共查询到20条相似文献，搜索用时 31 毫秒
1.
Günther Heinemann 《Boundary-Layer Meteorology》2002,103(1):49-81
Turbulence structures in the katabatic flow in the stable boundary layer (SBL) over the ice sheet are studied for two case studies with high wind speeds during the aircraft-based experiment KABEG (Katabatic wind and boundary layer front experiment around Greenland) in the area of southern Greenland. The aircraft data allow the direct determination of turbulence structures in the katabatic flow. For the first time, this allows the study of the turbulence structure in the katabatic wind system over the whole boundary layer and over a horizontal scale of 80 km.The katabatic flow is associated with a low-level jet (LLJ), with maximum wind speeds up to 25 m s-1. Turbulent kinetic energy (TKE) and the magnitude of the turbulent fluxes show a strong decrease below the LLJ. Sensible heat fluxes at the lowest level have values down to -25 W m-2. Latent heat fluxes are small in general, but evaporation values of up to +13 W m-2 are also measured. Turbulence spectra show a well-defined inertial subrange and a clear spectral gap around 250-m wavelength. While turbulence intensity decreases monotonously with height above the LLJ for the upper part of the slope, high spectral intensities are also present at upper levels close to the ice edge. Normalized fluxes and variances generally follow power-law profiles in the SBL.Terms of the TKE budget are computed from the aircraft data. The TKE destruction by the negative buoyancy is found to be very small, and the dissipation rate exceeds the dynamical production. 相似文献
2.
Mikio Nakanish 《Boundary-Layer Meteorology》2001,99(3):349-378
On the basis of data constructed with large-eddy simulation (LES), an attempt is made to improve the Mellor–Yamada (M–Y) turbulence closure model. Firstly, stably-stratified and convective planetary boundary layers without moisture are simulated by a LES model to obtain a database for the improvement. Secondly, based on the LES data, closure constants are re-evaluated and a new diagnostic equation for the master length scale L is proposed. The new equation is characterized by allowing L in the surface layer to vary with stability instead of constant kz, where k is the von Kármán constant, and z is height.The non-dimensional eddy-diffusivity coefficients calculated from the modifiedM–Y model are in satisfactory agreement with those from the LES data. It isfound that the modified M–Y model improves the original one largely, and thatthe improvement is achieved by considering buoyancy effects on the pressurecovariances andby using the newly proposed equation for L. 相似文献
3.
多普勒声雷达对化音地区风场结构探测的初步分析 总被引：2，自引：1，他引：1
本文利用1990年8月多普勒声雷达在“HEIFE”区化音站探测的资料,对该地区的PBL结构进行了初步分析。结果表明,用声雷达回波强度的双对数廓线确定大气边界层高度仍不失为一有效的方法。大气边界层高度演变过程的特点是:白天对流层边界发展较快,但维持时间较短;而夜间稳定边界层维持时间较长,且在其发展较高时,在近地层又发展出一个新的稳定层结,呈现多层次的SBL结构。ML内垂直风速方差的归一化结果与Lenschow大致相同。另外,在一次冷锋过境天气背景下风场出现低空急流结构。 相似文献
4.
Two cases of on-ice and off-ice air flow characterizing the opposite weather situations over the ice-edge zone in the northern Baltic Sea are analysed on the basis of aircraft observations, and modelled using atwo-dimensional mesoscale model. The stable boundary layer (SBL) during theon-ice flow exhibited little thermal modification, but a low-level jet (LLJ) was generated at the 250-m high top of the SBL. In the model, the LLJ was associated with inertial oscillations in space, while the baroclinicity explained the shape of the wind profile well above the SBL. Although the observed LLJ was most pronounced over the ice, the modelling suggests that it was not generated by the ice edge but by the coastline some 400 km upwind of the ice edge, where a much more drastic change in the thermal stratification and surface roughness took place. The generation, maintenance, and strength of the LLJ were very sensitive to the parameterization of turbulent mixing in the SBL. In the case of the off-ice flow, the modification of the air mass and the development of a convective boundary layer (CBL) both over the ice and open sea were reasonably well modelled. Sensitivity runs suggested that it was essential to take into account the effects of subgrid-scale leads, a forest in the archipelago (which was crossed by the air flow), and water vapour condensationinto ice crystals. The heat flux from leads was particularly important for the heatbudget of the CBL, and the observed growth of the CBL was partly due to theeffective mixing over the rough and relatively warm forest. 相似文献
5.
The structures and the vertical profiles of turbulent variance and covariance of the stably stratified boundary layer (SBL) are simulated with a second-order closure turbulence model. The results confirm that the vertical profiles of the dimensionless turbulence variance and covariance can be well represented by the form F = A(1 - Z / h)x. Here h is the height of SBL. and both exponent a and coefficient A are the functions of terrain, baroclinicity, radiation cooling and the state of temporal development of SBL. Comparing with Minnesota and Cabauw experiment data, we have analysed the value of a and expounded the main reasons that great difference in a exists among different literatures. 相似文献
6.
Zbigniew Sorbjan 《Boundary-Layer Meteorology》2014,151(3):407-428
A single-column model of the evolving stable boundary layer (SBL) is tested for self-similar properties of the flow and effects of ambient forcing. The turbulence closure of the model is diagnostic, based on the K-theory approach, with a semi-empirical form of the mixing length, and empirical stability functions of the Richardson number. The model results, expressed in terms of local similarity scales, are universal functions, satisfied in the entire SBL. Based on similarity expression, a realizability condition is derived for the minimum allowable turbulent heat flux in the SBL. Numerical experiments show that the development of “horse-shoe” shaped, fixed-elevation hodographs in the interior of the SBL around sunrise is controlled by effects imposed by surface thermal forcing. 相似文献
7.
The Coupling State of an Idealized Stable Boundary Layer 总被引：1，自引：1，他引：0
Otávio C. Acevedo Felipe D. Costa Gervásio A. Degrazia 《Boundary-Layer Meteorology》2012,145(1):211-228
The coupling state between the surface and the top of the stable boundary layer (SBL) is investigated using four different schemes to represent the turbulent exchange. An idealized SBL is assumed, with fixed wind speed and temperature at its top. At the surface, two cases are considered, first a constant temperature, 20 K lower than the SBL top, and later a constant 2 K h−1 cooling rate is assumed for 10 h after a neutral initial condition. The idealized conditions have been chosen to isolate the influence of the turbulence formulations on the coupling state, and the intense stratification has the purpose of enhancing such a response. The formulations compared are those that solve a prognostic equation for turbulent kinetic energy (TKE) and those that directly prescribe turbulence intensity as a function of atmospheric stability. Two TKE formulations are considered, with and without a dependence of the exchange coefficients on stability, while short and long tail stability functions (SFs) are also compared. In each case, the dependence on the wind speed at the SBL top is considered and it is shown that, for all formulations, the SBL experiences a transition from a decoupled state to a coupled state at an intermediate value of mechanical forcing. The vertical profiles of potential temperature, wind speed and turbulence intensity are shown as a function of the wind speed at the SBL top, both for the decoupled and coupled states. The formulation influence on the coupling state is analyzed and it is concluded that, in general, the simple TKE formulation has a better response, although it also tends to overestimate turbulent mixing. The consequences are discussed. 相似文献
8.
9.
Mesoscale measurements of the vertical dispersion coefficient 2 by using a composite turbulence water tank were validated through a comparison with CONDORS (Convective Diffusion Observed with Remote Sensors) field data, and were analysed with respect to the intensity of the thermal flux, mechanical turbulence, and plume release height.It seems possible to correct the plume z values for different release heights below 0.5zi (zi is the mixing height) by applying an equation expressing the height dependency of turbulence intensity. The downwind distance where the plume's mass centre height approaches its final level was also analysed with respect to the above three parameters, and an empirical equation to estimate the downwinddistance derived. 相似文献
10.
On the evolution of the height and temperature difference across the nocturnal stable boundary layer
Evidence is furnished which indicates that in some cases the height of the stable boundary layer (SBL) and the magnitude of the temperature difference across the SBL may be more appropriately described by an error function erf(t/) rather than the generally accepted square root time dependence. The time constant was observed to have values of one to three hours. The discovery has been found to be site dependent, however, as data from other sites follow the usual square root time evolution. 相似文献
11.
Analysis of Atmospheric Boundary Layer Height Characteristics over the Arctic Ocean Using the Aircraft and GPS Soundings 
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下载免费PDF全文 Utilizing aircraft sounding data collected from the Surface Heat Budget of the Arctic Ocean (SHEBA, 1998) campaign, the authors evaluated commonly-used profile methods for Arctic ABL height estimation by validating against the’true’ABL height from aircraft sounding profiles, where ABL height is defined as the top of the layer with significant turbulence. Furthermore, the best performing method was used to estimate ABL height from the one-year GPS soundings obtained during SHEBA (October 1997-October 1998). It was found that the temperature gradient method produces a reliable estimate of ABL height. Additionally, the authors determined optimal threshold values of temperature gradient for stable boundary layer (SBL) and convective boundary layer (CBL) to be 6.5 K/100 m and 1.0 K/100 m, respectively. The maximum ABL height during the year was 1150 m occurred in May. Median values of Arctic ABL height in May, June, July, and August were 400 m, 430 m, 180 m, and 320 m, respectively. Arctic ABL heights are clearly higher in the spring than in the summer. 相似文献
12.
西北干旱区夏季大气边界层结构及其陆面过程特征 总被引：18，自引：3，他引：15
在中国西北干旱区影响大气边界层形成和发展的气候环境和大气环流背景都具有一定特殊性.文中用外场观测试验资料,分析了位于西北干旱区的敦煌荒漠夏季大气边界层气象要素结构特征,发现该地区无论白天的对流边界层还是夜间的稳定边界层均比一般地区更深厚.在夏季晴天,夜间稳定边界层厚度超过900 m,最厚可以达到1750 m,其上的残余层一般能达到4000 m左右的高度;白天混合层最高达3700 m,混合层顶的逆温层顶盖的厚度大约450 m,甚至更厚,对流边界层厚度能够超过4000 m,对流边界层进入残余层后发展十分迅速.研究表明,白天深厚的对流边界层是夜间保持清晰而深厚的残余混合层的先决条件,夜间深厚的残余混合层又为白天对流边界层的发展提供了一个非常有利的热力环境条件.该地区经常性出现连续性晴天使得大气残余层的累积效应得以较长时间持续发展,创造了比较有利于大气对流边界层发展的大气热力环境条件.同时,该地区陆面过程和近地层大气运动特征也为这种独特的大气热力边界层结构提供了较好的支持.就该地区发展超厚大气对流边界层的物理机理而言,地表显著增温是强有力的外部热力强迫条件,近地层强感热通量提供了较充足的能量条件,较大的对流运动和湍流运动的速度是必要的运动学条件,大气残余层的累积效应提供了有利的热力环境条件. 相似文献
13.
Large-Eddy Simulation of stable boundary layers (SBLs) has been considered particularly difficult, indeed perhaps impossible with present computational resources. Here we present a new series of successful simulations of SBLs over uniform, flat terrain, using an approach previously successful for neutral and convective conditions, and showing that essentially the same model can handle all three main dry types of atmospheric boundary layer. We consider both technical requirements for successful and accurate SBL simulations and the observed characteristics of the simulated SBL. We discuss the evolution (in some cases to quasi-steady states) and compare with theory and experimental data. Effects of static-stability on the flow are analyzed using one-point and two-point statistics. Results show the development of a shear-driven SBL, with little sign of distinctively wavelike motions. The flow statistics are found to be consistent with local scaling, and that framework is used to compare with other data and theoretical models. 相似文献
14.
Yuji Ohya 《Boundary-Layer Meteorology》2001,98(1):57-82
Wind-tunnel simulations of theatmospheric stable boundary layer (SBL) developedover a rough surface were conducted by using athermally stratified wind tunnel at the Research Institutefor Applied Mechanics (RIAM), Kyushu University. Thepresent experiment is a continuation of the workcarried out in a wind tunnel at Colorado StateUniversity (CSU), where the SBL flows were developed over asmooth surface. Stably stratified flows were createdby heating the wind-tunnel airflow to a temperature ofabout 40–50°and by cooling the test-section floor toa temperature of about 10°. To simulate therough surface, a chain roughness was placed over thetest-section floor. We have investigated the buoyancyeffect on the turbulent boundary layer developed overthis rough surface for a wide range of stability,particularly focusing on the turbulence structure andtransport process in the very stable boundary layer.The present experimental results broadly confirm theresults obtained in the CSU experiment with the smoothsurface, and emphasizes the following features: thevertical profiles of turbulence statistics exhibitdifferent behaviour in two distinct stability regimes with weak and strong stability,corresponding to the difference in the verticalprofiles of the local Richardson number. The tworegimes are separated by the critical Richardsonnumber. The magnitudes in turbulence intensities andturbulent fluxes for the weak stability regime aremuch greater than those of the CSU experiments becauseof the greater surface roughness. For the very stableboundary layer, the turbulent fluxes of momentum andheat tend to vanish and wave-like motions due to theKelvin–Helmholtz instability and the rolling up andbreaking of those waves can be observed. Furthermore,the appearance of internal gravity waves is suggestedfrom cross-spectrum analyses. 相似文献
15.
The experiment IGLOS (Investigation of the Greenland Boundary Layer Over Summit) was conducted in June and July 2002 in the central plateau of the Greenland inland ice. The German research aircraft Polar2, equipped with the turbulence measurement system Meteopod, was used to investigate turbulence and radiation flux profiles near research station “Summit Camp”. Aircraft measurements are combined with measurements of radiation fluxes and turbulent quantities made from a 50 m tower at Summit Camp operated by Eidgenössische Technische Hochschule (ETH) Zürich. During all six flight missions, well-developed stable boundary layers were found. Even in high-wind conditions, the surface inversion thickness did not exceed roughly 100 m. The turbulent height of the stable boundary layer (SBL) was found to be much smaller than the surface inversion thickness. Above the surface layer, significant turbulent fluxes occurred only intermittently in intervals on the order of a few kilometres. Turbulent event fraction in the upper SBL shows the same dependence on gradient Richardson number as reported for near-surface measurements. Clear-air longwave radiation divergence was always found to contribute significantly to the SBL heat budget. In low-wind cases, radiative cooling even turned out to be dominant. 相似文献
16.
The Influence Of Urban Canopy Configuration On Urban Albedo 总被引：6，自引：0，他引：6
Akira Kondo Megumi Ueno Akikazu Kaga Katsuhito Yamaguchi 《Boundary-Layer Meteorology》2001,100(2):225-242
We propose a calculation method for shortwave radiation flux and longwave radiation flux within the urban canopy and investigate the influence of urban canopy configuration on net radiation flux. In the assumed urban configuration, buildings of equal size are arranged in a regular lattice within the urban canopy. The net shortwave radiation flux and longwave radiation flux within the urban canopy were calculated by the photon tracking method based on the Monte Carlo method. The albedo value obtained by this method shows close agreement with experimental data, and the average sky view factor shows almost perfect agreement with the theoretical value. Moreover, we calculated the urban albedo for the urban canopy configuration including roads and building height distribution.%Moreover, we calculated net radiation within the urban canopy in %consideration of roads and building height distribution.We found that the sky view factor of the ground surface is high when building coverage is low, building height is low, open space by roads exists, and building height is non-uniform. Moreover, we found that the albedo value is high when building height is small, open space by roads is wide, and building height is uniform. The albedo value was found to vary in a complicated manner with change in building coverage. 相似文献
17.
18.
J.R. Philip 《Boundary-Layer Meteorology》1997,84(1):85-98
Previous studies of blending heightsfor scalars, using periodic solutions of atmosphericconvection-diffusion equations, raise two questions. 1. What is the relation between growth of the internalboundary-layer height downwind of a step in surfaceconditions, and the increase of blending height withdownwind pattern wavelength? 2. Can the approach beextended to shear stress? The answers are: 1. Thedependence of boundary-layer height on fetch has thesame functional form as that of blending height onwavelength; but it is greater by a factor of about1.9. 2. Representing eddy diffusivity as the productof mean windspeed and a power of height gives a linearhorizontal momentum equation. A further approximationgives a convection-diffusion equation for shearstress, analogous to those used previously forscalars. 相似文献
19.
In this paper,the continuity and thermodynamic equations including moisture forcings were derived.Using these two equations and the basic momentum equation of local Cartesian coordinates,the budget equation of generalized moist potential vorticity(GMPV) was derived.The GMPV equation is a good generalization of the Ertel potential vorticity(PV) and moist potential vorticity(MPV) equations.The GMPV equation is conserved under adiabatic,frictionless,barotropic,or saturated atmospheric conditions,and it is closely associated with the horizontal frontogenesis and stability of the real atmosphere.A real case study indicates that term diabatic heating could be a useful diagnostic tool for heavy rainfall events. 相似文献
20.
The Role of Shear in the Morning Transition Boundary Layer 总被引：1，自引：1，他引：0
Robert J. Beare 《Boundary-Layer Meteorology》2008,129(3):395-410
We use large-eddy simulation (LES) to better define the early stages of the morning transition boundary layer. Previous LES
studies relating to the morning transition boundary layer focus on the role of the entraining convective boundary layer (CBL).
By using a combination of different domain sizes and grid lengths, the full evolution from the stable boundary layer (SBL)
to the CBL is modelled here. In the early stages of the morning transition the boundary layer is shown to be a combination
of a shallow mixed layer capped by a significant shear driven stable boundary layer (the so-called mixed CBL–SBL state). The
mixed CBL–SBL state is the key to understanding the sensitivity to shear. Turbulent kinetic energy budgets also indicate that
it is shear driven. The negative flux from the mixed CBL–SBL state extends much further above the minimum than is typically
found for the CBL later in the day, and the depth of penetration scales as w
m
/N
i
, where w
m
is the combined friction and convective velocity scale and N
i
the static stability at the inversion top. 相似文献