Stably-stratified Atmospheric Boundary Layer Research Articles

A stably-stratified atmospheric inflow assimilated to measured mean profiles interacting with a wind park

In this paper, a simulation chain is presented. This method comprises the precursor large-eddy simulation (LES) of a stably-stratified atmospheric boundary layer (ABL), the assimilation of the simulated mean velocities to measured wind profiles, and finally a simulation of the generated turbulent ABL flow passing through two wind turbines in a row. The high-resolved precursor simulation with a horizontal grid spacing of 3 m accounts for the characteristic turbulence of the stably-stratified ABL. Using an assimilation technique, the horizontal velocities are adapted accurately to the measured mean wind profiles for the WiValdi wind farm site at Krummendeich. The wakes of two wind turbines in a row with the assimilated inflow is successfully computed. With the simulation chain presented, it is possible to generate realistic atmospheric inflows for wind-turbine simulations with manageable computational effort.

Submeso Wave-Like Structures in the Atmospheric Boundary Layer and Their Parameters Measured with the Help of Sodar in Moscow Region

The paper presents study of the parameters of wave-like structures based on the data of long-term continuous sodar monitoring of the atmospheric boundary layer (ABL). Submesoscale internal gravity waves (IGWs) of non-orographic origin trapped in a stably stratified ABL (SBL) are considered. Statistical data on the parameters of two classes of IGWs are presented: internal gravity-shear waves (IGSWs) of the Kelvin-Helmholtz billow (KHB) type and buoyancy waves (BW). Identification and classification of IGWs was carried out by the visual analysis of sodar echograms. The measurements carried out in the Moscow region were used. The seasonal and diurnal variability of the frequency of registration of waves of both classes were studied, the values of the parameters of the observed waves were analyzed, and the ranges and average values of these quantities were compared.

Spatial Dependence of Stably Stratified Nocturnal Boundary-Layer Regimes in Complex Terrain

The stably stratified atmospheric boundary layer (SBL) has been found in previous studies to display distinct regimes of behaviour. In particular, a contrast is often drawn between the weakly (wSBL) and very (vSBL) stable boundary layers. Time series of SBL regime affiliation obtained from hidden Markov model analyses of data from three different towers at the Los Alamos National Laboratory are used to investigate the spatial dependence of SBL regime occupation and SBL transitions. The local topography influences the flow such that south-west to north-east flow prevails, for which wSBL and vSBL conditions respectively are more likely to occur. Joint probabilities of shared regime occupation at the three towers (with and without conditioning on wind direction) are much larger than would be expected from statistically independent regime sequences at the different locations. Very persistent wSBL nights (without any transitions to the vSBL) have a higher probability of occurring across the entire tower network domain than very persistent vSBL nights. Many regime transitions occur within a narrow time window between the different towers; occurrence probabilities of such events are much higher than would be expected from statistically independent regime transitions. Of such events, transitions occurring at exactly the same time across the tower network occur most often. Many co-occurring turbulence recovery events can be associated with high-intensity intermittent turbulence events. Our results imply that the scale on which the SBL regime occupation and transitions are dependent exceeds 10 km in this region of complex terrain.

FSI Simulation of two back-to-back wind turbines in atmospheric boundary layer flow

The paper presents aerodynamic and fluid–structure interaction (FSI) simulations of two back-to-back 5 MW horizontal-axis wind turbines (HAWTs) at full scale and with full geometrical complexity operating in a stably-stratified atmospheric boundary layer (ABL) flow. The numerical formulation for stratified incompressible flows is based on the ALE-VMS methodology, and is coupled to a Kirchhoff–Love thin-shell formulation employed to model the wind-turbine structure. A multi-domain method (MDM) is adopted for computational efficiency. In the simulations presented the wind turbines are positioned one behind the other at a distance of four rotor diameters, which results in a noticeable power production loss for the downstream turbine due to the wake velocity deficit of the upstream turbine. The importance of including FSI coupling in the modeling to better predict the unsteady aerodynamic loads acting on the wind-turbine blades is also highlighted.

Turbulent Winds and Temperature Fronts in Large-Eddy Simulations of the Stable Atmospheric Boundary Layer

Abstract The nighttime high-latitude stably stratified atmospheric boundary layer (SBL) is computationally simulated using high–Reynolds number large-eddy simulation on meshes varying from 2003 to 10243 over 9 physical hours for surface cooling rates Cr = [0.25, 1] K h−1. Continuous weakly stratified turbulence is maintained for this range of cooling, and the SBL splits into two regions depending on the location of the low-level jet (LLJ) and . Above the LLJ, turbulence is very weak and the gradient Richardson number is nearly constant: . Below the LLJ, small scales are dynamically important as the shear and buoyancy frequencies vary with mesh resolution. The heights of the SBL and Ri noticeably decrease as the mesh is varied from 2003 to 10243. Vertical profiles of the Ozmidov scale show its rapid decrease with increasing , with over a large fraction of the SBL for high cooling. Flow visualization identifies ubiquitous warm–cool temperature fronts populating the SBL. The fronts span a large vertical extent, tilt forward more so as the surface cooling increases, and propagate coherently. In a height–time reference frame, an instantaneous vertical profile of temperature appears intermittent, exhibiting a staircase pattern with increasing distance from the surface. Observations from CASES-99 also display these features. Conditional sampling based on linear stochastic estimation is used to identify coherent structures. Vortical structures are found upstream and downstream of a temperature front, similar to those in neutrally stratified boundary layers, and their dynamics are central to the front formation.

The impact of stable atmospheric boundary layers on wind-turbine wakes within offshore wind farms

In terms of predicting wind turbine wakes, the stably stratified atmospheric boundary layer (SABL) is taking an exceptional position as wake effects and thus loads on subsequent turbines are stronger. In this study we show the impact of the SABL on power production and wake effects (power deficits) in offshore wind farms by means of measurements as well as large-eddy simulations (LES). Measurements show enhanced wake effects in the SABL compared to the unstable situation. Another influence on the power generation of an offshore wind farm is the distance of the wind farm to the shore. This is accounted for in the LES by a modification of surface characteristics at the coastal discontinuity. In addition to the effect of the coast, the numerical case study also shows the existence of local jets between the turbines of the wind farm.

Detection of tip-vortex signatures behind a 2.5 MW wind turbine

The near-field signature of the vortical structures shed from the blade tips behind a 2.5MW horizontal axis wind turbine in a stably-stratified atmospheric boundary layer (ABL) was quantified for the first time. This study utilizes wind velocity measurements from sonic anemometers installed on a meteorological tower, which offers continuous characterization of wind conditions in the field, at elevations coinciding with the bottom, hub and top tip heights of the turbine. Using the stringent criteria on wind speed, direction and steadiness, we are able to subsample the dataset in which the sonic anemometers are positioned near the edge of the turbine wake. The spectral analysis of this dataset shows a distinct peak at the turbine rotational frequency (fT) for hub and top tip height measurements. Based on recent literature, we infer that this peak is the signature of tip-vortices, and the shift of this signature from blade-passing frequency (3fT) to fT is likely to be caused by vortex grouping phenomena. Slight changes of mean wind direction in other data samples result in the absence of the spectral peak, suggesting the very local extent of tip vortices.

Stable Atmospheric Boundary Layer Over a Uniform Slope: Some Theoretical Concepts

Theoretical arguments are developed to describe the effects of a uniform slope on the development of the stably stratified atmospheric boundary layer (SBL). A maximum sustainable surface buoyancy flux exists for the SBL overlying a uniform, non-sloping surface. In this study it is shown that the SBL overlying a uniform shallow slope (with gradient of the order of 1:1000) also supports a maximum sustainable buoyancy flux, B max, but that the value of B max is influenced by the gradient of the slope, γ. It is demonstrated that in the limit γ → 0, results for the SBL over a horizontal surface are recovered.

A LIMITED-LENGTH-SCALE k-ε MODEL FOR THE NEUTRAL AND STABLY-STRATIFIED ATMOSPHERIC BOUNDARY LAYER

Analytical and numerical models of the neutral and stably-stratifiedatmospheric boundary layer are reviewed. Theoretical arguments andcomputational models suggest that a quasi-steady state is attainable in aboundary layer cooled from below and it is shown how this may be incorporatedwithin a time-steady, one-dimensional model. A new length-scale-limitedk-e model is proposed for flows where a global maximum mixing length isimposed by the finite boundary-layer depth or, in stably-stratifiedconditions, by the Obukhov length, whilst still reducing to a form consistentwith the logarithmic law in the surface layer. Simulations compare favourablywith data from the Leipzig experiment and from Cardington airfield inEngland.

Large-Eddy Simulation of the stably-stratified atmospheric boundary layer

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.

Nieuwstadt's stable boundary layer revisited

Currently no theory of the stably stratified atmospheric boundary layer (SBL) is generally accepted as definitive even for idealized cases. Nieuwstadt's theory, though a promising candidate, faces objections relating to upper boundary and steady-state conditions, internal wave effects and the consistency of the model outside the strong-stability limit. the aim of this paper is to examine the objections, improve the model, draw further deductions and compare with numerical models. We shall deduce from Nieuwstadt's model that Bo = RfcG2|f|/√3, where Bo is the surface buoyancy-flux, G the geostrophic wind speed, f the Coriolis parameter and Rfc the critical value of the flux Richardson number Rf. Higher-order expansion shows this value is an upper bound corresponding to the stable limit L/h → 0. This is consistent with a similar bound on Bo derived from independent energy arguments. By contrast, within present idealizations there is no bound on the surface cooling rate. Comparisons with a new series of large eddy simulations (LES) and other numerical models support the interpretation of Nieuwstadt's SBL as an idealized limiting case. the theory explains from first principles the observed sensitivity to small slopes. Interaction between sharp inversions and slopes may cause turbulence in thin layers. Coupling with the surface boundary conditions is a likely cause of intermittent turbulence, and explains features of Brost and Wyngaard's second-order closure study. the formal singularity at the top of the SBL gives time-scales for approach to inertial and heat equilibrium. Both, for separate reasons, are O(|f|−1). A natural extension to moderately stable and near-neutral conditions agrees well with numerical model results, and provides a complete prediction of SBL structure from given surface heat flux and synoptic pressure gradient. the near-neutral regime is narrow at high Rossby number. Comparison with LES supports both the local scaling approach and the gross predictions of the theory. the model gives insight into the limitations of Rossby-similarity formulae. In particular, restrictions of domain size hd (or similar background stability effects) may not become negligible even when h/hd → 0. Hence matching to neutral conditions is important in predicting even quite stable boundary layers. Wave effects, though non-local in some respects, do not fundamentally change the Nieuwstadt picture for the mean structure. Retention in the model of the original value for Rfc is recommended even if wave radiation perturbs local Rf. In summary, the adapted Nieuwstadt theory seems to provide a definitive framework for the idealized SBL, from which other ‘perturbation’ effects may be assessed. the value of Bo/G2|f| gives a criterion for (a) quasi-steady or (b) intermittent SBL regimes.

Nieuwstadt's stable boundary layer revisited

AbstractCurrently no theory of the stably stratified atmospheric boundary layer (SBL) is generally accepted as definitive even for idealized cases. Nieuwstadt's theory, though a promising candidate, faces objections relating to upper boundary and steady‐state conditions, internal wave effects and the consistency of the model outside the strong‐stability limit. the aim of this paper is to examine the objections, improve the model, draw further deductions and compare with numerical models.We shall deduce from Nieuwstadt's model that Bo = RfcG2|f|/√3, where Bo is the surface buoyancy‐flux, G the geostrophic wind speed, f the Coriolis parameter and Rfc the critical value of the flux Richardson number Rf. Higher‐order expansion shows this value is an upper bound corresponding to the stable limit L/h → 0. This is consistent with a similar bound on Bo derived from independent energy arguments. By contrast, within present idealizations there is no bound on the surface cooling rate.Comparisons with a new series of large eddy simulations (LES) and other numerical models support the interpretation of Nieuwstadt's SBL as an idealized limiting case. the theory explains from first principles the observed sensitivity to small slopes. Interaction between sharp inversions and slopes may cause turbulence in thin layers. Coupling with the surface boundary conditions is a likely cause of intermittent turbulence, and explains features of Brost and Wyngaard's second‐order closure study. the formal singularity at the top of the SBL gives time‐scales for approach to inertial and heat equilibrium. Both, for separate reasons, are O(|f|−1).A natural extension to moderately stable and near‐neutral conditions agrees well with numerical model results, and provides a complete prediction of SBL structure from given surface heat flux and synoptic pressure gradient. the near‐neutral regime is narrow at high Rossby number. Comparison with LES supports both the local scaling approach and the gross predictions of the theory. the model gives insight into the limitations of Rossby‐similarity formulae. In particular, restrictions of domain size hd (or similar background stability effects) may not become negligible even when h/hd → 0. Hence matching to neutral conditions is important in predicting even quite stable boundary layers. Wave effects, though non‐local in some respects, do not fundamentally change the Nieuwstadt picture for the mean structure. Retention in the model of the original value for Rfc is recommended even if wave radiation perturbs local Rf.In summary, the adapted Nieuwstadt theory seems to provide a definitive framework for the idealized SBL, from which other ‘perturbation’ effects may be assessed. the value of Bo/G2|f| gives a criterion for (a) quasi‐steady or (b) intermittent SBL regimes.

Observations of an internal gravity wave in the lower troposphere at Halley, Antarctica

Observations of internal gravity waves in the stably-stratified atmospheric boundary layer at Halley, Antarctica are presented. These were made on 1 February, 1986 and take the form of temperature measurements from a 30 m mast and a Sodar record. The temperature record shows a clearly defined, dominant wave period of around 11 min. A high-resolution radiosonde ascent made during the period of wave activity exhibits thin layers of low Richardson number and it is suggested that these are regions of dynamic instability where the waves are generated. A linear stability analysis of the radiosonde data supports this idea. It is argued from simple theoretical ideas and by means of a numerical model that only waves with a wavelength greater than a certain critical value are likely to be observed at the surface. The observations are shown to be consistent with this hypothesis.