data sets given in the literature, the generalized scheme yields results which agree quite well with those obtained by Kramm (1989).  相似文献   

15.
A theory is offered for the drag and heat transfer relations in the statistically steady, horizontally homogeneous, diabatic, barotropic planetary boundary layer. The boundary layer is divided into three regionsR 1,R 2, andR 3, in which the heights are of the order of magnitude ofz 0,L, andh, respectively, wherez 0 is the roughness length for either momentum or temperature,L is the Obukhov length, andh is the height of the planetary boundary layer. A matching procedure is used in the overlap zones of regionsR 1 andR 2 and of regionsR 2 andR 3, assuming thatz 0 L h. The analysis yields the three similarity functionsA(),B(), andC() of the stability parameter, = u */fL, where is von Kármán's constant,u * is the friction velocity at the ground andf is the Coriolis parameter. The results are in agreement with those previously found by Zilitinkevich (1975) for the unstable case, and differ from his results only by the addition of a universal constant for the stable case. Some recent data from atmospheric measurements lend support to the theory and permit the approximate evaluation of universal constants.  相似文献   

16.
A numerical model of airflow in the lowest 50–100 m of the atmosphere above changes in surface roughness and temperature or heat flux has been developed based on boundary layer approximations, the Businger-Dyer hypotheses for the non-dimensional wind shear and heat flux and a mixing length hypothesis.Results have been obtained for several situations, in particular, airflow with neutral upstream conditions encountering a step change in surface temperature or heat flux with no roughness change. In these cases large increases in shear stress at the outer edge of the internal boundary layer are predicted. The case of unstable upstream flow encountering a step change to zero heat flux is also considered.Two situations that may be encountered near the shores of the Great Lakes are considered.Notation B Businger-Dyer constant (= 16.0) in form for M, H - c p Specific heat at constant pressure - g Acceleration due to gravity - H Upward vertical heat flux - H 0 , H 1 Surface heat fluxes for x < 0, x 0 - k von Kármán's constant ( = 0.4) - l Mixing length - L Monin-Obukhov length - L 0 Upstream value of L - m Ratio of roughness lengths (= z 1/z 0) - RL * Non-dimensional parameter, see Equations (20, 22 and 24) - RL 1 * Same as RL * but with z 1 scaling (= mRL *) - T Scaled temperature - T 0 (z) Upstream temperature profile - u 0, u 1(x) Surface friction velocities for x < 0, x 0 - U, W Horizontal and vertical mean velocities - U 0 (z) Upstream velocity profile - x, z Horizontal and vertical coordinates - z i Local roughness length  相似文献   

17.
Determination Of The Surface Drag Coefficient   总被引：1，自引：0，他引：1  
This study examines the dependence of the surface drag coefficienton stability, wind speed, mesoscale modulation of the turbulent flux and method of calculation of the drag coefficient. Data sets over grassland, sparse grass, heather and two forest sites are analyzed. For significantly unstable conditions, the drag coefficient does not depend systematically on z/L but decreases with wind speed for fixed intervals of z/L, where L is the Obukhov length. Even though the drag coefficient for weak wind conditions is sensitive to the exact method of calculation and choice of averaging time, the decrease of the drag coefficient with wind speed occurs for all of the calculation methods. A classification of flux calculation methods is constructed, which unifies the most common previous approaches.The roughness length corresponding to the usual Monin–Obukhovstability functions decreases with increasing wind speed. This dependence on wind speed cannot be eliminated by adjusting the stability functions. If physical, the decrease of the roughness length with increasing wind speed might be due to the decreasing role of viscous effectsand streamlining of the vegetation, although these effects cannot be isolated from existing atmospheric data.For weak winds, both the mean flow and the stress vector often meander significantly in response to mesoscale motions. The relationship between meandering of the stress and wind vectors is examined. For weak winds, the drag coefficient can be sensitive to the method of calculation, partly due to meandering of the stress vector.  相似文献   

18.
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.  相似文献   

19.
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20.
A statistical model, based on a method of Vulf'son, is used to examine some of the plume-like temperature structures formed in the unstable boundary layer. The model assumes that the plume diameter changes slowly with height so that a cylindrical approximation may be made. Measurements of the vertical velocity and temperature were used to determine the temperature dependent portion of the vertical velocity field. Temperature data were collected from sensors on a tower and from aircraft; velocity data were collected only from the tower.Using this model for analysis of the data indicates that: (1) the average isotherm diameter and the population of isotherms are a function ofz/L; (2) the distribution of core temperatures is approximately a uniform distribution.Independent of the model, a convective velocity was determined and found to have approximately the same profile as the temperature; from this the average velocity of the plumes was found to be a linear function ofz/L, fromz/L - 0.1 toz/L - 1.0. As a consequence of this functional dependence, the entrainment into the plumes is approximately constant over this range. The cumulative temperature distribution function was found to be an asymmetric function ofz/L. A simple relation which is independent ofu * is given to determine the heat flux.Contribution No. 269 Dept. of Atmospheric Sciences, University of Washington.  相似文献   

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