共查询到20条相似文献，搜索用时 31 毫秒
1.
Nine profiles of the temperature structure parameter C
T
2
and the standard deviation of vertical velocity fluctuations (
w) in the convective boundary layer (CBL) were obtained with a monostatic Doppler sodar during the second intensive field campaign of the First ISLSCP Field Experiment in 1987. The results were analyzed by using local similarity theory. Local similarity curves depend on four parameters: the height of the mixed layer (z
i
), the depth of the interfacial layer (), and the temperature fluxes at the top of the mixed layer (Q
i
) and the surface (Q
o). Values of these parameters were inferred from sodar data by using the similarity curve for C
T
2
and observations at three points in its profile. The effects of entrainment processes on the profiles of C
T
2
and
wnear the top of the CBL appeared to be described well by local similarity theory. Inferred estimates of surface temperature flux, however, were underestimated in comparison to fluxes measured by eddy correlation. The measured values of
wappeared to be slightly smaller than estimates based on available parmeterizations. These discrepancies might have been caused by experimental error or, more likely, by the distortion of turbulence structure above the site by flow over the nonuniform terrain at the observation site. 相似文献
2.
F. C. Medeiros Filho D. A. R. Jayasuriya R. S. Cole C. G. Helmis D. N. Asimakopoulos 《Meteorology and Atmospheric Physics》1988,39(3-4):197-202
Summary To investigate the effect of atmospheric turbulence on microwave communication links, temperature and water vapor pressure have been measured and radio refractivity has been computed, during different meteorological conditions, in the atmospheric boundary layer of an urban site. The cospectra between temperature (T) and water vapor pressure (e) have been found to be either negative over the whole range of frequencies, or the low-frequency end of the cospectrum is of opposite sign relative to higher frequency end. In both cases cospectra follow a–5/3 law in the inertial subrange, in agreement with the theoretical predictions. The coherence spectra clearly show that the temperature and humidity fluctuations are highly coherent within the inertial subrange under both convective and stable conditions. The relative contribution ofC
T
2
,C
eT
andC
e
2
to the real refractive index structure parameterC
n
2
is examined and discussed.With 4 Figures 相似文献
3.
Turbulence in the nocturnal boundary layer(NBL) is still not well characterized, especially over complex underlying surfaces. Herein, gradient tower data and eddy covariance data collected by the Beijing 325-m tower were used to better understand the differentiating characteristics of turbulence regimes and vertical turbulence structure of urban the NBL. As for heights above the urban canopy layer(UCL), the relationship between turbulence velocity scale(VTKE) and wind speed(V) was con... 相似文献
4.
T. Maitani 《Boundary-Layer Meteorology》1979,17(2):213-222
Turbulence statistics, including higher order moments, in the surface layer over plant canopies were compared with those observed over several different surfaces, using a nondimensional height (z – d)/z
0: The values of (z – d)/z
0extend over a very wide range from 10 over plant canopies to 107 over the ocean. Several properties such as intensities of turbulence and skewness factors show a remarkable height-dependency in the air layer below (z – d)/z
0 = 102, which is supposed to be much influenced by the underlying surface. In that layer, some peculiar phenomena, such as a downward energy transport and positive flux of shear stress, are frequently observed. 相似文献
5.
Turbulence Structure of the Unstable Atmospheric Surface Layer and Transition to the Outer Layer 总被引：4，自引：4，他引：0
K.G. McNaughton 《Boundary-Layer Meteorology》2004,112(2):199-221
We present a new model of the structure of turbulence in the unstable atmospheric surface layer, and of the structural transition between this and the outer layer. The archetypal element of wall-bounded shear turbulence is the Theodorsen ejection amplifier (TEA) structure, in which an initial ejection of air from near the ground into an ideal laminar and logarithmic flow induces vortical motion about a hairpin-shaped core, which then creates a second ejection that is similar to, but larger than, the first. A series of TEA structures form a TEA cascade. In real turbulent flows TEA structures occur in distorted forms as TEA-like (TEAL) structures. Distortion terminates many TEAL cascades and only the best-formed TEAL structures initiate new cycles. In an extended log layer the resulting shear turbulence is a complex, self-organizing, dissipative system exhibiting self-similar behaviour under inner scaling. Spectral results show that this structure is insensitive to instability. This is contrary to the fundamental hypothesis of Monin--Obukhov similarity theory. All TEAL cascades terminate at the top of the surface layer where they encounter, and are severely distorted by, powerful eddies of similar size from the outer layer. These eddies are products of the breakdown of the large eddies produced by buoyancy in the outer layer. When the outer layer is much deeper than the surface layer the interacting eddies are from the inertial subrange of the outer Richardson cascade. The scale height of the surface layer, z
s, is then found by matching the powers delivered to the creation of emerging TEAL structures to the power passing down the Richardson cascade in the outer layer. It is z
s = u
*
3
/ks, where u
* is friction velocity, k is the von Kármán constant and s is the rate of dissipation of turbulence kinetic energy in the outer layer immediately above the surface layer. This height is comparable to the Obukhov length in the fully convective boundary layer. Aircraft and tower observations confirm a strong qualitative change in the structure of the turbulence at about that height. The tallest eddies within the surface layer have height z
s, so z
s is a new basis parameter for similarity models of the surface layer. 相似文献
6.
The structure-function parametersC
T
2
andC
v
2
of temperature and velocity, respectively, from the 1973 Minnesota experiments and from large-eddy and direct numerical simulations show a smooth transition from M–O similarity to the local scaling hypothesized by Nieuwstadt for the outer regions of the stable boundary layer. Under that hypothesis, turbulence statistics aloft depend on the local vertical fluxes of momentum and temperature, so these results suggest that remote-sensing measurements ofC
T
2
andC
v
2
could be used to infer vertical profiles of those fluxes. We argue that the sensitivity of the fluxes to unsteadiness, baroclinity, terrain slope, and breaking gravity waves precludes the universality of the vertical profiles of structure-function parameters in the stable PBL. We find that theC
T
2
profile is particularly sensitive to these effects, which is consistent with observations that it varies considerably from case to case. 相似文献
7.
P. K. Pasricha R. Singh S. K. Sarkar H. N. Dutta B. M. Reddy P. K. Das 《Boundary-Layer Meteorology》1991,57(3):207-217
This paper presents meteorological measurements made during the antarctic summer period, on two 9 m and 3 m towers, on the rocky and ice shelf terrains of the Indian antarctic stations Maitri and Dakshin Gangotri, respectively. The measurements of fluctuations in temperature and wind speed made with relatively lesser precision instrumentation pertain to smaller wave numbers ~10-2 m-1 appropriate to outer scale L
0 of the atmospheric turbulence spectrum. Autocorrelation analysis of the fluctuations in temperature and wind speed has been performed. A new autoregressive scheme has been developed to represent the computed autocorrelation functions by a Yule statistical model, and to estimate the correlation period T
0 of the turbulent medium. Height profiles of outer scale L
0 of turbulence may be given in terms of T
0 and mean wind speed u. Further, the similarity theory of Monin-Obukhov has been used to compute height profiles of temperature structure parameter C
T
2. At Maitri, values of L
0 and C
T
2 are higher between 03–22 h local time than between 22–03 h. Values of L
0 and C
T
2 are smaller over the ice shelf terrain of the Dakshin Gangotri station, compared to those over the rocky terrain of the Maitri station. 相似文献
8.
Measurements of the humidity structure function parameters,C
q
2and C
Tq,over the ocean 总被引：1，自引：0，他引：1
Atmospheric turbulence measurements, including temperature and humidity fluctuations, were made from the R/V Acania off the coast of California in June, 1979. The purpose of the experiment was to investigate the scaling properties of the humidity structure function parameter (C
q
2) and temperaturehumidity cospectrum structure parameter (C
Tq) in the marine surface layer. The bulk parameterization method was used to obtain Monin-Obukhov Similarity (MOS) scaling parameters u
*, T
*, q
*and L. Assuming a neutral stability humidity drag coefficient c
qn = 1.3 × 10-3the dimensionless humidity structure function parameter C
q
2Z2/3/q*
2was found to be 18% lower than the corresponding temperature function obtained by Wyngaard et al. (1971). Furthermore, the measurements indicate that the temperature-humidity fluctuations are highly coherent well into the inertial subrange. The results have direct application to turbulent scattering of waves propagating in the atmosphere (particularly microwaves) and methods of estimating air-sea surface fluxes. 相似文献
9.
T. J. Moulsley D. N. Asimakopoulos R. S. Cole S. J. Caughey B. A. Crease 《Boundary-Layer Meteorology》1982,23(3):307-315
The comparison of C
infT
sup2
estimates in the atmospheric boundary layer, from spectral and differential temperature (T) measurements, is discussed. Measurements of C
infT
sup2
using these two methods are compared and the differences between the two are shown to be due to low-frequency enhancement of the T spectrum. Possible explanations for this effect are considered and attention is drawn to the significance of the resulting errors in boundary-layer turbulence measurements.Now at Department of Electrical and Electronic Engineering, Portsmouth Polytechnic, Anglesea Road, Portsmouth, U.K.Now at Department of Meteorology, University of Athens, Greece. 相似文献
10.
C. W. Fairall Ralph Markson G. E. Schacher K. L. Davidson 《Boundary-Layer Meteorology》1980,19(4):453-469
The dissipation rate of turbulent kinetic energy, , and the temperature structure function parameter, C
T
2, have been measured over water from the near surface (Z = 3 m) to the top of the boundary layer. The near surface values of and C
T
2 were used to calculate the velocity and temperature Monin-Obukhov scaling parameters u
* and T
*. The data collected during unstable lapse rates were used to evaluate the feasibility of extrapolating the values of and C
T
2 as a function of height with empirical scaling formulae. The dissipation rate scaling formula of Wyngaard et al. (l971 a) gave a good fit to an average of the data for Z < 0.8 Z
i. In the surface layer the scaling formula of Wyngaard et al. (1971b) disagreed with the C
T
2 values by as much as 50%. This disagreement is due to an unexpected reduction in the measured values of C
T
2 forZ < 30 m. At this point it is not clear if the discrepancy is a unique property of the marine boundary layer or if it is simply some unknown instrumental or analytical problem. The mixed layer scaling results were similar to the overland results of Kaimal et al. (1976). 相似文献
11.
Xiaofeng Guo Kun Yang Long Zhao Wei Yang Shenghai Li Meilin Zhu Tandong Yao Yingying Chen 《Boundary-Layer Meteorology》2011,139(2):307-332
We present a field investigation over a melting valley glacier on the Tibetan Plateau. In the ablation zone, aerodynamic roughness
lengths (z
0M
) vary on the order of 10−4–10−2 m, whose evolution corresponds to three melt phases with distinct surface cover and moisture exchange: snow (sublimation/evaporation),
bare ice (deposition/condensation), and ice hummocks (sublimation/evaporation). Bowen-ratio similarity is validated in the
stably stratified katabatic winds, which suggests a useful means for data quality check. A roughness sublayer is regarded
as irrelevant to the present ablation season, because selected characteristics of scalar turbulence over smooth snow are quite
similar to those over hummocky ice. We evaluate three parametrizations of the scalar roughness lengths (z
0T
for temperature and z
0q
for humidity), viz. key factors for the accurate estimation of sensible heat and latent heat fluxes using the bulk aerodynamic
method. The first approach is based on surface-renewal models and has been widely applied in glaciated areas; the second has
never received application over an ice/snow surface, despite its validity in (semi-)arid regions; the third, a derivative
of the first, is proposed for use specifically over rough ice defined as z
0M
> 10−3 m or so. This empirical z
0M
threshold value is deemed of general relevance to glaciated areas (e.g. ice sheet/cap and valley/outlet glaciers), above
which the first approach gives notably underestimated z
0T,q
. The first and the third approaches tend to underestimate and overestimate turbulent heat/moisture exchange, respectively,
frequently leading to relative errors higher than 30%. Comparatively, the second approach produces fairly low errors in energy
flux estimates both in individual melt phases and over the whole ablation season; it thus emerges as a practically useful
choice to parametrize z
0T,q
in glaciated areas. Moreover, we find all three candidate parametrizations unable to predict diurnal variations in the excess
resistances to humidity transfer, thus encouraging more efforts for improvement. 相似文献
12.
Temperature variance and temperature power spectra in the unstable surface layer have always presented a problem to the standard
Monin-Obukhov similarity model. Recently that problem has intensified with the demonstration by Smedman et al. (2007, Q J
Roy Meteorol Soc 133: 37–51) that temperature spectra and heat-flux cospectra can have two distinct peaks in slightly unstable
conditions, and by McNaughton et al. (2007, Nonlinear Process Geophys 14: 257–271) who showed that the wavenumber of the peak
of temperature spectra in a convective boundary layer (CBL), closely above the surface friction layer (SFL), can be sensitive
to the CBL depth, z
i. Neither the two-peak form at slight instability nor the dependence of peak position on z
i at large instability is compatible with the Monin-Obukhov model. Here we examine the properties of temperature spectra and
heat-flux cospectra from between these extremes, i.e. from within the unstable SFL, in two experiments. The analysis is based
on McNaughton’s model of the turbulence structure in the SFL. According to this model, heat is transported through most of
the SFL by sheet plumes, created by the action of impinging outer eddies. The smallest and most effective of these outer eddies
have sizes that scale on SFL depth, z
s. The z
s-scale eddies and plumes are organised within the overall convection pattern in the CBL, and in turn they organise the motion
of smaller eddies within the SFL, whose sizes scale on height, z. The main experimental results are: (1) the peak amplitudes of the temperature spectra in the SFL are collapsed with a scaling
factor (zsz)1/3eo2/3{(z_{\rm s}z)^{1/3}\varepsilon_{\rm o}^{2/3}} divided by the square of the surface temperature flux, where eo{\varepsilon_{\rm o}} is the dissipation rate of turbulent energy in the outer CBL (above the SFL); (2) the peak wavenumbers of the temperature
spectra are collapsed with the mixed length scale (z
i
z
s)1/2; (3) the peak wavenumbers of the heat-flux cospectra are collapsed with the doubly-mixed length scale (z
i
z
s)1/4
z
1/2; (4) for z/z
s < 0.03, the peak in the cospectrum is replaced by another peak at a wavenumber about a magnitude larger. This peak’s position
scales on z; (5) all these findings are consistent with the observations of Smedman et al. 相似文献
13.
In scintillometry Monin–Obukhov similarity theory (MOST) is used to calculate the surface sensible heat flux from the structure parameter of temperature (CT2){(C_{T^2})} . In order to prevent saturation a scintillometer can be installed at an elevated level. However, in that case the observation level might be located outside the atmospheric surface layer (ASL) and thus the validity of MOST questioned. Therefore, we examine two concepts to determine the turbulent surface sensible heat flux from the structure parameter at elevated levels with data obtained at 60-m height on the Cabauw tower (the Netherlands). In the first concept (MOSTs) CT2{C_{T^2}} is still scaled with the surface flux, whereas in the second (MOSTl) CT2{C_{T^2}} is scaled with the local sensible heat flux. The CT2{C_{T^2}} obtained from both concepts is compared with direct observations of CT2{C_{T^2}} using a sonic anemometer/thermometer. In the afternoon (when the measurement height is located within the ASL) both concepts give results that are comparable to the directly observed values of CT2{C_{T^2}} . In the morning (data outside the ASL), our data do not unequivocally support either of the two concepts. First, the peak in CT2{C_{T^2}} that occurs when the measurement height is located in the entrainment zone disqualifies the use of MOST. Second, during the morning transition, local scaling shows the correct pattern (zero flux and a minimum in CT2{C_{T^2}}) but underestimates CT2{C_{T^2}} by a factor of ten. Third, from the best linear fit a we found that the slope of MOSTl gave better results, whereas the offset is closer to zero for MOSTs. Further, the correlation between the direct observations and MOST-scaled results is low and similar for the two concepts. In the end, we conclude that MOST is not applicable for the morning hours when the observation level is above the ASL. 相似文献
14.
A numerical model of convective heat transfer due to isolated thermals in the atmospheric boundary layer is used to describe the temperature profile transformation in undisturbed conditions as a result of intensive dry free convection. Based on some assumptions, the heat transfer Equation (2) is transformed to the form (14) in which the coefficients and the function F are expressed by (d/dz)(ln ) and by parameters of thermals. Equation (14) has been solved numerically with the help of Equation (15) obtained from the statics equation because of Equation (8). The size distribution function f(z, r, t) of the thermals is discrete (Table I), according to Vulf'son (1961). On Figures 1 and 2 are plotted successive temperature profiles for a ground inversion, transformed due to free convection and turbulence (Figures 1a and 2a), and due to turbulence only (Figures 1b and 2b). The profiles are computed from Equation 14 (Figures 1a and 2a) and Equation 16 (Figures 1b and 2b) for k
z= 1 m2 s–1 (Figure 1) and k
z= 10 m2 s–1 (Figure 2). On Figure 3 the real temperature profiles in Sofia for June 22nd 1976 are compared with the profiles computed using the real initial profile for 4.30 h local time. Good qualitative agreement can be seen. 相似文献
15.
Experimental data of C
T
2, determined during various experiments in the surface layer, are compared with several functions giving the stability dependence of the temperature structure parameter. The universal function of the dimensionless temperature gradient by Skeib (1980) follows very well the experimental data and the empirical function by Wyngaard et al. (1971). This function can be used in an inertial-dissipation method. 相似文献
16.
Anitha Kumari Hegde R. Venkatesan C. V. Srinivas K. M. Balakrishna 《Meteorology and Atmospheric Physics》2010,107(3-4):161-172
Atmospheric surface layer meteorological observations obtained from 20-m-high meteorological tower at Mangalore, situated along the west coast of India are used to estimate the surface layer scaling parameters of roughness length (z o) and drag coefficient (C D), surface layer fluxes of sensible heat and momentum. These parameters are computed using the simple flux–profile relationships under the framework of Monin–Obukhov (M–O) similarity theory. The estimated values of z o are higher (1.35–1.54 m) than the values reported in the literature (>0.4–0.9 m) probably due to the undulating topography surrounding the location. The magnitude of C D is high for low wind speed (<1.5 m s?1) and found to be in the range 0.005–0.03. The variations of sensible heat fluxes (SHF) and momentum fluxes are also discussed. Relatively high fluxes of heat and momentum are observed during typical days on 26–27 February 2004 and 10–11 April 2004 due to the daytime unstable atmospheric conditions. Stable or near neutral conditions prevail after 1700 h IST with negative SHF. A mesoscale model PSU/NCAR MM5 is run using a high-resolution (1 km) grid over the study region to examine the influence of complex topography on the surface layer parameters and the simulated fluxes are compared with estimated values. Spatial variations of the frictional velocity (u *), C D, surface fluxes, planetary boundary layer (PBL) height and surface winds are noticed according to the topographic variations in the simulation. 相似文献
17.
Measurements of temperature and velocity microstructure near and downstream of a shallow seamount are used to compare fossil turbulence versus non-fossil turbulence models for the evolution of turbulence microstructure patches in the stratified ocean. According to non-fossil oceanic turbulence models, all overturn length scales LT of the microstructure grow and collapse in constant proportion to each other and to the turbulence energy (Oboukov) scale LO and the inertial buoyancy (Ozmidov) scale
of the patches; that is, with LTrms ≈1.2LR and viscous dissipation rate ≈ 0*. According to the Gibson fossil turbulence model, all microstructure originates from completely active turbulence with 0 ≈ 3LT2N3(≈ 280*) and LT/√6 ≈ LTrms, but this rapidly decays into a more persistent active-fossil state with 0F ≈ 30vN2, where N is the buoyancy frequency and v is the kinematic viscosity and, without further energy supply, finally reaches a completely fossil turbulence hydrodynamic state of internal wave motions, with F. The last turbulence eddies, with ≈ F, vanish at a buoyant-inertial-viscous (fossil Kolmogorov) scale LKF that is much smaller than the remnant overturn scales LT for large 0/F ratios. These density, temperature, and salinity overturns with LT ≈ 0.6 LR0 0.6 LR persist as turbulence fossils (by retaining the memory of o) and collapse very slowly. In the near wake below the summit depth of Ampere seamount, a much larger proportion of completely active turbulence patches was found than is usually found in the ocean interior away from sources. Dissipation rates and turbulence activity coefficients
of microstructure patches were found to decrease downstream, suggesting that the active turbulence indicated by the patches with AT 1 was caused by the presence of the seamount as a turbulence source. Therefore, the turbulence and mixing processes of ocean layers far away from turbulence sources probably have been undersampled by microstructure data sets lacking any AT 1 patches. This is because large fractions of the mixing and viscous dissipation of the patches occur in short-lived active turbulence regimes that are too brief to be detected. Consequently, large underestimates of the true space-time average turbulence fluxes and turbulence and scalar dissipation rates may result if non-fossil turbulence models are assumed in ocean microstructure data interpretation. 相似文献
18.
C
T
2measurements taken over a desert in stable conditions indicate that the atmosphere remains intermittently turbulent for Ri numbers as high as 10. This is in contrast to previous results which suggest that the atmosphere is essentially nonturbulent for Ri > 2. These measurements also indicate that time-averaged C
T
2measurements do not scale with the time-averaged mean Ri number in very stable conditions. However, the standard deviation of log10
C
T
2does appear to scale with Ri. 相似文献
19.
Sea salt spray contamination of a temperature sensor induces erroneous fluctuations due to the latent heat of evaporating or condensing water vapour associated with changes in humidity. A method is derived to correct this humidity sensitivity using data from a fast response humidiometer. When applied to aircraft measurements of turbulence in the mixed layer over the Coral Sea, spurious high frequency temperature variance is significantly corrected. Spatial separation of the thermometer and the humidiometer and the sampling rate limit the effectiveness of the method. The vertical profile of the structure function parameter C
T
2, determined from the corrected data, agrees well with results from other studies in a marine environment, but differences with measurements made over land remain. 相似文献
20.
K. G. Mcnaughton 《Boundary-Layer Meteorology》2006,118(1):83-107
We present a new account of the kinetic energy budget within an unstable atmospheric surface layer (ASL) beneath a convective
outer layer. It is based on the structural model of turbulence introduced by McNaughton (Boundary-Layer Meteorology, 112:
199–221, 2004). In this model the turbulence is described as a self-organizing system with a highly organized structure that
resists change by instability. This system is driven from above, with both the mean motion and the large-scale convective
motions of the outer layer creating shear across the surface layer. The outer convective motions thus modulate the turbulence
processes in the surface layer, causing variable downwards fluxes of momentum and kinetic energy. The variable components
of the momentum flux sum to zero, but the associated energy divergence is cumulative, increasing both the average kinetic
energy of the turbulence in the surface layer and the rate at which that energy is dissipated. The tendency of buoyancy to
preferentially enhance the vertical motions is opposed by pressure reaction forces, so pressure production, which is the work
done against these reaction forces, exactly equals buoyant production of kinetic energy. The pressure potential energy that
is produced is then redistributed throughout the layer through many conversions, back and forth, between pressure potential
and kinetic energy with zero sums. These exchanges generally increase the kinetic energy of the turbulence, the rate at which
turbulence transfers momentum and the rate at which it dissipates energy, but does not alter its overall structure. In this
model the velocity scale for turbulent transport processes in the surface layer is (kzɛ)1/3 rather than the friction velocity, u*. Here k is the von Kármán constant, z is observation height, ɛ is the dissipation rate. The model agrees very well with published experimental results, and provides
the foundation for the new similarity model of the unstable ASL, replacing the older Monin–Obukhov similarity theory, whose
assumptions are no longer tenable. 相似文献