Turbulence Characteristics In The Surface Boundary Layer Over The South American Pampa

This paper presents results of turbulence measurementsmade in the south of Brazil in the Pampa region.Data collected at 1Hz are used to calculatestandard deviations of temperature and velocities. Onthe other hand data collected at 10Hz areused to study the behaviour of spectra and cospectraof turbulence in the surface layer. Dimensionless dissipation rates of turbulent kinetic energy and temperature variance are also presented. The frameworkof Monin–Obukhov Similarity theory is used and allresults are compared with other experimentalstudies.     

Turbulence Characteristics Over Complex Terrain In West China

Meteorological data of velocity components and temperature have been measured on a mast of height 4.9 m at one site in the Heihe River Basin Field Experiment (HEIFE) conducted in west China. Mean and individual turbulence parameters, power spectra/cospectra, phase angles and their changes withfetch downwind of a change in surface roughness were analyzed. The turbulence characteristics depend strongly on the prevailing wind direction, which in turn is associated with changes in the upwind surface roughness pattern. The results show that values of horizontal velocity standard deviations sigma;_u,v scaled with local friction velocity u under different stratifications are larger than those over flat terrain, while the values of _w/u have the same values as over flat terrain. The differences between variance values of the horizontal velocity components, u and v, over inhomogeneous terrain were found to be significantly smaller than those over flat terrain. Since energy densities of the w spectra, uw and wT cospectra at low frequencies are relatively lower than those of longitudinal velocity spectra, total energies of w spectra, uw and wT cospectra tend to be in equilibrium with the local terrain. The values of phase angles at the low frequency end of the frequency showed obvious differences associated with changes of roughness.     

Observations Of The Turbulence Structure Within Two Stratocumulus-Topped, Marine Boundary Layers

Two situations observed during the second Aerosol Characterization Experiment (ACE-2) are analysed from aircraft measurements in the broken stratocumulus (Sc)-topped marine boundary layer. The first one (26 June 1997), characterized by a non-polluted, oceanic air mass, presents a decoupling between the Sc layer (1400–1520 m) and the turbulent mixed layer, this latter extending from the surface up to 580 m. In contrast, the second case (9 July 1997), during which continental air had been advected over the experimental area, presents a well-coupled layer extending from the surface up to the top of the Sc layer(910 m). This coupling, uncommon in this area in the middle of the day, isrelated to the relative shallowness of the boundary layer. For both situations,it is shown that the turbulent fluxes can be computed with reasonably goodaccuracy (better than 10 %), taking into account both the random and thesystematic errors involved in the eddy-correlation technique. Estimationof random error is based on the computation of the integral scale of thecovariance, and systematic error is estimated from the parameterizationof Mann and Lenschow. The fluxes show that the buoyancy, as a sourceof turbulence, is due to latent heat flux rather than sensible heat flux,with values comparable to previous experiments in the Azores-Canariesbasin. In addition, we propose a method to analyse, for coupled situations,the relationship between the fractional cloudiness and the organization ofthe turbulent field below the clouds. This method is based on a conditionalsampling technique. It is shown that this organization cannot be deducedfrom the analysis of the velocity signal, which is dominated by turbulence.However, when the signals are conditionally sampled according to thepresence or absence of clouds, a weak cloud-related organization can beshown, and the cloud-related transports quantified; the values found areof the order of 10 % of the total transfers, i.e. the same order of magnitude asthe errors on the total flux computation. The method developed is thereforepromising, provided that the uncertainties can be reduced by analyzing a highamount of data.     

Aircraft-Based Measurements Of Turbulence Structures In The Katabatic Flow Over Greenland

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.     

Simulation of a Stably Stratified Atmospheric Boundary Layer Using One-Dimensional Turbulence

One-dimensional turbulence (ODT) is a single-column simulation in which vertical motions are represented by an unsteady advective process, rather than their customary representation by a diffusive process. No space or time averaging of mesh-resolved motions is invoked. Molecular-transport scales can be resolved in ODT simulations of laboratory-scale flows, but this resolution of these scales is prohibitively expensive in ODT simulations of the atmospheric boundary layer (ABL), except possibly in small subregions of a non-uniform mesh.Here, two methods for ODT simulation of the ABL on uniform meshes are described and applied to the GABLS (GEWEX Atmospheric Boundary Layer Study; GEWEX is the Global Energy and Water Cycle Experiment) stable boundary-layer intercomparison case. One method involves resolution of the roughness scale using a fixed eddy viscosity to represent subgrid motions. The other method, which is implemented at lower spatial resolution, involves a variable eddy viscosity determined by the local mesh-resolved flow, as in multi-dimensional large-eddy simulation (LES). When run at typical LES resolution, it reproduces some of the key high-resolution results, but its fidelity is lower in some important respects. It is concluded that a more elaborate empirically based representation of the subgrid physics, closely analogous to closures currently employed in LES of the ABL, might improve its performance substantially, yielding a cost-effective ABL simulation tool. Prospects for further application of ODT to the ABL, including possible use of ODT as a near-surface subgrid closure framework for general circulation modeling, are assessed.     

Vertical Structure Of Turbulence In Offshore Flow During Rasex

The adjustment of the boundary layer immediately downstream froma coastline is examined based on two levels of eddy correlation data collected on a mast at the shore and six levels of eddy correlation data and profiles of mean variables collected from a mast 2 km offshore during the Risø Air-Sea Experiment. The characteristics of offshore flow are studied in terms of case studies and inter-variable relationships for the entire one-month data set. A turbulent kinetic energy budget is constructed for each case study.The buoyancy generation of turbulence is small compared to shear generation and dissipation. However, weakly stable and weakly unstable cases exhibit completely different vertical structure. With flow of warm air from land over cooler water, modest buoyancy destruction of turbulence and reduced shear generation of turbulence over the less rough sea surface cause the turbulence to rapidly weaken downstream from the coast. The reduction of downward mixing of momentum by the stratification leads to smaller roughness lengths compared to the unstable case. Shear generation at higher levels and advection of stronger turbulence from land often lead to an increase of stress and turbulence energy with height and downward transport of turbulence energy toward the surface.With flow of cool air over a warmer sea surface, a convective internal boundary layer develops downstream from the coast. An overlying relatively thick layer of downward buoyancy flux (virtual temperature flux) is sometimes maintained by shear generation in the accelerating offshore flow.     

Turbulence Characteristics Of The Stable Boundary Layer Over A Mid-Latitude Glacier. Part Ii: Pure Katabatic Forcing Conditions

Observations obtained over a glacier surface in a predominantlykatabatic flow and with a distinctwind maximum below 13-m height are presented. The data werecollected using a 13-m high profilemast and two sonic anemometers (at about 2.5-m and 10-m heights).The spectra at frequencies belowthat of the turbulence range appear to deviate considerably fromthe curves obtained by Kaimal andco-workers during the 1968 Kansas experiment. The characteristicsof these deviations are compared tothe observations of others in surface-layers disturbed by anykind of large-scale outer-layer (orinactive) turbulence. In our case the disturbances arelikely to be induced by the highmountain ridges that surround the glacier. Moreover, the deviationsobserved in the cospectra seemto result from an, as yet, unspecified interaction between theinactive outer-layer turbulenceand the local surface-layer turbulence. Near the distinctwind maximum turbulence production ceasedwhile turbulence itself did not, probably the result ofturbulence transport from other levels. Consequently, we studied thelocal similarity relations using _w instead of u_* as an alternative velocity scale. Wellbelow the wind maximum, and for relatively low stability(0< Ri_g <0.2), the flow behaves accordingto well established local-scaling similarity relationshipsin the stable boundary layer. For higherstability (Ri_g > 0.2), and near or above the wind maximum, the boundary-layer structure conforms tothat of z-less stratification suggesting that the eddy sizeis restricted by the local stability ofthe flow. In line with this we observed that the sensibleheat fluxes relate remarkably well to thelocal flow parameters.     

大气边界层阵风相干结构的产生条件

壁湍流相干结构的发现是近代湍流研究的重大进展之一,从20世纪50年代开始,在大气边界层湍流中也发现了相干结构——对流云街,并进行了系统的研究。近些年来,人们发现在近地层湍流中也存在相干结构。利用北京325 m气象塔对城市下垫面中大风和小风天气的风速分析,发现较有规律的周期3~6 min的阵风,且有明显的相干结构,而对不同下垫面的阵风研究,均发现存在这种相干结构,这种阵风相干结构对通量输送有不可忽视的作用。本文利用2012年4月甘肃省民勤县巴丹吉林沙漠观测塔的超声风速和平均场风速、温度观测资料,对阵风相干结构的产生条件进行了分析。采用傅立叶变换,将三维超声风速按频率分成基流(周期10分钟以上)、阵风扰动(周期1到10分钟)、湍流脉动(周期小于1分钟)三部分,结合平均场的资料分析发现:阵风相干结构出现在静力中性、不稳定甚至略微稳定的条件下,或者说机械作用主导的大气边界层,阵风区就会出现相干结构,热力作用对其有抑制和干扰的作用。从而,阵风的相干结构和壁面相干结构都出现在中性条件下,是机械湍流的现象,都主导着动量能量的输运。阵风区的相干结构并不等同于对流云街,他们出现在不同的大气稳定度条件下且尺度不同。     

Characterization Of Atmospheric Turbulence By Dynamical Systems Techniques

We describe a dimensional analysis for different time series of the vertical component of wind velocity from half-hourly sonic anemometermeasurements. The goal is a characterization of the atmospheric turbulence fromthe point of view of Dynamical Systems Theory, based on the correlation dimension of the strange attractor. Our results suggest that the convective turbulence correlation dimension (values 6) is lowerthan the mechanical one (values 7–9).     

Nonuniform Distribution Of High-Frequency Turbulence In The Unstable Boundary Layer

Turbulent flow data of wind velocity and temperature in the unstably stratifiedatmospheric boundary layer, derived from steel tower observations in the field and wind-tunnel experiments were used to study the relationship between the plumes and the small-scale eddies in the inertial subrange. Flow visualisation experiments in the wind tunnel were also conducted to observe the structure of the flow in the plumes, and time series data were analysed by using wavelet transforms. The results show that variances of velocity and temperature due to the small-scale eddies are large in the plumes and small outside of the plumes, and that the momentum and heat fluxes due to the small-scale eddies follow the same tendency as found in the variances. The ratios of the variances caused by the small-scale eddies in the plumes to the whole of the variances caused by the small-scale eddies in and out the plumes increase with non-dimensional height -z/L in which L is the local Obukhov length. Similar ratios of the fluxes caused by the small-scale eddies also show the same tendency. These ratios can be expressed as functions of -z/L for results based on field observationand the wind tunnel experiments. This relation hardly changes even if the wavelet function is changed. The flow visualisation experiments show that the plumes have a complicated structure in which mushroom type flows are stacked on top of each other. This characteristic structure seems to increase the energy of the small-scale eddies in the plumes.     

不同下垫面湍流统计特征研究

利用戈壁、草原、郊区和城郊等不同下垫面进行的湍流观测资料,使用相同的湍流数据采集和处理方法,研究了不同下垫面湍流统计特征。结果表明：在不稳定层结下,三个方向分量风速归一化标准差σu／u*,σv／u*,σw／u*与稳定度参数z／L呈1／3幂次关系;水平方向速度归一化标准差随地表粗糙度的增加而降低,垂直方向的速度、温度和湿度没有明显差异。文中给出了不同下垫面近中性层结下风速归一化标准差的数值。研究还发现：较湿润地区的湿度统计特征与温度的相似。     

Spatial and Temporal Occurrences of Intermittent Turbulence During CASES-99

Occurrences of intermittent turbulence in very stable conditions during theCASES-99 field study near Leon, Kansas were detected at several sites separatedby horizontal distances from 1 km to 25 km using sonic anemometers, minisodarsand a laser scintillometer. Periods with significant turbulent heat fluxes wereseparated by extended quiescent periods with little or no flux, and most of theflux during a night was realized in relatively small fractions (<20%) of thetotal time. There appeared to be no relationship between this intermittencyfraction and the median z/L (z being height and L the Obukhov length)value for the night, although overall sensible heat flux values on very stablenights were significantly less than those on less stable nights. The intermittencyfraction at 7 m was found to increase with mean wind speed at 20 m and, to alesser extent, with wind shear between 20 m and 30 m. While correspondenceof turbulent episodes at two sites separated by 1 km was common, it was less common at separations on the order of 20 km. There were time periods, however, during which enhanced turbulence levels were seen nearly simultaneously at large separation distances. Turbulence episodes were found to propagate upward or downward at different times with no readily defined large-scale controlling mechanism.     

Turbulence Structures Over The Marginal Ice Zone Under Flow Parallel To The Ice Edge: Measurements And Parameterizations

Three aircraft-based studies of boundary-layer fronts (BLFs) werecarried out during the experiment KABEG in April 1997 near thesea-ice edge over the Davis Strait. The boundary-layer flow wasparallel to the ice edge and hence two independent turbulent regimescould develop in an identical synoptic framework, separated by thefrontal zone along the ice edge. The zone of strongest crosswindhorizontal gradients was typically 20 km wide, while the downstreamscale of the BLF was observed to be several hundreds of kilometres.For two of the three cases the investigation of turbulence structureswas possible and the results are given herein.Horizontal and vertical profiles of turbulent fluxes and other turbulentquantities are presented. A spectral analysis reveals the coexistence ofsmall-scale turbulence with roll motions. These roll motions can behidden or can be visible as cloud streets. The associated transportmechanisms are highly relevant for the choice of suitable averagingintervals for turbulent flux calculations and model validation.Parameterizations for the vertical velocity variance, countergradienttransport, sea surface roughness and eddy diffusivity are evaluatedand compared for this baroclinic strong-wind convective boundary-layerenvironment. Analogously, drag coefficients and bulk transfer coefficientsare derived from measurements.     

Turbulence Closures In Neutral Boundary Layer Over Complex Terrain

In this work, three turbulence closure models, Mellor andYamada level 2.5, E - l and E - implemented in a circulation model, are compared in neutral condition over complex terrain. They are firstly applied to a one-dimensional case on flat terrain and then to a schematic two-dimensional valley. The simulation results, in terms of wind field and turbulent kinetic energy, are tested against measurements from a wind-tunnel experiment. The empirical constants defining the characteristic length scales of the closures are modified based on turbulence parameters estimated in the experiment. The formulation of the diffusion coefficients is analysed to explain the differences among the various closures in the simulation results. Regarding the mean flow, both on flat and complex terrain, all the closures yield satisfactory results. Concerning the turbulent kinetic energy, the best results are obtained by E - l and E - closures.     

Turbulent and Thermodynamic Structure of the Autumnal Arctic Boundary Layer Due to Embedded Clouds

We have studied the role of low-level clouds in modifying the thermodynamic and turbulence properties of the Arctic boundary layer during autumn. This was achieved through detailed analyses of boundary-layer properties in two regions, one with low-level cloud cover and the other free of clouds, using measurements from a research aircraft during the Beaufort and Arctic Storms Experiment (BASE). Both regions were measured on the same day under similar synoptic forcing. The cloudy region was characterized by strong horizontal inhomogeneity in low-level temperature and moisture that varied with the cloud-top height. The clear region was relatively homogeneous in temperature and specific humidity with a strong temperature inversion extending between heights of 100 m and 3 km. From measurements at the lowest levels, we also identified a shallow mixed layer below the deep stable layer in the clear region.Our spectral analyses revealed significant modifications of boundary-layer properties due to the presence of low-level clouds. In the cloudy region, turbulent perturbations dominated the boundary-layer flow and made large contributions to the scalar variances. In the clear boundary-layer, wave motion contributed significantly to the observed variances, while turbulent flow was relatively weak. The clear region was saturated, although no detectable clouds were measured.     

Intermittent Bursting of Turbulence in a Stable Boundary Layer with Low-level Jet

The atmospheric stable boundary layer (SBL) with a low-level jet is simulated experimentally using a thermally stratified wind tunnel. The turbulence structure and flow characteristics are investigated by simultaneous measurements of velocity and temperature fluctuations and by flow visualization. Attention is focused on the effect of strong wind shear due to a low-level jet on stratified boundary layers with strong stability. Occasional bursting of turbulence in the lower portion of the boundary layer can be found in the SBL with strong stability. This bursting originates aloft away from the surface and transports fluid with relatively low velocity and temperature upward and fluid with relatively high velocity and temperature downward. Furthermore, the relationship between the occurrence of turbulence bursting and the local gradient Richardson number (Ri) is investigated. The Ri becomes larger than the critical Ri, Ri_cr = 0.25, in quiescent periods. On the other hand, the Ri number becomes smaller than Ri_cr during bursting events.     

The Coherent Structure of Water Vapour Transfer in the Unstable Atmospheric Surface Layer

Observations of water vapour fluctuations over arice field show vapour ramps. Coherent structuresare first revealed by the frequently occurring ramp pattern in the vapourtrace. Wavelet and pseudo-wavelet analysis techniques were used inconditional sampling, and more than 100 hr of data have been analyzedto determine coherent structure characteristics. The most probablecoherent structure duration was in the range 2–12 sec andthe duration range of the most effective coherent structures shows somedifference between heat and water vapour transfers. Coherent structurescontribute to the major part of the total flux.     

The Turbulence Structure of the Stable Atmospheric Boundary Layer Around a Coastal Headland: Aircraft Observations and Modelling Results

The turbulence structure of a stable marine atmospheric boundary layer in the vicinity of a coastal headland is examined using aircraft observations and numerical simulations. Measurements are drawn from a flight by the NCAR C-130 around Cape Mendocino on the coast of northern California on June 7 1996 during the Coastal Waves 96 field program. Local similarity scaling of the velocity variances is found to apply successfully within the continuously turbulent layer; the empirical scaling function is similar to that found by several previous studies. Excellent agreement is found between the modelled and observed scaling results. No significant change in scaling behaviour is observed for the region within the expansion fan that forms downstream of the Cape, suggesting that the scaling can be applied to horizontally heterogeneous conditions; however, the precise form of the function relating scaled velocities and stability is observed to change close to the surface. This result, differences between the scaling functions found here and in other studies, and the departure of these functions from the constant value predicted by the original theory, leads us to question the nature of the similarity functions observed. We hypothesize that the form of the functions is controlled by non-local contributions to the velocity variance budgets, and that differences in the non-local terms between studies explain the differences in the observed scaling functions.     

The Adaptation of the Atmospheric Boundary Layer to a Change in Surface Roughness

The adaptation of the atmospheric boundary layer to a change in the underlying surface roughness is an interesting problem and hence much research, theoretical, experimental, and numerical, has been undertaken. Within the atmospheric boundary layer an accurate numerical model for the turbulent properties of the atmospheric boundary layer needs to be implemented if physically realistic results are to be obtained. Here, the adaptation of the atmospheric boundary layer to a change in surface roughness is investigated using a first-order turbulence closure model, a one-and-a-half-order turbulence closure model and a second-order turbulence closure model. Perturbations to the geostrophic wind and the pressure gradients are included and it is shown that the second-order turbulence closure model, namely the standard k - model, is inferior to a lower-order closure model if a modification to limit the turbulent eddy size within the atmospheric boundary layer is not included within the model.     

Impact of Turbulence Closures on Diurnal Temperature evolution for Clear Sky Situations Over Belgium

The aim of this study is to quantify the impact of turbulence closure on the simulation of surface air temperature at screen height (1.5 m) over Belgium. The mesoscale model MAR (Modèle Atmosphérique Régional), developed at the Université catholique de Louvain, is used to examine one-dimensional situations. A new second-order closure (level 2.5) is implemented containing prognostic equations for all three velocity variances, and diagnostic or prognostic formulations for the dissipation. This closure is compared with first and one-and-a-half order closures. Idealized nearly-neutral and convective cases underline the differences between first and second-order closures, and between diagnostic and prognostic equations for the dissipation. The one-and-a-half and second-order closures give satisfying results, but the first-order closure produces generally less appropriate vertical diffusion. Observed clear sky and weak horizontal advection situations have shown the sensitivity of 24 h temperature evolution to the choice of the turbulent closure.