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1.
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
2.
Turbulent Pressure and Velocity Perturbations Induced by Gentle Hills Covered with Sparse and Dense Canopies 总被引：1，自引：1，他引：0
How the spatial perturbations of the first and second moments of the velocity and pressure fields differ for flow over a train of gentle hills covered by either sparse or dense vegetation is explored using large-eddy simulation (LES). Two simulations are investigated where the canopy is composed of uniformly arrayed rods each with a height that is comparable to the hill height. In the first simulation, the rod density is chosen so that much of the momentum is absorbed within the canopy volume yet the canopy is not dense enough to induce separation on the lee side of the hill. In the second simulation, the rod density is large enough to induce recirculation inside the canopy on the lee side of the hill. For this separating flow case, zones of intense shear stress originating near the canopy-atmosphere interface persist all the way up to the middle layer, ‘contaminating’ much of the middle and outer layers with shear stress gradients. The implications of these persistent shear-stress gradients on rapid distortion theory and phase relationships between higher order velocity statistics and hill-induced mean velocity perturbations (Δu) are discussed. Within the inner layer, these intense shear zones improve predictions of the spatial perturbation by K-theory, especially for the phase relationships between the shear stress (~ ?Δu/?z) and the velocity variances, where z is the height. For the upper canopy layers, wake production increases with increasing leaf area density resulting in a vertical velocity variance more in phase with Δu than with ?Δu/?z. However, background turbulence and inactive eddies may have dampened this effect for the longitudinal velocity variance. The increase in leaf area density does not significantly affect the phase relationship between mean surface pressure and topography for the two simulations, though the LES results here confirm earlier findings that the minimum mean pressure shifts downstream from the hill crest. The increase in leaf area density and associated flow separation simply stretches this difference further downstream. This shift increases the pressure drag, the dominant term in the overall drag on the hill surface, by some 15%. With regards to the normalized pressure variance, increasing leaf area density increases ${\sigma_p/u_{*}^{2}}$ near the canopy top, where u * is the longitudinally averaged friction velocity at the canopy top and σ p is the standard deviation of the pressure fluctuations. This increase is shown to be consistent with a primitive scaling argument on the leading term describing the mean-flow turbulent interaction. This scaling argument also predicts the spatial variations in σ p above the canopy reasonably well for both simulations, but not inside the canopy. 相似文献
3.
The effect of topographical slope angle and atmospheric stratification on turbulence intensities in the unstably stratified surface layer have been parameterized using observations obtained from a three-dimensional sonic anemometer installed at 8 m height above the ground at the Seoul National University (SNU) campus site in Korea for the years 1999–2001. Winds obtained from the sonic anemometer are analyzed according to the mean wind direction, since the topographical slope angle changes significantly along the azimuthal direction. The effects of the topographical slope angle and atmospheric stratification on surface-layer turbulence intensity are examined with these data. It is found that both the friction velocity and the variance for each component of wind normalized by the mean wind speed decrease with increase of the topographical slope angle, having a maximum decreasing rate at very unstable stratification. The decreasing rate of the normalized friction velocity (u
* /U) is found to be much larger than that of the turbulence intensity of each wind component due to the reduction of wind shear with increase in slope angle under unstable stratification. The decreasing rate of the w component of turbulence intensity (σ
w
/U) is the smallest over the downslope surface whereas that of the u component (σ
u
/U) has a minimum over the upslope surface. Consequently, σ
w
/u
* has a maximum increasing rate with increase in slope angle for the downslope wind, whereas σ
u
/u
* has its maximum for the upslope wind. The sloping terrain is found to reduce both the friction velocity and turbulence intensity compared with those on a flat surface. However, the reduction of the friction velocity over the sloping terrain is larger than that of the turbulence intensity, thereby enhancing the turbulence intensity normalized by the friction velocity over sloping terrain compared with that over a flat surface. 相似文献
4.
A numerical study of stably stratified flow over a three-dimensional hill is presented. Large-eddy simulation is used here to examine in detail the laboratory experimental flows described in the landmark work of Hunt and Snyder about stratified flow over a hill. The flow is linearly stratified and U/Nh is varied from 0.2 to 1.0. Here N and U are the buoyancy frequency and freestream velocity respectively, and h is the height of the hill. The Reynolds number based on the hill height is varied from 365 to 2968. The characteristic flow patterns at various values of U/Nh have been obtained and they are in good agreement with earlier theoretical and experimental results. It is shown that the flow field cannot be predicted by Drazin's theory when recirculation exists at the leeside of the hill even at UNh 1. The wake structure agrees well with a two-dimensional wake assumption when U/Nh 1 but lee waves start to influence the wake structure as U/Nh increases. The dividing-streamline heights obtained in the simulation are in accordance with experimental results and Sheppard's formula. The energy loss along the dividing streamline due to friction/turbulence approximately offsets the energy gained from pressure field. When lee waves are present, linear theory always underestimates the amplitude and overestimates the wavelength of three-dimensional lee waves. The simulated variations of drag coefficients with the parameterK (=ND/ U) are qualitatively consistent with experimental data and linear theory. Here D is the depth of the tank. 相似文献
5.
George H. Fichtl 《Boundary-Layer Meteorology》1971,2(2):137-151
The standard deviation of vertical two-point longitudinal velocity fluctuation differences is analyzed experimentally with eleven sets of turbulence measurements obtained at the NASA 150-m ground-winds tower site at Cape Kennedy, Florida. It is concluded that /u
*0 is proportional to (fz/u
*0)0.22, where the coefficient of proportionality is a function of fz/u
*0 and u
*0/fL
0. The quantities f and L0 denote the Coriolis parameter and the surface Monin-Obukhov stability length, respectively; u
*0 is the surface friction velocity; z is the vertical distance between the two points over which the velocity difference is calculated; and zz is the mean height of the mid-point of the interval z above natural grade. The results of the analysis are valid for 20<-u
*0/fL
0<2000. 相似文献
6.
We examine the unsteady response of a neutral atmospheric boundary layer (ABL) of depth h and friction velocity u * when a uniform surface heat flux is applied abruptly or decreased rapidly over a time scale t<inf>θ</inf> less than about h /(10u *). Standard Monin–Obukhov (MO) relationships are used for the perturbed eddy viscosity profile in terms of the changes to
the heat flux and mean shear. Analytical solutions for changes in temperature, mean wind and shear stress profile are obtained
for the surface layer, when there are small changes in h /|LMO| over the time scale tMO~|L MO|/(10u*) (where L MO and t MO are the length and time scales, respectively). They show that a maximum in the wind speed profile occurs at the top of the
thermal boundary layer for weak surface cooling, i.e. a wind jet, whereas there is a flattening of the profile and no marked
maximum for weak surface heating. The modelled profiles are approximately the same as those obtained from the U.K. Met Office
Unified Model when operating as a mesoscale model at 12-km horizontal resolution. The theoretical model is modified when strong
surface heating is suddenly applied, resulting in a large change in h /|L MO| (>>1), over the time scale t MO. The eddy structure is predicted to change significantly and the addition of convective turbulence increases the shear turbulence
at the ground. A low-level wind jet can form, with convective turbulence adding to the mean momentum of the flow. This was
verified by our laboratory experiment and direct numerical simulations. Additionally, it is shown that the effects of Coriolis
acceleration diminish (rather than as suggested in the literature, amplify) the formation of the wind jets in the situations
considered here. Hence, only when the surface heat flux changes over time scales greater than 1/f (where f is the Coriolis parameter) does the ABL adjust monotonically between its equilibrium states. These results are also applicable
to the ABL passing over spatially varying surface heat fluxes. 相似文献
7.
The flows over four two-dimensional triangular hills and three two-dimensional bell-shaped hills have been investigated in a simulated rural atmospheric boundary layer modelled to a scale of 1:300: Further measurements were made over two of the triangular hills in a simulated rural boundary layer of 1: 3000 scale and in a simulated urban boundary layer modelled to a scale of 1:400. The effect of the model hill surface roughness was also investigated. Flow measurements were restricted to the mean velocity U, RMS velocity fluctuations u and the energy spectra for the streamwise velocity component Measurements were made at a number of longitudinal positions in the approach flow, over the model hills and downstream of the model hills. For each model hill, the crest was the region of largest mean velocity and smallest velocity fluctuations. The largest mean velocities over the model hills occurred for hills of intermediate slope rather than for the steepest hills. A decrease in the scale of the simulated atmospheric boundary layer led to a reduction in the amplification factors at the hill crests, whereas an increase in the surface roughness of the approach flow resulted in increased amplification factors at the hill crests. 相似文献
8.
临界起沙阈值可表征地表土壤的可蚀性,是风蚀起沙研究中非常重要的物理量之一。基于微气象学方法,将沙尘浓度和垂直沙尘通量均开始增加且至少持续0.5 h所对应的摩擦速度(或风速)确定为临界起沙摩擦速度u*t(或临界起沙风速Ut)。利用内蒙古科尔沁沙地地区2010-2013年春季大气环境综合观测资料,分析了不同沙尘天气过程(扬沙、沙尘暴和强沙尘暴)起沙阶段沙尘浓度和垂直沙尘通量随摩擦速度的演变特征,精细确定了该地区临界起沙摩擦速度(u*t)和临界起沙风速(Ut)的范围分别为0.45±0.20和6.5±3.0 m/s,同时讨论了不同起沙判据对确定临界起沙阈值产生的影响。相比而言,采用的起沙判据尽可能地排除了沙尘输送和沉降过程的影响,适用于不同的沙尘天气类型,使沙尘粒子进入大气的起沙结果更趋于合理,其结果可为建立统一、合理的起沙判据提供参考。 相似文献
9.
Edgar L. Andreas 《Boundary-Layer Meteorology》2011,141(3):333-347
A common parametrization over snow-covered surfaces that are undergoing saltation is that the aerodynamic roughness length
for wind speed (z
0) scales as au*2/g{\alpha u_\ast^2/g}, where u
* is the friction velocity, g is the acceleration of gravity, and α is an empirical constant. Data analyses seem to support this scaling: many published plots of z
0 measured over snow demonstrate proportionality to u*2{u_\ast^2 }. In fact, I show similar plots here that are based on two large eddy-covariance datasets: one collected over snow-covered
Arctic sea ice; another collected over snow-covered Antarctic sea ice. But in these and in most such plots from the literature,
the independent variable, u
*, was used to compute z
0 in the first place; the plots thus suffer from fictitious correlation that causes z
0 to unavoidably increase with u
* without any intervening physics. For these two datasets, when I plot z
0 against u
* derived from a bulk flux algorithm—and thus minimize the fictitious correlation—z
0 is independent of u
* in the drifting snow region, u
* ≥ 0.30 ms−1. I conclude that the relation z0 = au*2/g{z_0 = \alpha u_\ast^2/g} when snow is drifting is a fallacy fostered by analyses that suffer from fictitious correlation. 相似文献
10.
John A. T. Bye 《Boundary-Layer Meteorology》1988,44(4):407-410
It is shown that the observationally determined roughness relation z
0 = u
*
2/g in which g is the acceleration of gravity, u
*, is the friction velocity in air, and = 0.0185 (Wu, 1982) for the wind profile over the sea surface relative to the surface current, is consistent with the existence of a Richardson Number criterion at the air-sea interface in which the critical Richardson Number, Ric = 1, such that all the shear energy is converted into potential energy. 相似文献
11.
Displaced-Beam Small Aperture Scintillometer Test. Part Ii: Cases-99 Stable Boundary-Layer Experiment 总被引：3，自引：3，他引：0
O. K. Hartogensis H. A. R. De Bruin B. J. H. Van De Wiel 《Boundary-Layer Meteorology》2002,105(1):149-176
The performance of the Scintec displaced-beam small aperture scintillometer (DBSAS) in the stable boundary layer (SBL) is investigated using data gathered during the CASES-99 experiment in Kansas, U.S.A. The DBSAS is superior to the eddy-covariance method in determining vertical fluxes of sensible heat and momentumclose to the ground and/or over short (< 1 min) averaging intervals. Both aspects are of importance in the shallow and non-stationary SBL.The friction velocity, u*, the temperature scale, *, and from these the sensible heat flux, H, were calculated from the indirectly determined dissipation rate, , and the structure parameter of temperature, CT
2, by the DBSAS, which was operated over a path length of 112 m. All these variables are compared with eddy-covariance data for 10-minute time averages. Previously reported systematic errors in the DBSAS, overestimation of u* for low u* values and underestimation of u* for high u* values, have in part been dealt with by adjusting the beam displacement distance from 2.7 mm to 2.6 mm in the calculations. The latter adjustment is presented as a working hypothesis, not a general solution. 相似文献
12.
Scaling velocities relevant for turbulent flows in the planetary boundary layer are discussed. It is suggested that the scaling parameters should be determined by integrated bulk properties of the respective turbulent production terms. According to this concept, a new velocity scale, replacing the friction velocityu*, is proposed depending on bothu* and the geostrophic windu
g
. The convective velocity scalew* can be determined by the integral of the buoyancy production term and is therefore an appropriate velocity scale. Examination of Minnesota and Kansas data shows that these data do not give the possibility of verifying whether the new scaling velocity is more appropriate thanu*. This is because the range of variability of atmospheric stability during the field measurements is too small. However, theoretical considerations based on integrated properties of the turbulence, through the depth of the planetary boundary layer, are given in support of the new scaling velocity. 相似文献
13.
P. A. Taylor 《Boundary-Layer Meteorology》1970,1(1):18-39
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
14.
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
15.
Fluctuations in the vertical wind velocity and air temperature were measured with a 1-dimensional sonic anemometer and fine thermocouple over a flat agricultural site in the Rhone Valley, France. Strong Mistral winds with speeds up to 20 m s–1 kept atmospheric conditions very close to neutral and ensured stationarity. Friction velocities estimated both by eddy correlation (sonic plus Gill Bivane) and inertialdissipation (sonic only) methods agreed within 1 and 5 % respectively of traditional profile measurements over the measured range of 0.2 to 1.2 m s–1. The coefficient of eddy transport for heat exceeded that of momentum by a factor of 1.38 (± 0.05), a result almost identical to that obtained in the Kansas experiment (Businger et al., 1971). For - 0.15 >= z/L >= 0.05, the ratio
w
/u
* was 1.69 and 1.34 for unstable and stable conditions, respectively. For ¦z/L¦ >= 0.05, the ratio /T
* was 1.40 independent of whether neutrality was approached from either stable or unstable conditions. 相似文献
16.
P. K. Misra 《Boundary-Layer Meteorology》1979,17(1):93-99
The paper presents a simple method to compute the stability parameter Z/L and boundary-layer parameters such as friction velocity U
* and surface heat flux Q
0 on an operational basis. The method is based on the autocorrelation function of the vertical velocity which is relatively insensitive to averaging times except for very large lag times. Eddy correlation techniques on the other hand are very sensitive to averaging times. 相似文献
17.
This study investigates the impact of soil moisture availability on dispersion-related characteristics: surface fric-tion velocity (u*), characteristic scales of temperature and humidity (T* and q*), the planetary boundary layer height (h) and atmospheric stability classified by Monin-Obukhov length (L), Kazanski-Monin parameter (μ) and convective velocity scale (w*) during daytime convective condition using a one-dimensional primitive equation with a refined soil model. 相似文献
18.
K. L. Davidson 《Boundary-Layer Meteorology》1974,6(1-2):305-331
Turbulence data obtained over ocean waves during the BOMEX experiment of 1969 are presented. Procedures in measurement and analyses are described which include adjustments for possible platform, R/V FLIP, motion. Momentum transfer is shown to have been influenced by both stability and wind-wave coupling. The wind-wave coupling influence is separated from the stability influence and is described in terms of a linear dependence of the deviation from the logarithmic profile on C/u
*, where C is the phase speed corresponding to the wave spectrum peak. As observed by others, a value of C/u
* near 25 is associated with minimal wind-wave coupling influence. For C/u
* greater than 25, momentum transfer is decreased relative to the neutral profile prediction. Expressions are also presented for the wind-wave coupling influence on relative intensities,
u
/u
*,
u
/u
* and
w/u
*. Values of the relative intensities approximate neutral overland values when the expressions are written such that the wave influence is zero near a C/u
* value of 25. 相似文献
19.
S. A. Hsu 《Boundary-Layer Meteorology》1994,71(1-2):205-209
Under growing wind-wave conditions the shear velocity,u
*, over the water surface equalsg
2
H
s
2
B
a
2
C
p
3
, whereg is the gravitational acceleration,H
s
is the significant wave height,B
a
is a constant, andC
p
is the wave celerity. From an independent field experiment in a lake environment which provided all three parameters (u
*,H
s
, andC
p
), the value ofB
a
is found to be 0.89, which is slightly lower than but consistent (within 20%) with the literature value between 0.90 and 1.06 obtained from an oceanic environment. Since thisu
* equation does not include the wind speed,U
10, anotheru
* formulation withU
10 in addition to the wave information is also evaluated. It is shown that the latter equation which includesU
10 is superior to the former withoutU
10. 相似文献
20.
The paper reveals that the variations in parameters like u*, the scaling velocity and θ*. The scaling tempera-ture during the various phases of monsoon might be linked with subsynoptic features. The rise in u* is mainly connected with the presence of lower tropospheric cyclonic vorticity over a subsynoptic scale of the site. However the variations in θ* is mainly linked with the various phases of monsoon and θ* shows a sharp rise in presence of low level convective cloud.Besides the correlation studies of u and u*, θv and θv* , θv-θv0 and θv* are undertaken. The correlation be?tween θv and θv* is poor. In other two cases correlations are good. Besides u/u* , has shown good coefficient of variation values within the ζ range. 相似文献