Large-Eddy Simulations of the Stable Boundary Layer Using the Standard Kolmogorov Theory: Range of Applicability

Large-eddy simulations (LES) of the Stable Atmospheric Boundary Layer (SBL) are difficult because the turbulence is not isotropic for strong stratification and the Kolmogorov theory might be no longer valid. This fact compells us to work on modifications to the subgrid turbulence schemes, although there is not any widely accepted theory on anisotropic turbulence. In this work, a LES model is used to see what range of stable stratification can still be simulated with a subgrid turbulence scheme using the Kolmogorov theory for the dissipation. Twenty simulations of increasing stability have been performed using a horizontal resolution of 5 m. The model is able to simulate weakly and moderately stable conditions and experiences runaway cooling for strong stability. The goodness of the successful simulations is inspected through comparison to observations from the experimental campaigns SABLES-98 and CASES-99. Other supplementary tests have been performed on the resolution and the surface boundary condition.     

Theory And Measurements For Turbulence Spectra And Variances In The Atmospheric Neutral Surface Layer

Predictions from a new theory for high Reynolds number turbulent boundary layers during near-neutral conditions are shown to agree well with measurements of atmospheric surface-layer variances and spectra. The theory suggests surface-layer turbulence is determined by detached eddies that largely originate in the shearing motion immediately above the surface layer; as they descend into this layer, they are strongly distorted by the local shear and impinge onto the surface. Because the origin of these eddies is non-local, they are similar to those described in previous studies as `inactive' turbulence. However, they are, in fact, dynamically highly active, supplying the major mechanism for the momentum transport, including upward bursting on the time scale of the larger eddies. The vertical velocity results show that the variance and the low frequency parts of spectra increase with height in the surface layer, while in the self similar (k₁ ^-1) range the streamwise low frequency components are approximately constant with height. These large-scale longitudinal eddies extend to a length _s, which is equal to the boundary-layer height near the surface andincreases linearly to a maximum of about three times the boundary-layer height at roughly 15 m and decreases in the upper parts of the surface layer. This lower part of the surface layer, the eddy surface layer, is the region in which the eddies impinging from layers above are strongly distorted. This new result for the atmospheric boundary layer has practical application for calculating fluctuating wind loads on structures and lateral dispersion of pollution from local sources.     

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.     

On the Kinetic Energy Budget of the Unstable Atmospheric Surface Layer

We present a new account of the kinetic energy budget within an unstable atmospheric surface layer (ASL) beneath a convective outer layer. It is based on the structural model of turbulence introduced by McNaughton (Boundary-Layer Meteorology, 112: 199–221, 2004). In this model the turbulence is described as a self-organizing system with a highly organized structure that resists change by instability. This system is driven from above, with both the mean motion and the large-scale convective motions of the outer layer creating shear across the surface layer. The outer convective motions thus modulate the turbulence processes in the surface layer, causing variable downwards fluxes of momentum and kinetic energy. The variable components of the momentum flux sum to zero, but the associated energy divergence is cumulative, increasing both the average kinetic energy of the turbulence in the surface layer and the rate at which that energy is dissipated. The tendency of buoyancy to preferentially enhance the vertical motions is opposed by pressure reaction forces, so pressure production, which is the work done against these reaction forces, exactly equals buoyant production of kinetic energy. The pressure potential energy that is produced is then redistributed throughout the layer through many conversions, back and forth, between pressure potential and kinetic energy with zero sums. These exchanges generally increase the kinetic energy of the turbulence, the rate at which turbulence transfers momentum and the rate at which it dissipates energy, but does not alter its overall structure. In this model the velocity scale for turbulent transport processes in the surface layer is (kzɛ)^1/3 rather than the friction velocity, u_*. Here k is the von Kármán constant, z is observation height, ɛ is the dissipation rate. The model agrees very well with published experimental results, and provides the foundation for the new similarity model of the unstable ASL, replacing the older Monin–Obukhov similarity theory, whose assumptions are no longer tenable.     

Turbulence Structure of the Unstable Atmospheric Surface Layer and Transition to the Outer Layer

We present a new model of the structure of turbulence in the unstable atmospheric surface layer, and of the structural transition between this and the outer layer. The archetypal element of wall-bounded shear turbulence is the Theodorsen ejection amplifier (TEA) structure, in which an initial ejection of air from near the ground into an ideal laminar and logarithmic flow induces vortical motion about a hairpin-shaped core, which then creates a second ejection that is similar to, but larger than, the first. A series of TEA structures form a TEA cascade. In real turbulent flows TEA structures occur in distorted forms as TEA-like (TEAL) structures. Distortion terminates many TEAL cascades and only the best-formed TEAL structures initiate new cycles. In an extended log layer the resulting shear turbulence is a complex, self-organizing, dissipative system exhibiting self-similar behaviour under inner scaling. Spectral results show that this structure is insensitive to instability. This is contrary to the fundamental hypothesis of Monin--Obukhov similarity theory. All TEAL cascades terminate at the top of the surface layer where they encounter, and are severely distorted by, powerful eddies of similar size from the outer layer. These eddies are products of the breakdown of the large eddies produced by buoyancy in the outer layer. When the outer layer is much deeper than the surface layer the interacting eddies are from the inertial subrange of the outer Richardson cascade. The scale height of the surface layer, z _s, is then found by matching the powers delivered to the creation of emerging TEAL structures to the power passing down the Richardson cascade in the outer layer. It is z _s = u _* ³ /k_s, where u _* is friction velocity, k is the von Kármán constant and _s is the rate of dissipation of turbulence kinetic energy in the outer layer immediately above the surface layer. This height is comparable to the Obukhov length in the fully convective boundary layer. Aircraft and tower observations confirm a strong qualitative change in the structure of the turbulence at about that height. The tallest eddies within the surface layer have height z _s, so z _s is a new basis parameter for similarity models of the surface layer.     

AN EVALUATION OF SEVERAL TURBULENCE SCHEMES FOR THE PREDICTION OF MEAN AND TURBULENT FIELDS IN COMPLEX TERRAIN

A prognostic three-dimensional mesoscale model has been developed andused in one- and two-dimensional modes to evaluate ten local turbulenceclosure schemes. The schemes ranged from first-order to the two-equationprognostic schemes. Predictions by the models were compared for aone-dimensional convective boundary layer using mixed layer scaling andmeasurements to interpret the results. Two-dimensional simulations were alsoperformed for a sea-breeze flow and for flow over a hill. The results showedthat for all of the models considered, minor differences were produced in themean meteorological fields and in the vertical scalar fluxes, but majordifferences were apparent in the velocity variances and dissipation rate.Predicted tracer concentrations were very sensitive to the turbulence modelformulation for dispersion from a point source in the convective boundarylayer, particularly for the prediction of maximum concentrations. Predictedtracer concentrations from a surface volume source for the two-dimensionalsimulations were similar for all models, although the degree of mixing in themorning growth period produced some differences. Generally, good results forthe mean meteorological fields can be obtained with first-order schemes, evenif they underpredict the magnitude of turbulence in the convective boundarylayer, and reasonable tracer concentrations can also be obtained with thesemodels provided near-source effects are not important. The two-equationprognostic models performed best for the prediction of turbulence in theconvective boundary layer.     

湖南岳阳一次重度霾过程近地层物理量场异常特征分析

利用杭瑞高速公路洞庭湖大桥北岸测风塔的梯度风观测资料、三维超声风温仪资料以及岳阳气象站提供的逐小时气溶胶浓度和能见度观测资料,对湖南岳阳2017年1月28日的一次重度霾天气中的重污染过程的近地层物理量变化特征进行了分析,结果表明:（1）重污染来临前约130 min即28日01:50（北京时间,下同）,水平风速、垂直风速、高低层风切变都出现零值,大气处于静稳状态。重污染结束前180 min即28日09:00,上述物理量和高低层温度切变出现零值。（2）湍流强度在重污染来临前有强烈异常信号,其中水平纵向湍流强度异常信号最明显,于重污染发生前130 min出现异常峰值4.15,重污染结束前180 min出现异常峰值3.24。（3）湍流动能和动量通量都在重污染来临前130 min接近0.0 m2/s2,即湍流交换最弱,有利于污染物在近地面的持续堆积和重污染过程的发生。近地层的平均物理量和湍流特征量的异常信号的出现时间有较好的一致性,即出现在重污染来临前的130 min和结束前的180 min。揭示了重度霾污染天气的近地层物理量时间变化规律,着重分析了霾污染的生成、发展、消亡全过程的边界层湍流异常的前期信号,为深入认识霾污染天气进行有益的探索并为这类天气的预测预警提供科学依据。     

基于大涡模拟评估GRAPES模式对对流边界层的模拟性能

为检验GRAPES半拉格朗日动力框架在大涡尺度上的模拟性能,为未来发展千米及其以下高分辨尺度的数值模式奠定基础,并构造GRAPES大涡模式以检验和发展边界层湍流参数化提供科学工具。通过在GRAPES模式中加入Smagorinsky-Lilly小尺度湍涡参数化,并将模式分辨率提高至50 m,构建GRAPES大涡模式(GRAPES_LES),以便分析GRAPES模式在大涡尺度上的适用性。同时利用广泛应用的已有大涡模式UCLA_LES作为参考,通过对干对流边界层湍流的模拟分析及与UCLA_LES模拟结果的对比,得出如下主要结论:GRAPES半拉格朗日动力框架能够模拟出与已有的大涡模式相似的边界层湍流特征;同时,通过分析也证明GRAPES存在由于采用半拉格朗日平流计算而带来过度耗散的问题:当使用相同的滤波尺度(Smagorinsky 常数)时,GRAPES_LES模拟出的速度场更为平滑,小尺度湍流结构过于光滑,通过对湍流能量的能谱分析更清楚地表明了这一点。进一步,对不同的Smagorinsky常数(对应不同的滤波尺度)进行了敏感性试验,表明可以通过改变滤波尺度,有效地缓解半拉格朗日框架隐含的耗散问题,得到更接近UCLA_LES所模拟的湍流特征。     

Resolved-scale turbulence in the atmospheric surface layer from a large eddy simulation

Large eddy simulation has encountered difficulties in handling turbulence in the atmospheric surface layer due to deficiencies in sub-grid scale models. This paper addresses the possibility of resolving the turbulence in the upper part of the surface layer by a low-aspect ratio of grid spacing. Results show that resolved-scale shear stresses dominate over the sub-grid scale components so that effects due to the sub-grid scale model can be ignored in this region. The effects of the lower boundary condition on the resolved-scale turbulence in the upper part of the surface layer are discussed. It is concluded that the normalized mean velocity shear and resolved turbulence in the upper part of the surface layer are not affected by the specification of the lower boundary condition. In addition, the present work proposes a new independent model parameter, the Smagorinsky Reynolds Number (Re_SM), and demonstrates that this number determines the resolved turbulence in the upper part of the surface layer.     

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.     

Different Turbulent Regimes and Vertical Turbulence Structures of the Urban Nocturnal Stable Boundary Layer

Turbulence in the nocturnal boundary layer(NBL) is still not well characterized, especially over complex underlying surfaces. Herein, gradient tower data and eddy covariance data collected by the Beijing 325-m tower were used to better understand the differentiating characteristics of turbulence regimes and vertical turbulence structure of urban the NBL. As for heights above the urban canopy layer(UCL), the relationship between turbulence velocity scale(V_TKE) and wind speed(V) was con...     

Effects of precipitation on the eddy exchange in a wind-driven sea

Effects of precipitation on the temperature and salinity profiles in the top 2 or 3 m of the sea are calculated for four models of turbulent exchange: (1) constant eddy exchange with depth; (2) drift-current eddy exchange; (3) eddy exchange produced by shear in the orbital velocity under a surface wave; and (4) drift current and wave turbulence acting together. The surface temperature and salinity are only weakly affected, but the density gradient can become significantly stabler. The resulting influence on the eddy exchange is modeled with empirical formulas employing the Richardson number. Stability strongly damps drift-current eddy exchange but has much less effect on the wave-induced turbulence.The energy budget shows an increase in turbulent dissipation near the surface for weak stability when the surface stress in assumed to remain constant. Beyond a critical Richardson number of about 0.9 energy consumed by turbulent energy dissipation decreases. This occurs more due to a reduction in mechanical energy production than because of energy being used for the redistribution of mass.     

A parameterization of the stable atmospheric boundary layer

Two formulations of the stable atmospheric boundary layer are proposed for use in weather forecasting or climate models. They feature the log-linear profile near the surface, but are free from the associated critical Richardson number. The diffusion coefficients in the Ekman layer are a natural extension of the surface layer. They are locally determined using wind shear in one case and turbulent kinetic energy in the other. The parameterizations are tested in a one-dimensional model simulating the evolution of the nocturnal boundary layer with and without radiative cooling. Both formulations give very similar results, except near the top of the boundary layer where the transition to the free atmosphere is smoother with the wind shear formulation. A distinctive feature of these schemes is that they retain their simulating skill when resolution is reduced. This is verified for a wide range of situations. In practice, this means that there is no need for a large-scale model to have a level below 50 m or so.     

Variability Of The Stable And Unstable Atmospheric Boundary-Layer Height And Its Scales Over A Boreal Forest

Radiosondes releases during the NOPEX-WINTEX experiment carried out in late winter in Northern Finland were analysed for the determination of the height h of the atmospheric boundary layer. We investigate various possible scaling approaches, based on length scales using micrometeorological turbulence surface measurements and the background atmospheric stratification above h. Under stable conditions, the three previously observed turbulence regimes delineated by values of z/L (L is the Obukhov length) appears as a blueprint for understanding the departures found for the suitability of the Ekman scaling based on L_E = u_/f (u is the friction velocity and f the Coriolis parameter). The length scale L_N = u_/N (where N is the Brunt–Väisälä frequency) appears to be a useful scale under most stable conditions, especially in association with L. Under unstable conditions, shear production of turbulence is still significant, so that the three scales L, L_N and L_E are again relevant and the dimensionless ratios _N = L_N/L and L_N/L_E = N/f describe well the WINTEX data. Furthermore, in the classical scaling framework, the unstable domain may also be divided into three regimes as reflected by the dependence ofu_/f on instability (z/L).     

Turbulence structure in temperature inversions and in convection fields as observed by doppler sodar

The structure of turbulence in an inversion layer and in an homogeneous convective field of the planetary boundary layer is described. In the first part of the paper, we validate the sodar estimates of turbulent dissipation, by using measurements with an hot-wire anemometric system in situ. Limitations of an ε measurement technique using structure function calculations are given, taking account of atmospheric properties and acoustic Doppler instrumental effects. By comparison between isopleths of backscattering intensity and of turbulent dissipation rates, we observe that in the early morning, turbulence is advected by mechanical turbulence generated by wind shear. The same mechanism seems to be operating in the case of an inversion layer capping thermal instability, when the convective activity is not too greatly developed. A turbulent kinetic energy budget is examined using aircraft, sodar, and tower measurements. This indicates a constant turbulent dissipation profile through a deep convective layer.     

Role of land surface parameterizations on modeling cold-pooling events and low-level jets

Land surface parameterization schemes play a significant role in the accuracy of meso-local scale numerical models by accounting for the exchange of energy and water between the soil and the atmosphere. The role of land surface processes during large-scale cold-pooling events was studied with two land surface schemes (LSMs) in the Advanced Research Weather Forecasting model (ARW). Model evaluation was complex due to the surface and boundary layer interactions at different temporal and spatial scales as revealed by a scale dependent variance analysis. Wavelet analysis was used for the first time to analyze the model errors with specific focus on land surface processes. The ARW model was also evaluated for the formation of a low-level jet (LLJ). It is shown that vertical resolution in the model boundary layer played a significant role in determining the characteristics of LLJ, which influenced the lower boundary layer structure and moisture distribution. The results showed that the simulated low-level jet over southern Georgia was sensitive to the land surface parameterization and led to a significant difference in the boundary layer exchange. The jet shear played a crucial role in the maintenance of turbulence and weak shear caused excessive radiative cooling leading to unrealistic cold pools in the model. The results are important for regional downscaling as the excessive cold pools that are simulated in the model can go unnoticed.     

Single-Column Model Intercomparison for a Stably Stratified Atmospheric Boundary Layer

The parameterization of the stably stratified atmospheric boundary layer is a difficult issue, having a significant impact on medium-range weather forecasts and climate integrations. To pursue this further, a moderately stratified Arctic case is simulated by nineteen single-column turbulence schemes. Statistics from a large-eddy simulation intercomparison made for the same case by eleven different models are used as a guiding reference. The single-column parameterizations include research and operational schemes from major forecast and climate research centres. Results from first-order schemes, a large number of turbulence kinetic energy closures, and other models were used. There is a large spread in the results; in general, the operational schemes mix over a deeper layer than the research schemes, and the turbulence kinetic energy and other higher-order closures give results closer to the statistics obtained from the large-eddy simulations. The sensitivities of the schemes to the parameters of their turbulence closures are partially explored.     

Contrasting vertical structures of nocturnal boundary layers

This study analyzes eight levels of sonic anemometerdata collected on a 60-m towerduring CASES-99, toward the goal of understanding thevertical structure of thenocturnal boundary layer. Several different regimesare found. Thin boundarylayers are often observed where fluxes decrease with height and approximately vanish between 20 and 30 m aboveground. The flow above the thin boundary layeraccelerates and increasing shear oftengenerates significant turbulence in the middle ofthe night. Thisshear-generated turbulence is often stronger thanthat near the surface corresponding to an upside-downboundary layer. During these conditions,the turbulent transport of turbulence is downwardtoward the surface. The turbulence in this regimeshows features of z-less turbulence to the extentthat neither the height above groundnor the boundary-layer depth are primary scalingvariables. This layer isdifferent from a `residual layer' in thatturbulence is actively generated byshear associated with nocturnal accelerationsand often is stronger than that inthe surface-based boundary layer.In many cases, the turbulence does not varysignificantly across the towerlayer, implying that the boundary layer ismuch deeper than the 60-m towerlayer. Several case studies are presentedto illustrate the largevariation of vertical structure betweennights.     

Molecular dissipation of turbulent fluctuations in the convective mixed layer part II: Height variations of characteristic time scales and experimental test of molecular dissipation models

Extensive turbulence measurements from the Limagne and Beauce experiments were used to compute a characteristic time scale of the turbulence field (Τ = second moment/dissipation rate) for turbulent kinetic energy, temperature and humidity variances, and temperature-humidity covariance. The height variations of these time scales were analysed. The characteristic half-time scale Τ/2 of the turbulent velocity field was found, as expected, to be of the same order of magnitude as the large-eddy time scale Τ _L = Z_i/w*, showing that the turbulence structure is controlled by large eddies in the bulk of the mixed layer. The increase of Τ/2 above z/Z _i ～- 0.7 implies, however, that this time scale is no longer relevant to destruction of turbulent kinetic energy in the statically stable region with negative heat fluxes. An effective time scale Τ_eff, introduced by Zeman (1975), has been computed and its behaviour discussed. The scales for θ′ ², q′², and θ′q′ were found to be much shorter than Τ. Furthermore, a significant difference in behaviour was also revealed between the characteristic time scales of temperature and humidity fields in the stable layer. By using these experimental estimates, we tested some of the models for molecular dissipations, which are currently in use in higher order closure atmospheric boundary-layer models. The parameterized dissipation rates for θ′ ², and q′ ² agree well qualitatively with experimental estimates in the bulk of the mixed layer. In the stable layer, however, the parameterized dissipation rate ε _θ tends to become larger than the experimental ones although the parameterized dissipation rate ε _q still agrees with the experimental ones. For the molecular dissipation of θ′q′, this current model becomes physically inconsistent in the middle part of the mixed layer, because this term may become a production term for temperature-humidity covariance.