共查询到20条相似文献，搜索用时 15 毫秒
1.
The bulk aerodynamic formulation over heterogeneous surfaces 总被引：5，自引：2，他引：3
L. Mahrt 《Boundary-Layer Meteorology》1996,78(1-2):87-119
This interpretative literature survey examines problems with application of the bulk aerodynamic method to spatially averaged fluxes over heterogeneous surfaces. This task is approached by tying together concepts from a diverse range of recent studies on subgrid parameterization, the roughness sublayer, the roll of large inactive boundary-layer eddies, internal boundary-layer growth, the equilibrium sublayer, footprint theory and the blending height. Although these concepts are not completely compatible, qualitative scaling arguments based on these concepts lead to a tentative unified picture of the qualitative influence of surface heterogeneity for a wide spectrum of spatial scales.Generalization of the velocity scale is considered to account for nonvanishing heat and moisture fluxes in the limit of vanishing time-averaged wind speed and to account for the influence of subgrid mesoscale motions on the grid-averaged turbulent flux. The bulk aerodynamic relationship for the heat flux usually employs the surface radiation temperature or, equivalently, the temperature from the modelled surface energy budget. The corresponding thermal roughness length is quite variable and its dependence on available parameters is predictable only in special cases.An effective transfer coefficient to relate the spatially averaged surface fluxes to spatially averaged air-ground differences of temperature and other scalars can be most clearly defined when the blending height occurs below the reference level (observational level or first model level). This condition is satisfied only for surface heterogeneity occurring over horizontal scales up to a few times the boundary-layer depth, depending on the stability and height of the reference level. For surface heterogeneity on larger scales (small mesoscale), an effective transfer coefficient for the spatially averaged flow must be defined, for which predictive schemes are unavailable. For surface variations on large mesoscales, homogeneous subareas may be maintained where traditional similarity theory is locally applicable. Surface variations on these scales may generate thermally-driven mesoscale motions. 相似文献
2.
Koen De Ridder 《Boundary-Layer Meteorology》2010,134(2):257-267
In the roughness sublayer (RSL), Monin–Obukhov surface layer similarity theory fails. This is problematic for atmospheric
modelling applications over domains that include rough terrain such as forests or cities, since in these situations numerical
models often have the lowest model level located within the RSL. Based on empirical RSL profile functions for momentum and
scalar quantities, and scaling the height with the RSL height z
*, we derive a simple bulk transfer relation that accounts for RSL effects. To verify the validity of our approach, these relations
are employed together with wind speed and temperature profiles measured over boreal forest during the BOREAS experimental
campaign to estimate momentum and heat fluxes. It is demonstrated that, when compared with observed flux values, the inclusion
of RSL effects in the transfer relations yields a considerable improvement in the estimated fluxes. 相似文献
3.
Surface Heterogeneity Effects on Regional-Scale Fluxes in the Stable Boundary Layer: Aerodynamic Roughness Length Transitions 总被引：1，自引：1，他引：0
The effects of abrupt streamwise transitions of the aerodynamic roughness length ( $z_\mathrm{o}$ z o ) on the stable atmospheric boundary layer are evaluated using a series of large-eddy simulations based on the first Global Energy and Water Cycle Experiment Atmospheric Boundary Layer intercomparison study (GABLS1). Four $z_\mathrm{o}$ z o values spanning three orders of magnitude are used to create all possible binary distributions with each arranged into patches of characteristic length scales equal to roughly one-half, one, and two times the equivalent homogeneous boundary-layer height. The impact of the heterogeneity on mean profiles of wind speed and temperature, on surface fluxes of heat and momentum, and on internal boundary-layer dynamics are considered. It is found that $z_\mathrm{o}$ z o transitions do not significantly alter the functional relationship between the average surface fluxes and the mean profiles of wind speed and potential temperature. Although this suggests that bulk similarity theory is applicable for modelling the stable boundary layer over $z_\mathrm{o}$ z o heterogeneity, effective surface parameters must still be specified. Existing models that solve for effective roughness lengths of momentum and heat are evaluated and compared to values derived from the simulation data. The existing models are unable to accurately reproduce both the values of the effective aerodynamic roughness lengths and their trends as functions of patch length scale and stability. A new model for the effective aerodynamic roughness length is developed to exploit the benefits of the other models tested. It accurately accounts for the effects of the heterogeneity and stratification on the blending height and effective aerodynamic roughness length. The new model provides improved average surface fluxes when used with bulk similarity. 相似文献
4.
An efficient, pianetary boundary layer (PBL) model is developed and validated with empirical data for applications in general circulation models (GCMs). The purpose of this PBL model is to establish the turbulent surface fluxes as a function of the principal external PBL parameters in a numerically efficient way. It consists of a surface layer and a mixed layer matched together with the conditions of constant momentum and heat flux at the interface. An algebraic solution to the mean momentum equations describes the mixed-layer velocity profile and thus determines the surface wind vector. The velocity profile is globally valid by incorporating the effect of variable Coriolis force without becoming singular at the equator. Turbulent diffusion depends on atmospheric stability and is modeled in the surface layer by a drag law and with first-order closure in the mixed layer. Radiative cooling in the stably stratified PBL is considered in a simple manner. The coupled system is solved by an iterative method. In order to preserve the computational efficiency of the large-scale model, the PBL model is implemented into the GISS GCM by means of look-up tables with the bulk PBL Richardson number, PBL depth, neutral drag coefficient, and latitude as independent variables.A validation of the PBL model with observed data in the form of Rossby number similarity theory shows that the internal feedback mechanisms are represented correctly. The model, however, underpredicted the sensible heat-flux. A subsequent correction in the turbulence parameterization yields better agreement with the empirical data. The behavior of the principal internal PBL quantities is presented for a range of thermal stabilities and latitudes. 相似文献
5.
On the basis of the Monin-Obukhov similarity theory, a semi-empirical expression of universal functions is fitted by means of the iteration method. using the gradient observation data of wind and temperature in the surface layer. The characteristics of bulk transfer coefficient are studied and some empirical relationships among the bulk transfer coefficient, the wind speed and temperature are obtained. The applicability of the results is investigated using observation data. 相似文献
6.
Modelling surface turbulent fluxes in stable conditions 总被引：3，自引：0，他引：3
7.
广州地区近地层湍流输送的特征 总被引：1，自引：1，他引：0
利用风温梯度观测资料,以相似理论为基础,采用迭代方法得到普适函数Φ（ｚ／Ｌ）的半径验表达式,对近地层总体输送系数的牲进行了分析,求得了与风速、温度的拟合关系,并利用实测资料地其适用性作了检验。 相似文献
8.
G. L. Geernaert 《Boundary-Layer Meteorology》2002,105(3):433-450
In order to apply Monin–Obukhov similarity theory to estimatethe profiles of mean quantities and surface fluxes from bulk meteorological parameters, the assumptions of homogeneityand stationarity must be valid. Unfortunately, in coastal zones as well as many other regions of interest, theseassumptions are often violated. In this paper, an extension to the theory is presented that considers systematically varyingstate variables. Along-wind variations of windspeed, atmospheric stratification, and roughness are examined with respectto their relative importance to momentum flux divergence, and the drag coefficient is shown to be systematicallylower in coastal zones. For profiles of scalars, an analysis of the set of quasi-homogeneous terms is only speculated,and the relative importance for the terms will strongly depend on which scalar is of interest. 相似文献
9.
An important parameterization in large-eddy simulations (LESs) of high- Reynolds-number boundary layers, such as the atmospheric
boundary layer, is the specification of the surface boundary condition. Typical boundary conditions compute the fluctuating
surface shear stress as a function of the resolved (filtered) velocity at the lowest grid points based on similarity theory.
However, these approaches are questionable because they use instantaneous (filtered) variables, while similarity theory is
only valid for mean quantities. Three of these formulations are implemented in simulations of a neutral atmospheric boundary
layer with different aerodynamic surface roughness. Our results show unrealistic influence of surface roughness on the mean
profile, variance and spectra of the resolved velocity near the ground, in contradiction of similarity theory. In addition
to similarity-based surface boundary conditions, a recent model developed from an a priori experimental study is tested and it is shown to yield more realistic independence of the results to changes in surface roughness.
The optimum value of the model parameter found in our simulations matches well the value reported in the a priori wind-tunnel study. 相似文献
10.
William P. Kustas Martha C. Anderson John M. Norman Fuqin Li 《Boundary-Layer Meteorology》2007,122(1):167-187
In many land-surface models using bulk transfer (one-source) approaches, the application of radiometric surface temperature
observations in energy flux computations has given mixed results. This is due in part to the non-unique relationship between
the so-called aerodynamic temperature, which relates to the efficiency of heat exchange between the land surface and overlying
atmosphere, and a surface temperature measurement from a thermal-infrared radiometer, which largely corresponds to a weighted
soil and canopy temperature as a function of radiometer viewing angle. A number of studies over the past several years using
multi-source canopy models and/or experimental data have developed simplified methods to accommodate radiometric–aerodynamic
temperature differences in one-source approaches. A recent investigation related the variability in the radiometric–aerodynamic
relation to solar radiation using experimental data from a variety of landscapes, while another used a multi-source canopy
model combined with measurements over a wide range in vegetation density to derive a relationship based on leaf area index.
In this study, simulations by a detailed multi-source soil–plant–environment model, Cupid, which considers both radiative
and turbulent exchanges across the soil–canopy–air interface, are used to explore the radiometric–aerodynamic temperature
relations for a semi-arid shrubland ecosystem under a range of leaf area/canopy cover, soil moisture and meteorological conditions.
The simulated radiometric-aerodynamic temperatures indicate that, while solar radiation and leaf area both strongly affect
the magnitude of this temperature difference, the relationships are non-unique, having significant variability depending on
local conditions. These simulations also show that soil–canopy temperature differences are highly correlated with variations
in the radiometric–aerodynamic temperature differences, with the slope being primarily a function of leaf area. This result
suggests that two-source schemes with reliable estimates of component soil and canopy temperatures and associated resistances
may be better able to accommodate variability in the radiometric–aerodynamic relation for a wider range in vegetated canopy
cover conditions than is possible with one-source schemes. However, comparisons of sensible heat flux estimates with Cupid
using a simplified two-source model and a one-source model accommodating variability in the radiometric-aerodynamic relation
based on vegetation density gave similar scatter. On the other hand, with experimental data from the shrubland site, the two-source
model generally outperformed the one-source scheme. Clearly, vegetation density/leaf area has a major effect on the radiometric–aerodynamic
temperature relation and must be considered in either one-source or two-source formulations. Hence these adjusted one-source
models require similar inputs as in two-source approaches, but provide as output only bulk heat fluxes; this is not as useful
for monitoring vegetation conditions. 相似文献
11.
A dataset collected during a measurement campaign in the middle of the Po Valley, Italy, is used to investigate the boundary-layer structure in stable conditions. Empirical formulations for temperature and wind profiles derived from Monin–Obukhov similarity theory are used as regression curves to fit radiosounding profiles in the lower half of the boundary-layer. The best fitting parameters of the regression are then compared to the surface turbulent fluxes as measured by a co-located sonic anemometer. This comparison shows significant discrepancies and supports earlier results showing that surface fluxes, in the limit of high stability, are not adequate scalings for mean profiles. The most evident differences are found for cases for which the bulk Richardson number turns out to be quite large. One of the practical consequences is that boundary-layer height diagnostic formulations that mainly rely on surface fluxes are in disagreement with those obtained by inspecting the thermodynamic profiles recorded during the radiosounding ascent. Moreover the incorrect scaling of similarity profiles in stable conditions leads to the erroneous diagnosis of 2-m air temperatures used in numerical weather prediction validation. 相似文献
12.
Turbulent characteristics and bulk transfer coefficients over the desert in the HEIFE area 总被引：1，自引：0，他引：1
Ichiro Tamagawa 《Boundary-Layer Meteorology》1996,77(1):1-20
Observations of surface-layer turbulence and turbulent fluxes were made over a desert in northwestern China as a part of HEIFE (HEIhe river Field Experiment). These show that the normalized variations of the vertical wind component and of the air temperature obey Monin-Obukhov similarity well, especially in free convective conditions. However, the variations of specific humidity do not obey Monin-Obukhov similarity.Mean bulk transfer coefficients of sensible heat and momentum flux are obtained as functions of stability over a wide stability range from the observed data of turbulent fluxes and mast profiles. However, the bulk transfer coefficient for water vapor could not be obtained because of the large scatter of the data. In free convective conditions, the sensible heat flux was found to be approximately proportional to the 1.4 power of temperature difference between the surface and 20m. The bulk transfer coefficient of sensible heat is also obtained as a function of the bulk Richardson number for practical convenience. 相似文献
13.
Spectral Structure of Small-Scale Turbulent and Mesoscale Fluxes in the Atmospheric Boundary Layer over a Thermally Inhomogeneous Land Surface 总被引：1，自引：0，他引：1
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
14.
近地层湍流通量计算对于中尺度数值模式有重要意义, 湍流通量的参数化是当前大气边界层研究的重要课题之一。选择青藏高原东缘大理观象台边界层通量观测系统, 离线测试了WRF区域模式中的两种常用的近地层参数化方案(MM5相似理论非迭代方案A和ETA 相似理论迭代方案B), 并将参数化方案计算结果与边界层铁塔涡动相关法的观测值进行对比分析。在大理观象台观测场不同植被随季节交替的状况下, 根据边界层铁塔4层高度风速拟合, 发现近地层空气动力学粗糙度随季节变化特征明显。将拟合的空气动力学粗糙度输入模式参数化方案进行通量计算。结果表明:稳定度是影响近地层参数化方案精度的重要因素, 在不稳定条件下方案B低估了动量通量, 方案A优于方案B, 而在稳定条件下方案A低估了动量通量, 方案B优于方案A, 两种方案总体来看误差不大。对于大理边界层通量观测场地农田植被交替的环境条件, 不同季节下垫面植被类型的差异, 以及植被的稀疏对近地层参数化方案湍流通量计算结果的精度有显著影响。方案B考虑了空气动力学粗糙度z0和热量粗糙度z0h的差异, 不稳定条件下感热通量计算结果在裸土或稀少植被条件下明显优于方案A。针对方案B不稳定条件下感热通量计算结果在裸土下垫面仍出现高估的现象, 在使用了(Zeng, et al, 1998)提出的对于使用辐射地表温度在裸土下垫面时的订正方法后, 计算结果也有明显改善。 相似文献
15.
近地层湍流通量计算对于中尺度数值模式有重要意义,湍流通量的参数化是当前大气边界层研究的重要课题之一.选择青藏高原东缘大理观象台边界层通量观测系统,离线测试了WRF区域模式中的两种常用的近地层参数化方案(MM5相似理论非迭代方案A和ETA相似理论迭代方案B),并将参数化方案计算结果与边界层铁塔涡动相关法的观测值进行对比分析.在大理观象台观测场不同植被随季节交替的状况下,根据边界层铁塔4层高度风速拟合,发现近地层空气动力学粗糙度随季节变化特征明显.将拟合的空气动力学粗糙度输入模式参数化方案进行通量计算.结果表明:稳定度是影响近地层参数化方案精度的重要因素,在不稳定条件下方案B低估了动量通量,方案A优于方案B,而在稳定条件下方案A低估了动量通量,方案B优于方案A,两种方案总体来看误差不大.对于大理边界层通量观测场农田植被交替的环境条件,不同季节下垫面植被类型的差异,以及植被的稀疏对近地层参数化方案湍流通量计算结果的精度有显着影响.方案B考虑了空气动力学粗糙度z0和热量粗糙度z0h的差异,不稳定条件下感热通量计算结果在裸土或稀少植被条件下明显优于方案A.针对方案B不稳定条件下感热通量计算结果在裸土下垫面仍出现高估的现象,使用了Zeng等1998年提出的用辐射地表温度订正裸土下垫面感热能量方法后,计算结果也有明显改善. 相似文献
16.
The need for a thorough knowledge of boundary conditions over the vast oceanic surfaces makes remote sensing appear as the most suitable tool for future development. Remote sensing techniques require calibration data and consequently in-situ experiments to yield those data. Over the sea, aircraft seem to be the most convenient means of conducting validation experiments because they allow exploration of a large range of scales (from local measurements to measurements of pixel scale). This paper reports on some in-situ atmospheric aircraft measurements that were conducted as part of the TOSCANE-T experiment organized by the European Space Agency to calibrate the scatterometer for launch on ERS-1 in the early 1990's for global ocean wind measurement.The data analysed are the thermodynamic and turbulent variables measured by an instrumented aircraft, the Hurel Dubois, flying at a constant level of 50 m over the sea surface. Special attention was drawn to the variability of the boundary conditions within areas of 25 × 30 km. In fact, the two-dimensional fields of fluxes and thermodynamic parameters were inhomogeneous with some rather strong wind distortion.The eddy fluxes were parameterized with the aid of bulk aerodynamic formulations at a basic scale of 30 km samples, which corresponds to the aircraft flux computation legs. The bulk aerodynamic coefficients for momentum and heat were found to remain independent of windspeed (for wind velocities less than 12 m/s). Fluxes and thermodynamic parameters were also investigated at two other scales: the integration scale was either reduced to small areas of which size always remains larger than several characteristic lengths of turbulent transfer, or extended to a large area of 25 × 30km. The results of the bulk aerodynamic relationships appeared to be scale invariant, which would therefore justify the application of average values, within the range of scales under study. 相似文献
17.
Fred C. Bosveld A. A. M. Holtslag B. J. J. M. Van Den Hurk 《Boundary-Layer Meteorology》1999,92(3):429-451
Infrared crown radiation temperatures as observed over a dense Douglas fir forest are analyzed in the context of similarity theory and the concept of transport resistances. As such we obtain a rather high value of the roughness length for heat, which is about equal to the roughness length for momentum. This value can be explained by the more efficient transport of heat relative to momentum in the roughness sublayer of the forest. Correcting for this effect we arrive at the classic value for homogeneous terrain of about 0.1 times the roughness length for momentum. For unstable cases the presence of enhanced mixing of heat in the roughness sublayer leads to a modified integral stability function for the dimensionless potential temperature difference between the surface and the top of the roughness sublayer. The observations give some evidence for this different stability behaviour. The analysis suggests that during daytime the radiative surface temperature and the aerodynamic surface temperature are not significantly different when used to estimate fluxes. Daytime trunk space air temperature is satisfactory parameterized with the concept of gusts and with surface renewal analysis. As such it is related to the sensible heat flux and the storage heat flux. Night time radiation temperatures at times strongly deviate from the expected behaviour based on similarity theory and the roughness length for heat, suggesting that the concept of a single surface temperature is too simple for such cases. 相似文献
18.
Gary K. Greenhut 《Boundary-Layer Meteorology》1982,24(2):253-264
Turbulence data obtained aboard a NOAA P-3 research aircraft during flights over the eastern equatorial Pacific Ocean are used to obtain mean vertical profiles of water vapor density, potential temperature, wind speed and fluxes of latent heat, buoyancy and momentum. The variation of eddy fluxes and bulk transfer coefficients as a function of atmospheric stability are plotted for two of the flights. The observed transfer coefficients generally agree with those obtained from parameterizations based on surface-layer similarity theory (Deardorff, 1968; Kondo, 1975). 相似文献
19.
This case study introduces measurements of turbulent fluxes in a nocturnal boundary layer in North Germany with the new helicopter-borne turbulence measurement system HELIPOD, a detailed data analysis and examination in regard of systematic errors of the instrument, and some comparison with local similarity theory and experiments of the past, in order to confirm the occurrence of small vertical turbulent fluxes. The examined nocturnal boundary layer offered excellent conditions to analyse the quality of the measurement system. In this connection, a detailed look at a strong ground-based inversion disclosed small turbulent fluxes with a spectral maximum at ten metres wavelength or less, embedded in intermittent turbulence. For verification of these fluxes, the measurements were compared with well established results from past experiments. Local similarity theory was applied to calculate dimensionless variances of the turbulent quantities, which were found in good agreement with other observations. Since shear and stratification varied significantly on the horizontal flight legs due to global intermittency, a method was developed to determine vertical gradients on a horizontal flight pattern, by use of small fluctuations of the measurement height. With these locally determined gradients, gradient transport theory became applicable and the turbulent diffusivities for heat and momentum, the Richardson number, and the flux Richardson number were estimated within isolated strong turbulent outbursts. Within these outbursts the flux Richardson number was found between 0.1 and 0.2. The functional relationship between the gradient Richardson number and the turbulent Prandtl number agreed well with observations in past experiments and large eddy simulation. The impact of the stratification on the vertical turbulent exchange, as already described for the surface layer using Monin–Obukhov similarity, was analogously observed in the very stably stratified bulk flow when local scaling was applied. 相似文献