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1.
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
2.
Summary In this paper the results of an urban measurement campaign are presented. The experiment took place from July 1995 to February
1996 in Basel, Switzerland. A total of more than 2000 undisturbed 30-minute runs of simultaneous measurements of the fluctuations
of the wind vector u′, v′, w′ and the sonic temperature θ
s
′ at three different heights (z=36, 50 and 76 m a.g.l.) are analysed with respect to the integral statistics and their spectral behaviour. Estimates of the
zero plane displacement height d calculated by the temperature variance method yield a value of 22 m for the two lower levels, which corresponds to 0.92 h
(the mean height of the roughness elements). At all three measurement heights the dimensionless standard deviation σ
w
/u
* is systematically smaller than the Monin-Obukhov similarity function for the inertial sublayer, however, deviations are smaller
compared to other urban turbulence studies. The σθ/θ* values follow the inertial sublayer prediction very close for the two lowest levels, while at the uppermost level significant
deviations are observed. Profiles of normalized velocity and temperature variances show a clear dependence on stability. The
profile of friction velocity u
* is similar to the profiles reported in other urban studies with a maximum around z/h=2.1. Spectral characteristics of the wind components in general show a clear dependence on stability and dimensionless measurement
height z/h with a shift of the spectral peak to lower frequencies as thermal stability changes from stable to unstable conditions and
as z/h decreases. Velocity spectra follow the −2/3 slope in the inertial subrange region and the ratios of spectral energy densities
S
w
(f)/S
u
(f) approach the value of 4/3 required for local isotropy in the inertial subrange. Velocity spectra and spectral peaks fit
best to the well established surface layer spectra from Kaimal et al. (1972) at the uppermost level at z/h=3.2.
Received September 26, 1997 Revised February 15, 1998 相似文献
3.
Measured spectra ofθ
x,θ
y,θ
z, the derivatives of temperature in streamwise, lateral and vertical directions, respectively, indicate that the spectral
densities ofθ
z andθ
y are nearly similar but significantly different from the spectral density ofθ
x. The high-frequency parts of the three spectra satisfy, in a qualitative sense, local isotropy requirements. In the high-frequency
end of the inertial subrange, the relative behaviour of spectra ofθ
x,θ
y andθ
z is also consistent with local isotropy. 相似文献
4.
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
5.
C. R. Wood A. Lacser J. F. Barlow A. Padhra S. E. Belcher E. Nemitz C. Helfter D. Famulari C. S. B. Grimmond 《Boundary-Layer Meteorology》2010,137(1):77-96
Flow and turbulence above urban terrain is more complex than above rural terrain, due to the different momentum and heat transfer
characteristics that are affected by the presence of buildings (e.g. pressure variations around buildings). The applicability
of similarity theory (as developed over rural terrain) is tested using observations of flow from a sonic anemometer located
at 190.3 m height in London, U.K. using about 6500 h of data. Turbulence statistics—dimensionless wind speed and temperature,
standard deviations and correlation coefficients for momentum and heat transfer—were analysed in three ways. First, turbulence
statistics were plotted as a function only of a local stability parameter z/Λ (where Λ is the local Obukhov length and z is the height above ground); the σ
i
/u
* values (i = u, v, w) for neutral conditions are 2.3, 1.85 and 1.35 respectively, similar to canonical values. Second, analysis of urban mixed-layer
formulations during daytime convective conditions over London was undertaken, showing that atmospheric turbulence at high
altitude over large cities might not behave dissimilarly from that over rural terrain. Third, correlation coefficients for
heat and momentum were analyzed with respect to local stability. The results give confidence in using the framework of local
similarity for turbulence measured over London, and perhaps other cities. However, the following caveats for our data are
worth noting: (i) the terrain is reasonably flat, (ii) building heights vary little over a large area, and (iii) the sensor
height is above the mean roughness sublayer depth. 相似文献
6.
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
7.
The parameterization of the dimensionless entrainment rate (w
e
/w
*) versus the convective Richardson number (Ri
δθ
) is discussed in the framework of a first-order jump model (FOM). A theoretical estimation for the proportionality coefficient
in this parameterization, namely, the total entrainment flux ratio, is derived. This states that the total entrainment flux
ratio in FOM can be estimated as the ratio of the entrainment zone thickness to the mixed-layer depth, a relationship that
is supported by earlier tank experiments, and suggesting that the total entrainment flux ratio should be treated as a variable.
Analyses show that the variability of the total entrainment flux ratio is actually the effect of stratification in the free
atmosphere on the entrainment process, which should be taken into account in the parameterization. Further examination of
data from tank experiments and large-eddy simulations demonstrate that the different power laws for w
e
/w
* versus Ri
δθ
can be interpreted as the variability of the total entrainment flux ratio. These results indicate that the dimensionless
entrainment rate depends not only on the convective Richardson number but also upon the total entrainment flux ratio. 相似文献
8.
The turbulent structure of the lake breeze penetration and subsequent development of the thermal internal boundary layer (TIBL) was observed using a kytoon-mounted ultrasonic anemometer-thermometer. The lake breeze penetrated with an upward rolling motion associated with the upward flow near the lake breeze front. After the lake breeze front passed, the behaviors of the velocity and temperature at the top of the lake breeze layer were similar to those found in convective boundary layers (CBL). Comparing gq/*,
u
/w
* and
w
/w
* between the present observation of TIBL development after the passage of the lake breeze front and CBL data from the literature, the /* values showed reasonable agreement; however,
u
/w
* and
w
/W* had smaller values in the TIBL than in the CBL at higher altitudes. This is due to the differences in the mean velocity profiles. While the CBL has a uniform velocity profile, the TIBL has a peak at lower elevation due to the lake breeze penetration; the velocity then decreases with height.Present address: The Institute of Behavioral Science, 1-35-7 Yoyogi, Tokyo 151, Japan. 相似文献
9.
Summary ?Evapotranspiration characteristics on the point-scale (several hundred square meters) and the local scale (several square
kilometers) are analysed by comparing a deterministic and a statistical – deterministic surface energy balance model. The
vegetation surface variability is represented by both the surface heterogeneity and inhomogeneity. Heterogeneity means the
mosaic of wet (wif ≠ 0) and dry (1-wif) fractions of vegetation surface, while inhomogeneity addresses small scale variations of soil moisture content.
The microscale characteristics of evapotranspiration are considered in terms of analysing evapotranspiration E
v
versus soil moisture content θ, relative frequency distribution characteristics of E
v
(θ) and the aggregation algorithms for its estimation. The analyses are performed for loam soil type under different atmospheric
forcing conditions.
The main result is as follows: For dry vegetation surface (wif = 0), the relationship between the aggregated (θagg) and the area-averaged (θ
m
) soil moisture content is nonlinear and depends on both the states of the surface and the atmospheric forcing conditions.
In the study, we assumed that there are no advective effects and mesoscale circulation patterns induced by surface discontinuities.
Based on this fact it seems unlikely to be able to construct an aggregation algorithm for calculating θagg without inclusion of the atmospheric forcing conditions. This means that it will be difficult to construct a simple formula
for calculating area-averaged transpiration, if it is possible at all.
Received May 3, 2001; revised May 31, 2002; accepted June 3, 2002 相似文献
10.
Flux-Variance Method for Latent Heat and Carbon Dioxide Fluxes in Unstable Conditions 总被引:4,自引:0,他引:4
Xiaofeng Guo Hongsheng Zhang Xuhui Cai Ling Kang Tong Zhu Monique Y. Leclerc 《Boundary-Layer Meteorology》2009,131(3):363-384
Applied previously to momentum and heat fluxes, the present study extends the flux-variance method to latent heat and CO2 fluxes in unstable conditions. Scalar similarity is also examined among temperature (θ), water vapour (q), and CO2 (c). Temperature is adopted as the reference scalar, leading to two feasible strategies to estimate latent heat and CO2 fluxes: the first one relies on flux-variance similarity relations for scalars, while the second is based on the parameterization
of relative transport efficiency in terms of scalar correlation coefficient and a non-dimensional quantity. The relationship
between the θ-to-q transport efficiency (λ
θ
q
) and θ-q correlation coefficient (R
θ
q
) is used to describe the intermediate hydrological conditions. We also parameterize the θ-to-c transport efficiency (λ
θ
c
) as a function of the θ-c correlation coefficient (R
θ
c
) by introducing a new non-dimensional ratio (α). The flux-variance method is a viable technique for flux gap-filling, when turbulence measurements of wind velocity are
not available. It is worth noting that the extended method is not exempt from a correction for density effects when used for
estimating water or carbon exchange. 相似文献
11.
An attempt has been made to investigate the role of vertical wind shear, corrective instability and the thermodynamic parameter (θes - θe) below the first lifting condensation level (FLCL) in the occurrence of instanta-neous premonsoon thunderstorm over Agartala (AGT) and Ranchi (RNC) at 12 GMT Radiosonde data of 1988 have been utilized here. The study has however been confined to 1000 hPa-500 hPa range at most Here the convectively unstable layers with positive vertical wind shear upto 500 hPa have been termed as ‘Fa?vourable Layers’ (FL) and the level at which an initially stable layer turns out to be convectively unstable for the first time has been termed as ‘Transition Level’ (TL). It is observed that the changes in vertical wind shear are positive at TL at the time of occurrence of thunderstorm (TS) and the corresponding change is negative on fair-weather situa?tion Moreover, the 90% confidence interval for (θes - θe) reveals that for AGT the upper layer thermodynamic characteristic is important at the time of occurrence of TS whereas for RNC, the value of (θes - θe) at the surface is much more effective 相似文献
12.
Monin-Obukhov Functions for Standard Deviations of Velocity 总被引:2,自引:2,他引:0
J. D. Wilson 《Boundary-Layer Meteorology》2008,129(3):353-369
The origins of Monin-Obukhov similarity theory (MOST) are briefly reviewed, as a context for the analysis of signals from
sonic anemometers operating in the surface layer over a Utah salt flat. At this site (over the interval of these measurements)
the neutral limit for the normalized vertical velocity standard deviation (σ
w
/u
*) deviates markedly from what has generally been regarded as the standard value (i.e. about 1.3), suggesting (since others
have also reported such deviations) that this Monin-Obukhov constant is not, in fact, universal. New (but tentative) formulae
are suggested for σ
w
and for the longitudinal standard deviation σ
u
. 相似文献
13.
Gerhard Kramm 《Boundary-Layer Meteorology》1989,48(3):315-327
The estimation of the surface-layer parameters u
* (friction velocity), * and q
* (temperature and humidity scales),
r
and q
r (temperature and humidity reference values), z
o (roughness length) and d (zero-displacement) from vertical profiles of wind velocity, temperature and humidity by least-squares methods is described. The estimation is based on the flux-gradient relationships and the constant flux assumption for the transfer of momentum, sensible heat and matter near the Earth's surface.Test calculations were carried out with the vertical profile data from the GREIV I 1974 experiment and the Great Plains Turbulence Project. 相似文献
14.
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
15.
Data collected during the SHEBA and CASES-99 field programs are employed to examine the flux–gradient relationship for wind
speed and temperature in the stably stratified boundary layer. The gradient-based and flux-based similarity functions are
assessed in terms of the Richardson number Ri and the stability parameter z/Λ*, z being height and Λ* the local Obukhov length. The resulting functions are expressed in an analytical form, which is essentially unaffected by
self-correlation, when thermal stratification is strong. Turbulence within the stably stratified boundary layer is classified
into four regimes: “nearly-neutral” (0 < z/Λ* < 0.02), “weakly-stable” (0.02 < z/Λ* < 0.6), “very-stable” (0.6 < z/Λ* < 50), and “extremely-stable” (z/Λ* > 50). The flux-based similarity functions for gradients are constant in “nearly-neutral” conditions. In the “very-stable”
regime, the dimensionless gradients are exponential, and proportional to (z/Λ*)3/5. The existence of scaling laws in “extremely-stable” conditions is doubtful. The Prandtl number Pr decreases from 0.9 in nearly-neutral conditions and to about 0.7 in the very-stable regime. The necessary condition for the
presence of steady-state turbulence is Ri < 0.7. 相似文献
16.
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
17.
The impact of the PBL scheme and the vertical distribution of model layers on simulations of Alpine foehn 总被引:1,自引:0,他引:1
Summary This paper investigates the influence of the planetary boundary-layer (PBL) parameterization and the vertical distribution
of model layers on simulations of an Alpine foehn case that was observed during the Mesoscale Alpine Programme (MAP) in autumn
1999. The study is based on the PSU/NCAR MM5 modelling system and combines five different PBL schemes with three model layer
settings, which mainly differ in the height above ground of the lowest model level (z
1). Specifically, z
1 takes values of about 7 m, 22 m and 36 m, and the experiments with z
1 = 7 m are set up such that the second model level is located at z = 36 m. To assess if the different model setups have a systematic impact on the model performance, the simulation results
are compared against wind lidar, radiosonde and surface measurements gathered along the Austrian Wipp Valley. Moreover, the
dependence of the simulated wind and temperature fields at a given height (36 m above ground) on z
1 is examined for several different regions.
Our validation results show that at least over the Wipp Valley, the dependence of the model skill on z
1 tends to be larger and more systematic than the impact of the PBL scheme. The agreement of the simulated wind field with
observations tends to benefit from moving the lowest model layer closer to the ground, which appears to be related to the
dependence of lee-side flow separation on z
1. However, the simulated 2 m-temperatures are closest to observations for the intermediate z
1 of 22 m. This is mainly related to the fact that the simulated low-level temperatures decrease systematically with decreasing
z
1 for all PBL schemes, turning a positive bias at z
1 = 36 m into a negative bias at z
1 = 7 m. The systematic z
1-dependence is also observed for the temperatures at a fixed height of 36 m, indicating a deficiency in the self-consistency
of the model results that is not related to a specific PBL formulation. Possible reasons for this deficiency are discussed
in the paper. On the other hand, a systematic z
1-dependence of the 36-m wind speed is encountered only for one out of the five PBL schemes. This turns out to be related to
an unrealistic profile of the vertical mixing coefficient.
Correspondence: Günther Z?ngl, Meteorologisches Institut der Universitat München, 80333 München, Germany 相似文献
18.
A large-eddy simulation (LES) model, using the one-equation subgrid-scale (SGS) parametrization, was developed to study the
flow and pollutant transport in and above urban street canyons. Three identical two-dimensional (2D) street canyons of unity
aspect ratio, each consisting of a ground-level area source of constant pollutant concentration, are evenly aligned in a cross-flow
in the streamwise direction x. The flow falls into the skimming flow regime. A larger computational domain is adopted to accurately resolve the turbulence
above roof level and its influence on the flow characteristics in the street canyons. The LES calculated statistics of wind
and pollutant transports agree well with other field, laboratory and modelling results available in the literature. The maximum
wind velocity standard deviations σ
i
in the streamwise (σ
u
), spanwise (σ
v
) and vertical (σ
w
) directions are located near the roof-level windward corners. Moreover, a second σ
w
peak is found at z ≈ 1.5h (h is the building height) over the street canyons. Normalizing σ
i
by the local friction velocity u
*, it is found that σ
u
/u
* ≈ 1.8, σ
v
/u
* ≈ 1.3 and σ
w
/u
* ≈ 1.25 exhibiting rather uniform values in the urban roughness sublayer. Quadrant analysis of the vertical momentum flux
u′′w′′ shows that, while the inward and outward interactions are small, the sweeps and ejections dominate the momentum transport
over the street canyons. In the x direction, the two-point correlations of velocity R
v,x
and R
w,x
drop to zero at a separation larger than h but R
u,x
(= 0.2) persists even at a separation of half the domain size. Partitioning the convective transfer coefficient Ω
T
of pollutant into its removal and re-entry components, an increasing pollutant re-entrainment from 26.3 to 43.3% in the x direction is revealed, suggesting the impact of background pollutant on the air quality in street canyons. 相似文献
19.
Factors controlling the magnitudes of, and short-term variations in, the potential temperatures of the snow surface and the
air at the height of 2 m θS and θ2 m over Arctic sea ice in winter are analysed. The study addresses the winters of 1986–1987 and 1987–1988, and is based on the
temperature, wind, and cloud observations made by Russian drifting ice stations. It also relies on the ERA40 re-analyses of
the European Centre for Medium-Range Weather Forecasts, which were utilised to calculate the lateral heat advection at the
sites of the ice stations. The cloud cover and wind speed were more important than the heat advection in controlling the magnitudes
of θ2 m and θS, while on a time scale of 24 h, during steady forcing conditions, the heat advection was the most important factor affecting
the changes in θS and θ2 m. During changing conditions, and considering individual factors separately, the monthly mean 24-h temperature changes were
less than ± 5 °C: the effect of the cloud cover was the largest, and that of the heat advection was the smallest. When simultaneous
changes in the three factors were analysed, the seasonal mean temperature changes were even of the order of ±15 °C, with the
strongest warming events exceeding 35 K in a single day. The difference θS − θ2 m reached its lowest seasonal mean values during conditions of clear skies (−1.3 °C), light winds (−1.3 °C) and warm-air advection
(−0.8 °C). θS and θ2 m followed each other closely, even during major synoptic-scale temperature variations. 相似文献
20.
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