共查询到20条相似文献，搜索用时 231 毫秒
1.
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
2.
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
3.
Abstract The relationships between monthly anomalies of sea surface temperature (SST) and monthly anomalies of several surface wind parameters are examined using ten years of data from the mid‐latitude North Pacific Ocean. The wind parameters involve both u3 * and curl τ, where u* is the atmospheric friction velocity and τ the surface stress. These quantities are calculated from surface wind components analysed on synoptic (6‐hourly) maps. In order to examine the effect of synoptic disturbances, the time series of surface wind components at each grid point is high‐pass filtered (passing periods less than 10 days) and the above wind parameters are calculated from both filtered and unfiltered wind components. Two statistically significant relationships are found between monthly anomalies of SST and those of the various wind parameters. The first is a large coherent negative correlation between monthly anomalies of u3 * calculated from the high‐pass filtered wind components and month‐to‐month changes in the SST anomalies in the Central Pacific. This relationship is attributed to the production of turbulent vertical mixing in the ocean by synoptic disturbances in the atmosphere. The second relationship is a large positive correlation between curl τ calculated from the unfiltered wind components and SST anomaly changes in the Eastern Pacific. This relationship, which is opposite to that expected from Ekman pumping, is attributed to a negative association between the wind stress curl and the meridional advection of heat by the eastern boundary current system. It is shown that these atmospheric forcing mechanisms explain up to 10 per cent of the variance of monthly SST anomalies in a large part of the mid‐latitude North Pacific Ocean. This amount is in addition to, but certainly less than, that which can be explained by anomalous horizontal advection through statistical relationships with sea‐level pressure anomalies (Davis, 1976). 相似文献
4.
A wind tunnel investigation of the wind erosion of uranium mine-tailings material typical of a northern Ontario site has been carried out. The aim of the study was to measure the effects of various parameters, including mean and turbulent wind characteristics of the boundary layer and surface moisture content, upon the erosion process. The analysis of experimental data has yielded a mathematical model for predicting the net vertical mass fluxes. The results show that the dry vertical flux is proportional to u
*
2.3and the wet flux to u
*
5.0 Partical size analysis was also carried out. 相似文献
5.
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
6.
The atmospheric surface layer over sea has a density stratification which varies with moisture content and air/sea temperature difference. This influences the growth of water waves. To study the effect quantitatively, the Reynolds equations are solved numerically. For given wind speed and surface roughness, wave growth is found to be more rapid in unstably stratified conditions than in stable conditions. This is due to an increase in turbulence, primarily caused by an increase of mixing length.Under the assumption of a Charnock relation between surface roughness and friction velocity, it is found that for large inverse wave age (u
*/c>0.07), the effect of stratification on wave growth is weell described by Monin-Obukhov scaling of the friction velocity. For smaller values ofu
*/c, Monin-Obukhov scaling overpredicts.The effect on duration-limited wave growth is studied with the third-generation WAM surface wave model driven by 10 m winds. Effects of stratification on the significant wave height are found to be of the order of 10%. The results are comparable to those of a recent reanalysis of field measurements, although the measured stratification effect is somewhat stronger. Implementation of a stratification-dependent growth in wave models is recommended, as it can lead to small but significant improvements in wave forecasts when accurate air and sea temperatures are available. 相似文献
7.
Gary A. Briggs 《Boundary-Layer Meteorology》1988,45(1-2):117-135
It is suggested that convective scaling, with appropriate extensions, provides the most useful framework for estimating the effects of urban-scale surface inhomogeneities on diffusion in convective conditions. Strong contrasts in surface heat flux exist between cropland, forests, urban areas, and water or marshland surfaces. It is argued that a typical fetch for convective turbulence to readjust to changed heat (or buoyancy) input from the surface below is 2(U/w
*)h, where U is the mean wind speed in the mixing layer, w
* is the convective scaling velocity, and h is the mixing depth. In contrast, the fetch required for wind speed to readjust to new underlying surface roughness is of the order (U/u
*)2h/2, where u
* is the friction velocity.The ratio w
*/U is the best index of diffusion rates in moderately to very unstable conditions. General urban effects on heat flux, h, and U are discussed separately, then their combined effects on w
*/U are estimated. While this ratio can double over a large city during light winds, its increase is much less for small cities, or during moderate winds. Finally, some examples of heat flux in- homogeneities causing stationary convective features are presented. Steady downdrafts associated with these features are of the order of 0.4w
*, and could significantly increase surface concentrations from elevated sources.On assignment from the National Oceanic and Atmospheric Administration, U.S. Department of Commerce.This paper is based on a presentation made at the AMS Specialty Conference on Air Quality Modeling of the Urban Boundary Layer, in Baltimore, late 1983. 相似文献
8.
Abstract Six‐hourly surface wind analyses over the North Pacific Ocean covering the 10‐year period 1969–78 are used to describe synoptic storm activity in terms of parameters that are directly related to the atmospheric forcing of the ocean. The cube of the atmospheric friction velocity, u3 * and the curl of the surface wind stress, curl τ, are used because of their relationship to turbulent vertical mixing and Ekman pumping in the ocean, respectively. In an attempt to isolate synoptic disturbances from mean fields, the time series of surface wind components at each individual grid point are partitioned into “high‐pass” (periods shorter than 10 days) and “low‐pass” (periods longer than 10 days) components by means of conventional filtering procedures. The two quantities u3 * and curl τ are then calculated from (a) the high‐pass filtered wind components only, (b) a combination of the filtered wind components that include the interaction between the high‐ and low‐pass fields, and (c) the unfiltered wind components. These quantities describe the atmospheric forcing of the ocean that is attributable to (a) synoptic storm activity by itself, (b) synoptic storm activity in the presence of the low‐pass (mean) flow, and (c) the total spectrum of wind forcing, respectively. Maps of the long‐term (10‐year) monthly mean u3 * calculated from (a) and (b) are coherent across the mid‐latitude North Pacific and appear to coincide with the normal seasonal evolution of synoptic storm activity in that region. In mid‐latitudes, the values of u3 * calculated from (a) and (b) are 27 and 83%, respectively, of the value of u3 * itself. Thus, a major fraction of the production of turbulent energy available for mixing in the upper layers of the ocean comes from synoptic disturbances with a period shorter than 10 days. Maps of the long‐term monthly mean wind stress curl are quite different in that the mean wind stress curl calculated from (a) is essentially negligible. However, the mean curl calculated from (b) closely resembles the pattern of total curl (c), but with a magnitude of only 41% of (c). Thus, synoptic disturbances with a period shorter than 10 days are also responsible for a significant fraction of the Ekman pumping of the ocean. Future studies with these data will attempt to determine whether a relationship exists between synoptic storm activity, as measured by the parameters developed in this study, and large‐scale sea‐surface temperature anomalies. 相似文献
9.
Observational evidence for the Monin-Obukhov similarity under all stability conditions 总被引：2，自引：0，他引：2
Data collected in the surface layer in a northern suburban area of Nanjing from 15 November to 29 December 2007 were analyzed to examine the Monin-Obukhov similarity for describing the turbulent fluctu- ations of 3D winds under all stability conditions and to obtain the turbulence characteristics under different weather conditions. The results show that the dimensionless standard deviations of turbulent velocity com- ponents (σ u /u* , σ v /u* , σ w /u * ) and dimensionless turbulent kinetic energy (TKE) can be well described by "1/3" power law relationships under stable, neutral, and unstable conditions, with σ u /u * > σ v /u * > σ w /u* . Land use and land cover changes mainly impact dimensionless standard deviations of horizontal component fluctuations, but they have very little on those of the vertical component. The dimensionless standard devi- ations of wind components and dimensionless TKE are remarkably affected by different weather conditions; the deviations of horizontal wind component and dimensionless TKE present fog day > clear sky > overcast > cloudy; the trend of the vertical wind component is the reverse. The surface drag coefficient at a Nan- jing suburban measurement site during the observation period was obviously higher than at other reported plains and plateau areas, and was approximately one order larger in magnitude than the reported plains areas. Dimensionless standard deviation of temperature declined with increasing |z /L| with an approximate "-1/3" slope in unstable stratification and "-2/3" slope in stable stratification. 相似文献
10.
11.
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
12.
F. Nieuwstadt 《Boundary-Layer Meteorology》1978,14(2):235-246
A method is given to calculate the surface layer parameters: u
*
(friction velocity) and T
*
(temperature scale) from wind speed and temperature profiles.The problem is formulated as a minimization of a least-square function, which is constructed from the difference between the measured profiles and the well-known Kansas profile relations.The wind speed and temperature profiles are treated simultaneously in this procedure. All the available wind speed and temperature measurements are used in order to reduce the effect of measurement errors.Estimates of the goodness of fit and confidence limits on the estimated parameters are discussed.The method has been applied to data obtained during experiments in a wide variety of conditions: Project Prairie Grass, experiments over Lake Flevo and experiments at the meteorological tower at Cabauw, the last two in the Netherlands. 相似文献
13.
A quantative transposition model is introduced which determines hourly wind speeds in a representative tropical region (Central Sudan). The model consists of two parts. Firstly, a local boundary-layer model, based on the energy balance equation and the Businger-Dyer equations, is used to compute the average diurnal cycle of various characteristic boundary-layer parameters. Secondly, a horizontal transposition method is introduced to calculate wind speed behaviour at an arbitrary station from that at a reference station. This method is based on assumed spatial constancy of the turbulence parameter u
* in the period November–April in a region of about (700 × 800) km2 in Central Sudan. The constancy of u
* is concluded from the very stationary character of the climate. Model-computed hourly wind speeds are consistent with the potential wind speeds (at 10 m over open country) calculated from the measured data, and provide better local wind estimates than the conventional procedure which assumes constant regional hourly wind speeds. 相似文献
14.
A model is developed to simulate the potential temperature and the height of the mixed layer under advection conditions. It includes analytic expressions for the effects of mixed-layer conditions upwind of the interface between two different surfaces on the development of the mixed layer downwind from the interface. Model performance is evaluated against tethersonde data obtained on two summer days during sea breeze flow in Vancouver, Canada. It is found that the mixed-layer height and temperature over the ocean has a small but noticeable effect on the development of the mixed layer observed 10 km inland from the coast. For these two clear days, the subsidence velocity at the inversion base capping the mixed layer is estimated to be about 30 mm s–1 from late morning to late afternoon. When the effects of subsidence are included in the model, the mixed-layer height is considerably underpredicted, while the prediction for the mean potential temperature in the mixed layer is considerably improved. Good predictions for both height and temperature can be obtained when values for the heat entrainment ratio,c, 0.44 and 0.68 for these two days respectively for the period from 1000 to 1300 LAT, were used. These values are estimated using an equation including the additional effects on heat entrainment due to the mechanical mixing caused by wind shear at the top of the mixed layer and surface friction. The contribution of wind shear to entrainment was equal to, or greater than, that from buoyant convection resulting from the surface heat flux. Strong wind shear occurred near the top of the mixed layer between the lower level inland flow and the return flow aloft in the sea breeze circulation.Symbols
c
entrainment parameter for sensible heat
-
c
p
specific heat of air at constant pressure, 1010 J kg–1 K–1
-
d
1
the thickness of velocity shear at the mixed-layer top, m
-
Q
H
surface sensible heat flux, W m–2
-
u
m
mean mixed-layer wind speed, m s–1
-
u
*
friction velocity at the surface, m s–1
-
w
subsidence velocity, m s–1
-
W
subsidence warming,oC s–1
-
w
e
entrainment velocity, m s–1
-
w
*
convection velocity in the mixed layer, m s–1
-
x
downwind horizontal distance from the water-land interface, m
-
y
dummy variable forx, m
-
Z
height above the surface, m
-
Z
i
height of capping inversion, m
-
Z
m
mixed-layer depth, i.e.,Z
i–Zs, m
-
Z
s
height of the surface layer, m
-
lapse rate of potential temperature aboveZ
i, K m–1
-
potential temperature step atZ
i, K
- u
h
velocity step change at the mixed-layer top
-
m
mean mixed-layer potential temperature, K 相似文献
15.
Siri Jodha Singh Khalsa 《Boundary-Layer Meteorology》1980,19(1):3-17
The indirect dissipation technique is used to estimate 1-min averages of friction velocity u
*in the surface layer over the tropical ocean. These estimates are compared to estimates of u
*obtained using a drag coefficient and the relative difference between the two is examined in relation to stability and averaging time. Plumes and downdrafts are found to be responsible for an anomalous behavior of the drag coefficient estimates. Certain factors relating to plume properties, derived using conditional sampling as described in Khalsa (1980), are shown to be related to the variance between the two estimates of friction velocity. An investigation into the effects of increasing the averaging time reveals that plume spacing, which is dependent on stability, and the mean wind speed determine the minimum time for smoothing the influence of plumes and downdrafts.Department of Atmospheric Sciences contribution number 513. 相似文献
16.
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
17.
Mean wind speed profiles were measured by tracking radiosondes in the unstable atmospheric boundary layer (ABL) over the forested Landes region in southwestern France. New Monin-Obukhov stability correction functions, recently proposed following an, analysis by Kader and Yaglom, as well as the Businger-Dyer stability formulation were tested, with wind speeds in the surface sublayer to calculate the regional shear stress. These profile-derived shear stresses were compared with eddy correlation measurements gathered above a mature forest stand, at a location roughly, 4.5 km from the radiosonde launch site. The shear stress values obtained by means of the newly proposed stability function were in slightly better agreement with the eddy correlation values than those obtained by means of a Businger-Dyer type stability function. The general robustness of the profile method can be attributed in part to prior knowledge of the regional surface roughness (z
0=1.2 m) and the momentum displacement height (d
0=6.0 m), which were determined from neutral wind profile analysis. The 100 m drag coefficient for the unstable conditions above this broken forest surface was found to beu
*
2
/V
100
2
=0.0173. 相似文献
18.
High frequency measurements of wind velocity and temperature were made during the Ocean Storms Project in November 1987. The dissipation method was applied to the resulting time series in order to determine friction velocities,u
*, and the characteristic temperature scale,t
*, at 1-min intervals. These values were then compared to the 1-min mean wind speed and air-sea temperature differences to determine relationships for the drag coefficient (C
d
) and Stanton number (C
h
). The drag coefficient was comparable to other values reported in the literature, although the variation with wind speed was greater than reported by other investigators. An examination of the residual time series indicated a systematic low frequency periodicity of about 2-hr duration which was attributed to a fluctuating wind interacting with the surface gravity wave field. The temperature fluctuations did not produce meaningful estimates ofC
h
for stable conditions. For unstable conditions, a value of 1.09±0.02×10–3 was found. 相似文献
19.
Air-sea bulk transfer coefficients in diabatic conditions 总被引：13，自引：0，他引：13
Junsei Kondo 《Boundary-Layer Meteorology》1975,9(1):91-112
On the basis of recent data for the roughness Reynolds number of the sea surface, and using the Owen-Thomson theory on the transfers of heat and mass between a rough surface and the flow above it, the bulk transfer coefficients of the sea surface have been estimated. For a reference height of 10 m, the neutral-lapse transfer coefficient for water vapor is larger by only a few percent than that for sensible heat. When the wind speed at the 10-m height is u
10>3 m s–1, the coefficient for sensible heat C
H
is larger by about 10% than that for momentum C
D
. For u
10<5 m s–1, however, the value of C
D
exceeds the value of C
H
, and for u
10=15 m s–1 it is shown that C
H
0.8C
D
. It may be also proposed that 103
C
D
=1.11 to 1.70, 103
C
E
=1.18 to 1.30, and 103
C
H
=1.15 to 1.26 for a range of u
10=4 to 20 m s–1. A plot of diabatic transfer coefficients versus wind speed is obtained by using a parameter of the sea-air temperature difference. For practical purposes, the coefficients are approximated by empirical formulae. 相似文献
20.
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