首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 781 毫秒
1.
An atmospheric surface-layer (ASL) experiment conducted at a meteorological site in the Oostelijk-Flevoland polder of the Netherlands is described. Turbulent fluctuations of wind velocity, air temperature and static pressure were measured, using three 10 m towers.Simultaneous turbulent signals at several heights on the towers were used to investigate the properties of the turbulent structures which contribute most significantly to the turbulent vertical transports in the unstable ASL. These turbulent structures produce between 30 and 50% of the mean turbulent vertical transport of horizontal alongwind momentum and they contribute to between 40 and 50% of the mean turbulent vertical heat transport; in both cases this occurs during 15 to 20% of the total observation time.The translation speed of the turbulent structures equals the wind speed averaged over the depth of the ASL, which scales on the surface friction velocity. The inclination angle of the temperature interface at the upstream edge of the turbulent structures to the surface is significantly smaller than that of the internal shear layer, which is associated with the temperature interface. The turbulent structures in the unstable ASL are determined by a large-scale temperature field: Convective motions, which encompass the whole depth of the planetary boundary layer (PBL), penetrate into the ASL. The curvature of the vertical profile of mean horizontal alongwind velocity forces the alignment of the convective cells in the flow direction (Kuettner, 1971), which have an average length of several hundreds of metres and an average width of a few tens of metres. This mechanism leads to the formation of turbulent structures, which extend throughout the depth of the ASL.  相似文献   

2.
大气对流边界层中热力湍流过程的研究   总被引：5，自引：1，他引：5  
周明煜  吕乃平 《气象学报》1987,45(4):451-458
本文利用实测资料讨论了对流边界层内的湍流结构,重点分析复盖递温层附近风速和温度的方差以及垂直速度三阶矩的垂直分布。此外,还讨论了对流边界层顶部附近温度的垂直分布,得出温度梯度随高度分布呈—2次幂关系。  相似文献   

3.
Observations of wind velocity and air temperature fluctuations were made in the nocturnal surface inversion layer over a sorghum field. Wave-like fluctuations of temperature and wind velocity with a period of 15–20 min were observed for about 2 hours, 3 to 5 hours before sunrise. Wave-like fluctuations of temperature were observed in the air layer above and within a plant canopy and were most noticeable at the top of the plant canopy. Spectral analysis of temperature and wind velocity fluctuations reveals a separation of energy into wave-like and turbulent fluctuations. Cospectral analysis shows that for both momentum and heat, vertical transports are partitioned almost equally in the frequency ranges characteristic of wave-like and turbulent fluctuations. This suggests wave- turbulence interactions at low frequencies in the air layer near a plant canopy.  相似文献   

4.
An observational analysis of the structures and characteristics of a windy atmospheric boundary layer during a cold air outbreak in the South China Sea region is reported in this paper. It is found that the main structures and characteristics are the same as during strong wind episodes with cold air outbreaks on land. The high frequency turbulent fluctuations(period<1 min) are nearly random and isotropic with weak coherency, but the gusty wind disturbances(1 min相似文献   

5.
The turbulence data measured at two levels (i.e., 8.7 and 2.7?m) in the Energy Balance Experiment (EBEX), which was conducted in San Joaquin Valley in California during the period from July 20 to August 24, 2000, are used to study the characteristics of coherent structures over an irrigated cotton field. Patch-to-patch irrigation in the field generated the dry-to-wet horizontal advection and the oasis effects, leading to the development of a stably internal boundary layer (SIBL) in the late mornings or the early afternoons. The SIBL persisted in the rest of the afternoons. Under this circumstance, a near-neutral atmospheric surface layer (ASL) developed during the period with a stratification transition from the unstable to stable conditions during the daytime. Therefore, EBEX provides us with unique datasets to investigate the features of coherent structures that were generated over the patches upstream and passed by our site in the unstable ASL, the near-neutral ASL, and the SIBL. We use an objective detection technique and the conditional average method that is developed based on the wavelet analysis. Our data reveal some consistencies and inconsistencies in the characteristics of coherent structures as compared with previous studies. Ramp-like structures and sweep?Cejection cycles under the daytime SIBL have similar patterns to those under the nocturnal stable ASL. However, some features (i.e., intermittence) are different from those under the nocturnal stable ASL. Under the three stratifications, thermal and mechanical factors in the ASL perform differently in affecting the ramp intensity for different quantities (i.e., velocity components, temperature, and specific humidity), leading to coherent structures that modulate turbulence flow and alter turbulent transfer differently. It is also found that coherent structures contribute about 10?C20% to the total fluxes in our case with different flux contributions under three stratifications and with higher transporting efficiency in sensible heat flux than latent heat and momentum fluxes.  相似文献   

6.
Large-scale turbulence structures in the near-neutral atmospheric boundary layer (ABL) are investigated on the basis of observations made from the 213-m tall meteorological tower at Tsukuba, Japan. Vertical profiles of wind speed and turbulent fluxes in the ABL were obtained with sonic anemometer-thermometers at six levels of the tower. From the archived data, 31 near-neutral cases are selected for the analysis of turbulence structures. For the typical case, event detection by the integral wavelet transform with a large time scale (180 s) from the streamwise velocity component (u) at the highest level (200 m) reveals a descending high-speed structure with a time scale of approximately 100 s (a spatial scale of 1 km at the 200-m height). By applying the wavelet transform to the u velocity component at each level, the intermittent appearance of large-scale high-speed structures extending also in the vertical is detected. These structures usually make a large contribution to the downward momentum transfer and induce the enhancement of turbulent kinetic energy. This behaviour is like that of “active” turbulent motions. From the analysis of the two-point space–time correlation of wavelet coefficients for the u velocity component, the vertical extent and the downward influence of large-scale structures are examined. Large fluctuations in the large-scale range (wavelet variance at the selected time scale) at the 200-m level tend to induce the large correlation between the higher and lower levels.  相似文献   

7.
Large-eddy simulation of turbulent flow above and within a forest   总被引：10，自引：22，他引：10  
A large-eddy simulation has been performed of an atmospheric surface layer in which the lower third of the domain is occupied by a drag layer and heat sources to represent a forest. Subgridscale processes are treated using second-order closure techniques. Lateral boundaries are periodic, while the upper boundary is a frictionless fixed lid. Mean vertical profiles of wind velocity derived from the output are realistic in their shape and response to forest density. Similarly, vertical profiles of Reynolds stress, turbulent kinetic energy and velocity skewness match observations, at least in a qualitative sense. The limited vertical extent of the domain and the artificial upper boundary, however, cause some departures from measured turbulence profiles in real forests. Instantaneous turbulent velocity and scalar fields are presented which show some of the features obtained by tower instrumentation in the field and in wind tunnels, such as the vertical coherence of vertical velocity and the slope of structures revealed by temperature patterns.  相似文献   

8.
Higher-order moments, minima and maxima of turbulent temperature and water vapour mixing ratio probability density functions measured with an eddy-covariance system near the ground were related to each other and to vertical boundary-layer profiles of the same scalars obtained through airborne soundings. The dependence of kurtosis on squared skewness showed a kurtosis intercept below the Gaussian expectation, suggesting a compression of the probability density function by the presence of natural boundaries. This hypothesis was corroborated by comparing actual minima and maxima of turbulent fluctuations to estimates obtained from the first four sample moments by fitting a four-parameter beta distribution. The most sharply defined boundaries were found for the minima of temperature datasets during the day, indicating that negative temperature fluctuations at the sensor are limited by the availability of lower temperatures in the boundary layer. By comparison to vertical profiles, it could be verified that the turbulent minimum of temperature near the ground is close to the minimum of potential temperature in the boundary layer. The turbulent minimum of water vapour mixing ratio was found to be equal to the mixing ratio at a height above the minimum of the temperature profile. This height roughly agrees with the top of the non-local unstable domain according to bulk Richardson number profiles. We conclude that turbulence statistics measured near the surface cannot be solely explained by local effects, but contain information about the whole boundary layer including the entrainment zone.  相似文献   

9.
A collaborative experimental effort employing the minimally perturbed atmospheric surface-layer flow over the salt playa of western Utah has enabled us to map coherence in turbulent boundary layers at very high Reynolds numbers, \({Re_{\tau}\sim\mathcal{O}(10^6)}\) . It is found that the large-scale coherence noted in the logarithmic region of laboratory-scale boundary layers are also present in the very high Reynolds number atmospheric surface layer (ASL). In the ASL these features tend to scale on outer variables (approaching the kilometre scale in the streamwise direction for the present study). The mean statistics and two-point correlation map show that the surface layer under neutrally buoyant conditions behaves similarly to the canonical boundary layer. Linear stochastic estimation of the three-dimensional correlation map indicates that the low momentum fluid in the streamwise direction is accompanied by counter-rotating roll modes across the span of the flow. Instantaneous flow fields confirm the inferences made from the linear stochastic estimations. It is further shown that vortical structures aligned in the streamwise direction are present in the surface layer, and bear attributes that resemble the hairpin vortex features found in laboratory flows. Ramp-like high shear zones that contribute significantly to the Reynolds shear-stress are also present in the ASL in a form nearly identical to that found in laboratory flows. Overall, the present findings serve to draw useful connections between the vast number of observations made in the laboratory and in the atmosphere.  相似文献   

10.
An examination of boundary-layer meteorological and avian aerodynamic theories suggests that soaring birds can be used to measure the magnitude of vertical air motions within the boundary layer. These theories are applied to obtain mixed-layer normalized thermal updraft intensity over both flat and complex terrain from the climb rates of soaring American white pelicans and from diagnostic boundary-layer model-produced estimates of the boundary-layer depth zi and the convective velocity scale w*. Comparison of the flatland data with the profiles of normalized updraft velocity obtained from previous studies reveals that the pelican-derived measurements of thermal updraft intensity are in close agreement with those obtained using traditional research aircraft and large eddy simulation (LES) in the height range of 0.2 to 0.8 zi. Given the success of this method, the profiles of thermal vertical velocity over the flatland and the nearby mountains are compared. This comparison shows that these profiles are statistically indistinguishable over this height range, indicating that the profile for thermal updraft intensity varies little over this sample of complex terrain. These observations support the findings of a recent LES study that explored the turbulent structure of the boundary layer using a range of terrain specifications. For terrain similar in scale to that encountered in this study, results of the LES suggest that the terrain caused less than an 11% variation in the standard deviation of vertical velocity.  相似文献   

11.
A flow situation over coastal waters of the Baltic Sea is studied. The boundary layer was characterized by stable stratification and the presence of a pronounced low level jet at very low height, 30–150 m, above the surface of the sea. The atmospheric surface layer was apparently extremely shallow; thus the non-dimensional wind gradients and temperature gradients derived from measurements at 8 m do not show adherence to Monin-Obukhov similarity, in sharp contrast to findings from the same site at similar stability conditions but with no low level jet. Instead these quantities are shown to be governed by scales characteristic of stable shear flow away from the surface. The height to the jet centre appears to be an important quantity. Thus, for the cases with the lowest jet height values (30–50 m), some turbulent characteristics of the flow (non-dimensional velocity standard deviations and the correlation between the longitudinal and vertical velocity) have values similar to those found for the zero pressure-gradient laboratory boundary layer over a flat plate (the so called canonical boundary layer) rather than the typical values found in atmospheric boundary-layer flow. It was inferred that the large scale fluctuations known as inactive turbulence, as well as gravity waves, were suppressed in this case.  相似文献   

12.
Effects of wind on quasi-steady, shallow convection in the Martian boundary layer are studied using a large-eddy simulation model. Convection in the model is generated by the radiative flux divergence and the strength of the surface heat flux, which do not vary in time. The resulting convective boundary layer exhibits transient, irregular, horizontal cellular structures, transported by wind, and a lack of well-pronounced regular horizontal rolls, observed for analogous conditions on Earth. The dimensionless statistics of turbulence are generally similar to those generated in the windless conditions, and depend on the ratio F, defined in terms of the integrated radiative and turbulent heating rates in the boundary layer. The simulations show that variations of the radiative heating influence the temperature statistics, while their effects on the wind velocity are relatively small. The horizontal velocity variances do not show a strong dependence on parameter F, in contrast with the vertical velocity variances, which are strongly dependent on F.  相似文献   

13.
Surface-layer convection is investigated by analyzing multi-point measurements of temperature and velocity fluctuations at different sets of spatial points.The visual analysis of temperature and velocity fluctuations measured by sensors mounted on a mast of 36-m height clearly reveals the presence of large-scale convective cells (known as ramp structures) making large contributions to the heat transfer from the ground to lower atmosphere. The vertical temperature variability is described with the aid of empirical orthogonal functions derived from temperature covariance matrices for the heights of 1, 2, 5, 10, 18 and 36 m. Temporal-spatial correlation functions obtained allow estimates of a characteristic velocity scale, which may be interpreted as the downwind velocity of ramp structures.  相似文献   

14.
以壁面脉冲扰动来构建湍流边界层近壁区对称与非对称相干结构理论模式,采用直接数值模拟的方法,研究了湍流边界层近壁区对称与非对称相干结构形成的理论机制和演化规律.计算结果显示非对称相干结构比对称相干结构更容易增长,这是由于非对称型结构能在湍流边界层近壁区域诱导产生较大的展向扰动速度,且不同初始结构类型分布对湍流边界层近壁区相干结构形成的动力学机制不同所致,从而加深了对近壁湍流边界层单个相干结构形成的某种动力学机制的认识理解.  相似文献   

15.
We present a Lagrangian stochastic model of vertical dispersion in the convective boundary layer (CBL). This model is based on a generalized Langevin equation that uses the simplifying assumption that the skewed vertical velocity probability distribution is spatially homogeneous. This approach has been shown to account for two key properties of CBL turbulence associated with large-scale coherent turbulent structures: skewed vertical velocity distributions and long velocity correlation time. A 'linear-skewed' form of the generalized Langevin equation is used, which has a linear (in velocity) deterministic acceleration and a skewed random acceleration. 'Reflection' boundary conditions for selecting a new velocity for a particle that encounters a boundary were investigated, including alternatives to the standard assumption that the magnitudes of the particle incident and reflected velocities are positively correlated. Model simulations were tested using cases for which exact, analytic statistical properties of particle velocity and position are known, i.e., well-mixed spatial and velocity distributions. Simulations of laboratory experiments of CBL dispersion show that (1) the homogeneous linear-skewed Langevin equation model (as well as an alternative 'nonlinear-Gaussian' Langevin equation model) can simulate the important aspects of dispersion in the CBL, and (2) a negatively-correlated-speed reflection boundary condition simulates the observed dispersion of material near the surface in the CBL significantly better than alternative reflection boundary conditions. The homogeneous linear-skewed Langevin equation model has the advantage that it is computationally more efficient than the homogeneous nonlinear-Gaussian Langevin equation model, and considerably more efficient than inhomogeneous Langevin equation models.  相似文献   

16.
The role of thermals in the convective boundary layer   总被引：1，自引：0，他引：1  
Detailed measurements of the structure of thermals throughout the convective boundary layer were obtained from the NCAR Electra aircraft over the ocean during the Air Mass Transformation Experiment (AMTEX). Humidity was used as an indicator of thermals. The variables were first high-pass filtered with a 5 km cutoff digital filter to eliminate mesoscale variations. Segments of the 5 min (30 km length) horizontal flight legs with humidity greater than half the standard deviation of humidity fluctuations for that leg were defined as thermals. This was found to be a better indicator of thermals than temperature in the upper part of the boundary layer since the temperature in a thermal is cooler than its environment in the upper part of the boundary layer. Using mixed-layer scaling, the normalized length and number of thermals were found to scale with the 1/3 and -1/3 powers, respectively, of normalized height, while vertical velocity and temperature scaled according to similarity predictions in the free convection region of the surface layer. The observational results presented here extend throughout the entire mixed layer. Using these results in the equation for mean updraft velocity of a field of thermals, the sum of the vertical pressure gradient and edge-effect terms can be estimated. This residual term is found to be important throughout most of the boundary layer. The magnitude of the divergence of vertical velocity variance within a thermal is found to be larger than the magnitude of the mean updraft velocity term throughout most of the mixed layer.Part of this work was completed while visiting Risø National Laboratory, Denmark.The National Center for Atmospheric Research is sponsored by the National Science Foundation.  相似文献   

17.
Large-scale turbulent motions enhancing horizontal gas spread in an atmospheric boundary layer are simulated in a wind-tunnel experiment. The large-scale turbulent motions can be generated using an active grid installed at the front of the test section in the wind tunnel, when appropriate parameters for the angular deflection and the rotation speed are chosen. The power spectra of vertical velocity fluctuations are unchanged with and without the active grid because they are strongly affected by the surface. The power spectra of both streamwise and lateral velocity fluctuations with the active grid increase in the low frequency region, and are closer to the empirical relations inferred from field observations. The large-scale turbulent motions do not affect the Reynolds shear stress, but change the balance of the processes involved. The relative contributions of ejections to sweeps are suppressed by large-scale turbulent motions, indicating that the motions behave as sweep events. The lateral gas spread is enhanced by the lateral large-scale turbulent motions generated by the active grid. The large-scale motions, however, do not affect the vertical velocity fluctuations near the surface, resulting in their having a minimal effect on the vertical gas spread. The peak concentration normalized using the root-mean-squared value of concentration fluctuation is remarkably constant over most regions of the plume irrespective of the operation of the active grid.  相似文献   

18.
The relationship between surface pressure fluctuations and the velocity field associated with turbulent coherent structures is examined for flow within and above a deciduous forest. Measurements were taken with tower-mounted sonic anemometer/thermometers at six heights, Lyman-alpha humidiometers at three heights, and a pressure sensor at the forest floor. We find a strong, near-linear relationship between the mean square turbulent velocity and the standard deviation of the high-pass-filtered pressure fluctuations. Lagged cross-correlations between vertical velocity fluctuations and those of pressure show maximum correlations of ± 0.5 but with a phase offset. Examination of surface pressure during the passage of coherent structures, which are characterized by a transition from ejection to sweep, reveals a period of overpressure about 20 s in duration roughly centered on the time of passage of the scalar microfront at the top of the canopy. Pressure patterns associated with coherent structures appear to be largely responsible for the form of the correlations stated above.Pressure patterns calculated from an integrated Poisson equation, using observed velocity and temperature signals during coherent structures, match the main features of the observed pressure. Retrieval of the pressure fluctuations in this manner reveals that the mean wind shear/turbulence interaction term is dominant, but that important contributions arise from two other terms in the equation. Buoyancy effects are negligible. We show that the surface pressure signal is mainly created by the velocity field near the top of the forest, and present evidence to suggest that features of the sub-crown air movement result directly from this pressure field.  相似文献   

19.
Spectral analysis was performed on aircraft observations of a convective boundary layer (CBL) that developed over a thermally inhomogeneous, well-marked mesoscale land surface. The observations, part of the GAME-Siberia experiment, were recorded between April and June 2000 over the Lena River near Yakutsk City. A special integral parameter termed the ‘reduced depth of the CBL’ was used to scale the height of the mixed layer with variable depth. Analysis of wavelet cospectra and spectra facilitated the separation of fluxes and other variables into small-scale turbulent fluctuations (with scales less than the reduced depth of the CBL, approximately 2 km) and mesoscale fluctuations (up to 20 km). This separation approach allows for independent exploration of the scales. Analyses showed that vertical distributions obeyed different laws for small-scale fluxes and mesoscale fluxes (of sensible heat, water vapour, momentum and carbon dioxide) and for other variables (wind speed and air temperature fluctuations, coherence and degree of anisotropy). Vertical profiles of small-scale turbulent fluxes showed a strong decay that differed from generally accepted similarity models for the CBL. Vertical profiles of mesoscale fluxes and other variables clearly showed sharp inflections at the same relative (with respect to the reduced depth of the CBL) height of approximately 0.55 in the CBL. Conventional similarity models for sensible heat fluxes describe both small-scale turbulent and mesoscale flows. The present results suggest that mesoscale motions that reach up to the relative level of 0.55 could be initiated by thermal surface heterogeneity. Entrainment between the upper part of the CBL and the free atmosphere may cause mesoscale motions in that region of the CBL.  相似文献   

20.
Coherent structures in turbulent flow above a midlatitude deciduous forest are identified using a wavelet analysis technique. Coupling between motions above the canopy (z/h=1.5, whereh is canopy height) and within the canopy (z/h=0.6) are studied using composite velocity and temperature fields constructed from 85 hours of data. Data are classified into winter and summer cases, for both convective and stable conditions. Vertical velocity fluctuations are in phase at both observation levels. Horizontal motions associated with the structures within the canopy lead those above the canopy, and linear analysis indicates that the horizontal motions deep in the canopy should lead the vertical motions by 90°. On average, coherent structures are responsible for only about 40% of overall turbulent heat and momentum fluxes, much less than previously reported. However, our large data set reveals that this flux fraction comes from a wide distribution that includes much higher fractions in its upper extremes. The separation distanceL s between adjacent coherent structures, 6–10h, is comparable to that obtained in previous observations over short canopies and in the laboratory. Changes in separation between the summer and winter (leafless) conditions are consistent withL s being determined by a local horizontal wind shear scale.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号