共查询到20条相似文献，搜索用时 406 毫秒
1.
Structure functions are used to study the dissipation and inertial range scales of turbulent energy, to parametrize remote turbulence measurements, and to characterize ramp features in the turbulent field. Ramp features are associated with turbulent coherent structures, which dominate energy and mass fluxes in the atmospheric surface layer. The analysis of structure functions to identify ramp characteristics is used in surface renewal methods for estimating fluxes. It is unclear how commonly observed different scales of ramp-like shapes (i.e., smaller ramps and spikes embedded in larger ramps) influence structure function analysis. Here, we examine the impact of two ramp-like scales on structure function analysis using artificially generated data. The range of time lags in structure function analysis was extended to include time lags typically associated with isotropic turbulence to those larger than the ramp durations. The Van Atta procedure (Arch Mech 29:161–171, 1977) has been expanded here to resolve the characteristics of two-scale ramp models. This new method accurately, and in some cases, exactly determines the amplitude and duration of both ramp scales. Spectral analysis was applied to the structure functions for a broad range of time lags to provide qualitative support for the expanded Van Atta procedure results. The theory reported here forms the foundation for novel methods of analyzing turbulent coherent structures. 相似文献
2.
High frequency temperature measurements were recorded at five heights and surface renewal (SR) analysis was used to estimate sensible heat flux density (H) over 0.1 m tall grass. Traces of the temperature data showed ramp-like structures, and the mean amplitude and duration of these ramps were used to calculate H using structure functions. Data were compared with H values measured with a sonic anemometer. Latent heat flux density (E) was calculated using an energy balance and the results were compared with E computed from the sonic anemometer data. SR analysis provided good estimates of H for data recorded at all heights but the canopy top and at the highest measurement level, which was above the fully adjusted boundary layer. 相似文献
3.
Wenjun Chen Michael D. Novak T. Andrew Black Xuhui Lee 《Boundary-Layer Meteorology》1997,84(1):99-124
Air temperature time series within and above canopies reveal ramp patternsassociated with coherent eddies that are responsible for most of thevertical transport of sensible heat. Van Atta used a simple step-changeramp model to analyse the coherent part of air temperature structurefunctions. However, his ocean data, and our own measurements for aDouglas-fir forest, straw mulch, and bare soil, reveal that even withoutlinearization his model cannot account for the observed decrease of thecubic structure function for small time lag. We found that a ramp model inwhich the rapid change at the end of the ramp occurs in a finite microfronttime can describe this decrease very well, and predict at least relativemagnitudes of microfront times between different surfaces. Averagerecurrence time for ramps, determined by analysis of the cubic structurefunction with the new ramp model, agreed well with values determined usingthe Mexican Hat wavelet transform, except at lower levels within theforest. Ramp frequency above the forest and mulch scaled very well withwind speed at the canopy top divided by canopy height. Within the forest,ramp frequency did not vary systematically with height. This is inaccordance with the idea that large-scale canopy turbulence is mostlygenerated by instability of the mean canopy wind profile, similar to aplane mixing layer. The straw mulch and bare soil experiments uniquelyextend measurements of temperature structure functions and ramp frequencyto the smallest scales possible in the field. 相似文献
4.
Step-like temperature fluctuations associated with inverted ramps in a stable surface layer 总被引：1，自引：0，他引：1
Ensemble averages of temperature before and after step-like temperature fluctuations reveal the presence of inverted ramps in a stable surface layer. Normalized frequency of upward steps increases with increasing stability, whereas normalized magnitude of the temperature step decreases with stability and becomes constant at about R
i
= 1. These results suggest that the significance of temperature steps increases as stability increases. In moderate stability, the temperature pattern shows a gradual decrease after an upward step, which can be called a time-inverted ramp. Descending air and large downward heat flux are observed in a time-inverted ramp, suggesting a contribution from an ordered motion in wind. On the other hand, the temperature steps are related to gravity waves in strong stability. 相似文献
5.
A conditional sampling technique using a multilevel scheme was applied to the detection of temperature and humidity microfronts and organized ejection/sweep motions under different atmospheric stabilities. Data were obtained with seven triaxial sonic anemometer/thermometers and three Lyman-alpha hygrometers within and above a deciduous forest. Both temperature and humidity microfronts were identified in unstable cases, but only humidity microfronts could be detected under neutral conditions. Inverted temperature ramps occurred under slightly stable conditions. Occasionally, wave-like patterns appeared within the canopy, seemingly coupled with inverse ramps occurring above the forest. The frequency of occurrence of scalar microfronts appears to have no clear dependence on atmospheric stability, and averages 74–84 s per cycle with a mode of about 50 s per cycle. However, the strength of ejections and sweeps, shown by the vertical velocity averaged within structures, was reduced by increasing atmospheric stability. Structures identified under different stabilities show many similarities in their patterns of scalar ramps, and associated velocity and surface pressure. Profiles of short-term averaged longitudinal velocity at different times during the microfront passage show that the air within the canopy was retarded and an intensified shear above the canopy occurred prior to the passage of the microfront. Results from the present conditional analysis strongly suggest an important role of shear instability in the formation of canopy coherent structure. 相似文献
6.
Monin–Obukhov similarity functions for the structure parameters of temperature and humidity are needed to derive surface heat and water vapour fluxes from scintillometer measurements and it is often assumed that the two functions are identical in the atmospheric surface layer. Nevertheless, this assumption has not yet been verified experimentally. This study investigates the dissimilarity between the turbulent transport of sensible heat and water vapour, with a specific focus on the difference between the Monin–Obukhov similarity functions for the structure parameters. Using two datasets collected over homogeneous surfaces where the surface sources of sensible heat and water vapour are well correlated, we observe that under stable and very unstable conditions, the two functions are similar. This similarity however breaks down under weakly unstable conditions; in that regime, the absolute values of the correlations between temperature and humidity are also observed to be low, most likely due to large-scale eddies that transport unsteadiness, advection or entrainment effects from the outer layer. We analyze and demonstrate how this reduction in the correlation leads to dissimilarity between the turbulent transport of these two scalars and the corresponding Monin–Obukhov similarity functions for their structure parameters. A model to derive sensible and latent heat fluxes from structure parameters without measuring the friction velocity is tested and found to work very well under moderately to strongly unstable conditions (−z/L > 0.5). Finally, we discuss the modelling of the cross-structure parameter over wet surfaces, which is crucial for correcting water vapour effects on optical scintillometer measurements and also for obtaining surface sensible and latent heat fluxes from the two-wavelength scintillometry. 相似文献
7.
An investigation is made of the mechanics of amplitude vacillation in a numerically simulated rotating annulus flow system. Amplitude vacillation is characterized by a periodic change of vertical wave structure in concert with growth and decay of wave amplitude and phase speed. The temperature wave amplitude profile for the dominant component consists of three local maxima: (1) lower boundary layer, (2) upper half layer and (3) lower half layer. The lower layer waves lead the time-dependent structural variation during vacillation. Two types of amplitude vacillation found in the experimental measurements (Buzyna et al., 1989: J. Atmos. Sci. 46, 2716–2729) can be distinguished in the temperature wave by whether the lower layer waves split from and travel behind the upper layer waves by one wave period in each cycle of vacillation. Linear eigenvalue analyses with respect to the instantaneous axisymmetric state at various points in time are performed to elucidate the simple interaction between the dominant wave and the zonal mean state. During the vacillation cycle, the zonal mean state is modified by the wave, which causes a change in growth rate and vertical structure of the linearly most unstable eigenmode. This, in turn, forces the actual changes of the nonlinear solutions. 相似文献
8.
This review of the last three years of progress in the understanding of wind profiles and the structure of turbulence in the planetary boundary layer is divided into three parts. The first part, by N. E. Busch, deals with the atmospheric surface layer below 30 m. It is shown that the Monin-Oboukhov similarity hypotheses fail at low frequencies and large wave-lengths, probably due to mesoscale influences. Also, it is suggested that the neutral surface layer is a poor reference state in some respects, because the structure of turbulence in unstable conditions is quite different from that in stable stratification. The second part, by H. Tennekes, is concerned with the intermittency of the dissipative structure of turbulence and its effects on the velocity and temperature structure functions. It is shown that the modified Kolmogorov-Oboukhov theory, which attempts to explain the consequences of the dissipative intermittency, is unable to predict the shape of the temperature structure functions. The third part of this review, by H. A. Panofsky, deals with wind profiles and turbulence structure above 30 m. It is shown that between 30 and 150 m, surface-layer formulas can be used, if such mesoscale effects as changes of terrain roughness are taken into account where needed. Experimental data on turbulence above 150 m are quite sparse; some of the current scaling laws that can be used in this region are described. 相似文献
9.
Gerard Kiely John D. Albertson Marc B. Parlance William E. Eichinger 《Boundary-Layer Meteorology》1996,77(3-4):267-284
The flux of sensible heat from the land surface is related to the average rate of dissipation of temperature fluctuations in the atmospheric surface layer through the temperature variance budget equation. In many cases it is desirable to estimate the heat flux from measurement or inference of the dissipation rate. Here we study how the dissipation rate scales with atmospheric stability, using three inertial range methods to calculate the dissipation rate: power spectra, second order structure functions, and third order structure functions. Experimental data are analyzed from a pair of field experiments, during which turbulent fluctuations of velocity and temperature were measured over a broad range of neutral and unstable atmospheric flows. It is shown that the temperature dissipation rate scales with a single convective power law continuously from near-neutral to strongly unstable stratification. The dissipation scaling is found to nearly match production in the near-neutral region, but to be consistently lower than production in the more convective regimes. The convective scaling is shown to offer a simplified means of computing sensible heat flux from the dissipation rate of temperature variance.Also at Johns Hopkins University, Baltimore, MarylandAlso at Los Alamos National Laboratory, Los Alamos, New Mexico. 相似文献
10.
B. M. Koprov V. M. Koprov T. I. Makarova G. S. Golitsyn 《Boundary-Layer Meteorology》2004,111(1):19-32
Simultaneous measurements of the instantaneous values of absolute temperatureat seven heights within the lower 36 m of the atmospheric boundary layer underdifferent stability conditions were carried out, accompanied by measurements ofthe wind velocity components at two levels and of solar radiation flux at the surface.The data obtained allow one to investigate individual convective cells known ascoherent structures (CS). Outside the CS, i.e., during quiet periods, an instanttemperature profile is in close agreement with the dry-adiabatic lapse rate, butwithin CS the temperature changes much faster with height, and the shape ofthe profile varies significantly.A method was developed to transform temperature records from sensors atseveral heights into an other form, namely, into temporal variations of theheights of isothermal surfaces. Since coherent structures were found to advectwith the mean wind velocity, these temporal height variations may be transformedinto the spatial ones, i.e., into the xoz-plane section of the temperature field.In such a pictorial presentation coherent structures look like asymmetric columnsof heat, penetrating the whole atmospheric surface layer.Coherent structures also exist in the stable stratified surface layer, but they have aninverse asymmetry and occupy only the lower several metres. Wavelike activitydominates in the upper part of the stable surface layer.The characteristic time of surface-layer adjustment to the rapid changes of solarradiation (due to cloud shadows or cloud gaps) was found to be on the order ofone minute. Such a time interval is required for coherent structure to reach the topof surface layer. 相似文献
11.
ENERGY-INERTIAL SCALE INTERACTIONS FOR VELOCITY AND TEMPERATURE IN THE UNSTABLE ATMOSPHERIC SURFACE LAYER 总被引：1，自引：1，他引：0
Triaxial sonic anemometer velocity and temperature measurements were used to investigate the local structure of the velocity and temperature fluctuations in the unstable atmospheric surface layer above a grass-covered forest clearing. Despite the existence of a 2/3 power law in the longitudinal velocity (2 decades) and temperature (1 decade) structure functions, local isotropy within the inertial subrange was not attained by the temperature field, although a near-isotropic state was attained by the velocity field. It was found that sources of anisotropy were due to interactions between the large-scale and small-scaleeddy motion, and due to localvelocity-thermal interactions. Statistical measures were developed and used to quantify these types of interactions. Other types of interactions were also measured but were less significant. The temperature gradient skewness was measured and found to be non-zero in agreement with other laboratory flow types for inertial subrange scales. Despite these interactions and anisotropy sources in the local temperature field, Obukhovs 1949hypothesis for the mixed velocity-temperature structure functions was found to be valid. Finally, our measurements show that while a 2/3 power-law in the longitudinal velocity structure function developed at scales comparable to five times the height from the ground surface (z), near-isotropic conditions wereachieved at scales smaller than z/2. 相似文献
12.
Urban heat island diagnosis using ASTER satellite images and ‘in situ’ air temperature 总被引：1，自引：0，他引：1
This study demonstrates that thermal satellite images combined with ‘in situ’ ground data can be used to examine models of heat island genesis and thus identify the main causes of urban heat islands (UHIs). The models, although proposed over 30 years ago, have not been thoroughly evaluated due to a combination of inadequate ground data and the low resolution of thermal satellite data. Also there has been limited understanding of the relevance of satellite-derived surface temperatures to local and regional scale air temperatures. A cloud-free ASTER thermal image of urban and rural areas of Hong Kong was obtained on a winter night with a well-developed heat island, accompanied by a 148 km vehicle traverse of air temperatures. Over the whole traverse a high R2 of 0.80 was observed between surface and air temperatures, with the two datasets showing a similar amplitude and general trend, but with the surface exhibiting much higher local variability than air temperature. Gradients in both surface and air temperature could be related to differences in land cover, with little evidence of large scale advection, thus supporting the population/physical structure model of UHI causation, rather than the advection model. However, the much higher surface and air temperatures observed over the largest urban area, Kowloon, than over any smaller urban centre with similar physical structure in the New Territories, would seem more indicative of the advection model. The image and ground data suggest that Kowloon's urban canopy layer climate is mainly influenced by local city structure, but it is also modified by a strongly developed, regional scale urban boundary layer which has developed over the largest urban centre of Kowloon, and reinforces heating from both above and below. 相似文献
13.
14.
WRF模式对青藏高原那曲地区大气边界层模拟适用性研究 总被引：2，自引：0，他引：2
采用WRF（Weather Research and Forecasting）模式4种边界层参数化方案对青藏高原那曲地区边界层特征进行了数值模拟,并利用"第三次青藏高原大气科学试验"在青藏高原那曲地区5个站点的观测资料对模拟结果进行验证,分析不同参数化方案在那曲地区的适用性。研究表明,YSU、MYJ、ACM2和BouLac方案对2 m气温和地表温度的模拟偏低。BouLac方案模拟的地表温度偏差较小。通过对能量平衡各分量的对比分析发现,温度模拟偏低可能是向下长波辐射模拟偏低以及感热通量和潜热通量交换过强导致的。对于边界层风、位温和相对湿度垂直结构的模拟,局地方案的模拟效果均优于非局地方案。BouLac方案对那曲地区近地层温度、边界层内位温和相对湿度的垂直分布模拟效果较好。 相似文献
15.
采用MODIS资料和美国发展的MODIS大气温、湿度廓线统计反演算法,估算大气温度、湿度廓线作为初始场,应用101层快速透过率模式(PFAAST)估算了大气透过率,并采用Newton非线性迭代算法反演中国西北荒漠戈壁地区大气温度廓线。结果表明:该方法对边界层高度及以上部分的大气温度反演得比较好,误差基本都在2 K范围内,边界层范围内的温度反演误差较大,反演误差与气溶胶光学厚度增量和地表温度估算误差呈显著正相关关系,与大气水汽混合比的关系较差。文中从敏感性试验和理论分析角度阐述了地表温度和气溶胶光学厚度估算误差对大气温度反演误差的影响,发现不同光谱波段的地表温度权重均随地表温度的增加有不同程度增加,地表温度反演误差增加将增加地表温度权重,提高地表温度估算误差有助于提高地表温度权重的精度;荒漠戈壁地区大气边界层中气溶胶浓度较高,光学厚度较大,使边界层大气透过率降低,进而降低卫星红外遥感波段的地表温度权重和空气温度权重。由于该模式没有很好地考虑边界层中沙尘气溶胶的影响,使卫星反演的大气透过率偏高,以至于高估地表温度权重和大气温度权重,使得反演的表面温度和空气温度偏低。该研究结合太阳光度计获得的光学厚度资料,采用统计方法对气溶胶效应引起的大气透过率误差和表面温度估算误差进行校正,并对物理算法进行本地化改进,实现了边界层温度廓线的反演。 相似文献
16.
Two surface layer parameterization schemes along with five planetary boundary layer (PBL) schemes in the Weather Research and Forecasting model (WRF) are analyzed in order to evaluate the performance of the WRF model in simulating the surface variables and turbulent fluxes over an Indian sub-continent region. These surface layer schemes are based on the fifth-generation Pennsylvania State University—National Center for Atmospheric Research Mesoscale Model (MM5) parameterization; (a) Old MM5 scheme having Businger-Dyer similarity functions and (b) revised MM5 scheme utilizing the functions that are valid for full ranges of atmospheric stabilities. The study suggests that each PBL scheme can reproduce the diurnal variation of 2 m temperature, momentum flux and sensible heat flux irrespective of the surface layer scheme used for the simulations. However, a comparison of model-simulated values of surface variables and turbulent fluxes with observed values suggests that each PBL scheme is found to systematically over-estimate the nocturnal 2 m temperature and 10 m wind speed with both the revised and old schemes during stable conditions. 相似文献
17.
The simulation of horizontally homogeneous boundary layers that have characteristics of weakly and moderately stable atmospheric flow is investigated, where the well-established wind engineering practice of using ‘flow generators’ to provide a deep boundary layer is employed. Primary attention is given to the flow above the surface layer, in the absence of an overlying inversion, as assessed from first- and second-order moments of velocity and temperature. A uniform inlet temperature profile ahead of a deep layer, allowing initially neutral flow, results in the upper part of the boundary layer remaining neutral. A non-uniform inlet temperature profile is required but needs careful specification if odd characteristics are to be avoided, attributed to long-lasting effects inherent of stability, and to a reduced level of turbulent mixing. The first part of the wind-tunnel floor must not be cooled if turbulence quantities are to vary smoothly with height. Closely horizontally homogeneous flow is demonstrated, where profiles are comparable or closely comparable with atmospheric data in terms of local similarity and functions of normalized height. The ratio of boundary-layer height to surface Obukhov length, and the surface heat flux, are functions of the bulk Richardson number, independent of horizontal homogeneity. Surface heat flux rises to a maximum and then decreases. 相似文献
18.
提要：利用2013年春季在巴丹吉林沙漠北缘拐子湖地区的沙尘暴加强观测资料,对比分析该地区典型流动沙面晴天、扬沙和沙尘暴三种天气背景下各气象要素的变化特征及差异,同时对沙尘暴过程中近地层风沙活动特征进行分析。结果表明,随风速增加沙尘天气强度逐步提升且沙尘天气来临前风速、风向均表现出明显的调整现象,此后爆发过程中风速、风向相对稳定。随沙尘天气强度的增加气温逐渐减小且沙尘天气过程中地面呈利于沙尘起动的暖干状态,同时地面气压不断升高。悬浮的沙尘会导致拐子湖流动沙地各层地温有减小趋势,但减小程度相对较弱,使沙尘天气下各层地温仍保持良好的梯度变化和正弦型日变化趋势。拐子湖流沙地春季起沙风速为6.5m/s,输沙通量垂直分布状况在20cm左右具有明显的分段现象。地表100cm内总输沙量的50%和90%分别集中在地表20cm和56cm高度以内。观测期间整个5月地表0～100cm高度内的输沙通量为420.96kg/m。 相似文献
19.
Aimé Druilhet Pierre Durand Alberte Fischer Frédérique Said 《Boundary-Layer Meteorology》1990,52(4):397-414
Through simultaneous measurements, it is possible to demonstrate the relation between sea swell and surface temperature as measured by a thermoradiometer onboard an aircraft. Thermodynamic measurements made on board the aircraft enable analysis of the interaction between swell and surface marine layer dynamics. The cospectral functions of vertical turbulent transfer for sensible and latent heat indicate large-scale dynamic structures which would seem to be initiated by inhomogeneities in the humidity field. A cross-spectral examination of temperature and humidity produces a structure that is common to both parameters and is caused by swell propagation. The data used here were collected during the TOSCANE-T experiment, whose aim was to validate remote sensing scatterometer techniques, by means of an instrumented aircraft designed for atmospheric research. 相似文献
20.
Turbulence measurements in the lower half of the convective boundary layer (CBL), which includes both mixed layer and surface layer, were carried out with five sonic anemometers mounted on a 213-m tower over a complex flat suburban area with patches of forest, agricultural land, houses and buildings. Also made were radiosoundings of temperature, humidity and wind speed, to determine the CBL height. The sonic anemometer data of wind speed and temperature were processed to derive the second-moment turbulent statistics and were analyzed to investigate the applicability of variance methods to estimate regional surface fluxes of sensible heat. It was found that the temperature variances in the lower mixed layer, coupled with universal functions, produced sensible heat fluxes H over the area with an rms error of the order of 40 Wm-2 when compared with H values derived from the eddy correlation method. The variance of the vertical wind speed did not produce as good a result. In contrast, the surface-layer temperature variances yielded H values with rms error of the order of 20 Wm-2, even though the underlying surface was non-uniform and highly non-isothermal, above which enhanced temperature variances could be suspected. 相似文献