On the Structure and Adjustment of Inversion-Capped Neutral Atmospheric Boundary-Layer Flows: Large-Eddy Simulation Study

Pedersen, Jesper Grønnegaard; Gryning, Sven‐Erik; Kelly, Mark

doi:10.1007/s10546-014-9937-z

Cited by 29 publications

(57 citation statements)

References 35 publications

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“…The ratio of u * /G ≈ 0.025 can be inferred from table 2 given G = 12 m s −1 for all cases. Figure 4 (a) shows that all cases develop a supergeostrophic jet near the boundary-layer top with a maximum wind speed of approximately 1.05 G, similar to Pedersen et al (2014). Further, the vertical profiles of stress and wind direction (see figure 4b,c) follow the expected trends, i.e.…”

Section: Initialisation Resultsmentioning

confidence: 58%

“…Near the end of the first phase, the boundary-layer growth attains a small constant value (less than 2 m h −1 ), indicating that the boundary layer has reached a quasi-steady state. For comparison, Pedersen et al (2014) found average growth rates for CNBLs in quasi-steady state between 0.28 and 0.58 m h −1 . Further, the hub-height velocity M hub and the friction velocity u * are found to be nearly constant for the different cases, and the geostrophic angle α increases with decreasing boundary-layer height.…”

Section: Initialisation Resultsmentioning

confidence: 99%

“…In the literature, equilibration times for the CNBL are reported to be between 16 and 24 model hours (see, e.g. Zilitinkevich et al 2007;Pedersen et al 2014), so the duration of the spinup phase is set to 20 h. The precursor domain does not contain any wind turbines, so there are no mechanisms to trigger large-scale stationary gravity-wave structures. Gravity waves can still be excited in the precursor domain by turbulent motions near the top of the boundary layer (see, e.g.…”

Section: Boundary-layer Initialisationmentioning

confidence: 99%

“…For instance, Zilitinkevich & Esau (2003 and Zilitinkevich, Esau & Baklanov (2007) continued to improve equilibrium height formulations and similarity theory predictions for the CNBL, and Esau (2004a,b) showed the importance of the inversion-layer height for flow properties such as turbulent kinetic energy and surface drag. Further, Taylor & Sarkar (2007, 2008a investigated stratification effects in the bottom Ekman layer of the ocean and Pedersen, Gryning & Kelly (2014) recently examined the dynamic evolution of the CNBL towards a statistically steady-state, fully turbulent boundary layer.…”

Section: Introductionmentioning

confidence: 99%

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Boundary-layer development and gravity waves in conventionally neutral wind farms

2017

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While neutral atmospheric boundary layers are rare over land, they occur frequently over sea. In these cases they are almost always of the conventionally neutral type, in which the neutral boundary layer is capped by a strong inversion layer and a stably stratified atmosphere aloft. In the current study, we use large-eddy simulations (LES) to investigate the interaction between a large wind farm that has a fetch of 15 km and a conventionally neutral boundary layer (CNBL) in typical offshore conditions. At the domain inlet, we consider three different equilibrium CNBLs with heights of approximately 300 m, 500 m and 1000 m that are generated in a separate precursor LES. We find that the height of the inflow boundary layer has a significant impact on the wind farm flow development. First of all, above the farm, an internal boundary layer develops that interacts downwind with the capping inversion for the two lowest CNBL cases. Secondly, the upward displacement of the boundary layer by flow deceleration in the wind farm excites gravity waves in the inversion layer and the free atmosphere above. For the lower CNBL cases, these waves induce significant pressure gradients in the farm (both favourable and unfavourable depending on location and case). A detailed energy budget analysis in the turbine region shows that energy extracted by the wind turbines comes both from flow deceleration and from vertical turbulent entrainment. Though turbulent transport dominates near the end of the farm, flow deceleration remains significant, i.e. up to 35 % of the turbulent flux for the lowest CNBL case. In fact, while the turbulent fluxes are fully developed after eight turbine rows, the mean flow does not reach a stationary regime. A further energy budget analysis over the rest of the CNBL reveals that all energy available at turbine level comes from upwind kinetic energy in the boundary layer. In the lower CNBL cases, the pressure field induced by gravity waves plays an important role in redistributing this energy throughout the farm. Overall, in all cases entrainment at the capping inversion is negligible, and also the work done by the mean background pressure gradient, arising from the geostrophic balance in the free atmosphere, is small.

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Section: Initialisation Resultsmentioning

confidence: 58%

Section: Initialisation Resultsmentioning

confidence: 99%

Section: Boundary-layer Initialisationmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Boundary-layer development and gravity waves in conventionally neutral wind farms

2017

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“…The same is seen for the wind speed—also averaged over the domain and over the final 2 h—shown in Figure in the same format as Figure . Increasing

Δ x

tends to decrease wind shear in the middle region of the STBL (

100 m \leq z \leq 500 m

approximately); an effect very similar to that of slightly increasing the surface heat flux [ Pedersen et al ., ]. Regarding both scalar concentration and wind speed we also see the tendency of better mixed profiles with increasing

Δ x

when using

Δ z = 10

m, but not with

Δ z = 5

m; the horizontally averaged passive scalar concentration is nearly constant with height within the STBL in both

A_{20, 5}, A_{35, 5}

, and

A_{70, 5}

(dark‐red dashed, solid, and dash‐dot lines in Figure b), but decreases throughout the STBL with increasing

Δ x

.…”

Section: Resultsmentioning

confidence: 90%

Resolution and domain‐size sensitivity in implicit large‐eddy simulation of the stratocumulus‐topped boundary layer

Pedersen

Malinowski

Grabowski

2016

J Adv Model Earth Syst

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As a complement to measurements, numerical modeling facilitates improved understanding of the complex turbulent processes in the stratocumulus-topped boundary layer (STBL). Due to limited computational resources simulations are often run at too coarse resolutions to resolve details of cloud-top turbulence and potentially in computational domains too small to account for the largest scales of boundary layer circulations. The effects of such deficiencies are not fully understood. Here the influence of resolution/ anisotropy of the computational grid and domain size in under-resolved implicit large-eddy simulation of the STBL is investigated. The performed simulations are based on data from the first research flight of the DYCOMS-II campaign. Regarding cloud cover and domain-averaged liquid water path, simulations with horizontal/vertical grid spacing of 35/5 m, 70/10 m, and 105/15 m are found to agree better with measurements than more computationally expensive simulations with isotropic grid boxes, e.g., with 10/10 m or 15/15 m grid spacing. While decreasing the vertical grid spacing allows more representative simulation of the thin, turbulent, stably stratified interfacial layer between the STBL and the free troposphere, coarsening the horizontal resolution dampens vertical velocity fluctuations in this region and mimics the observed anisotropy of stably stratified small-scale turbulence near the cloud top. The size of the computational domain is found to have almost no impact on mean cloud properties. However, increasing it from 3:533:5 km 2 to 14314 km 2 does lead to the occurrence of larger coherent updraft structures. Increasing it further to 21321 km 2 shows little or no increase in the updraft size.

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A universal wind profile for the inversion‐capped neutral atmospheric boundary layer

Kelly

Cersosimo

Berg

2019

Quart J Royal Meteoro Soc

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Wind profiles above the atmospheric surface layer are not accurately described by classic similarity theories. Far from the surface, the underlying assumptions of such surface‐layer theories break down due to the stronger influence of buoyancy forces induced by the temperature inversion that caps the atmospheric boundary layer (ABL), as well as the Coriolis force. This paper examines the influence of these forces on the mean flow and presents a new similarity theory to predict mean wind profiles in and above the surface layer for an ABL with zero surface heat flux and capped by an inversion of potential temperature, that is, the conditionally neutral ABL. The analysis here is based on the results of 17 large‐eddy simulations (LES) over a flat homogeneous rough surface, which leads to and supports the new similarity theory. The development is based on two applications of the Buckingham Π theorem. A first application allows determination of the entrainment‐induced heat flux profile through the ABL and into the surface layer, which is then used within a second dimensional argument for the vertical shear of mean wind speed. We subsequently find a new dimensionless group (Π2) depending on the capping inversion strength, the Coriolis parameter, the surface stress and the ABL depth, which is correlated to the dimensionless shear (Π1) through a universal function β. Integrating the functional relation between Π1 and Π2 produces an equation for the mean wind speed profile; it effectively includes an additive “correction” to the log‐law in terms of Π2, analogous to the Monin–Obukhov profile correction function. Unlike surface‐layer similarity, the new form accounts for the influences of both the surface and the ABL top. Relative to the LES, the new profile form exhibits errors in mean wind speed below 5% for heights below 90% of the ABL depth; this is relevant for applications above the surface layer (e.g., wind energy).

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On the Structure and Adjustment of Inversion-Capped Neutral Atmospheric Boundary-Layer Flows: Large-Eddy Simulation Study

Cited by 29 publications

References 35 publications

Boundary-layer development and gravity waves in conventionally neutral wind farms

Boundary-layer development and gravity waves in conventionally neutral wind farms

Resolution and domain‐size sensitivity in implicit large‐eddy simulation of the stratocumulus‐topped boundary layer

A universal wind profile for the inversion‐capped neutral atmospheric boundary layer

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