Pressure-Gradient Forcing Methods for Large-Eddy Simulations of Flows in the Lower Atmospheric Boundary Layer

Turbulent flows over forest canopies have been successfully modeled using Large-Eddy Simulations (LES). Simulated winds result from the balance between a simplified pressure gradient forcing (e.g., a constant pressure-gradient or a canonical Ekman balance) and the dissipation of momentum, due to vegetation drag. Little attention has been paid to the impacts of these forcing methods on flow features, despite practical challenges and unrealistic features, such as establishing stationary velocity or streak locking. This study presents a technique for capturing the effects of a pressure-gradient force (PGF), associated with atmospheric patterns much larger than the computational domain for idealized simulations of near-surface phenomena. Four variants of this new PGF are compared to existing forcings, for turbulence statistics, spectra, and temporal averages of flow fields. Results demonstrate that most features of the turbulent flow are captured. The variants can either enable modelers to prescribe a wind speed and direction at a reference height close to the ground as required in wildfire simulations, and/or mitigate streaks locking by reproducing the stability of the Ekman balance. Conditions of use, benefits, and drawbacks are discussed. PGF approaches, therefore, provide a viable solution for precursor inflows, including for the specific domains used in fire simulations.

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On the fine vertical structure of the low troposphere over the coastal margins of East Antarctica

10.5194/acp-2018-1197 ◽

2019 ◽

Author(s):

Étienne Vignon ◽

Olivier Traullé ◽

Alexis Berne

Keyword(s):

Pressure Gradient ◽

Vertical Structure ◽

East Antarctica ◽

Radiosonde Data ◽

Gradient Force ◽

Pressure Gradient Force ◽

Near Surface ◽

Ice Shelves ◽

Low Level ◽

Humidity Profiles

Abstract. Eight years of high-resolution radiosonde data at nine Antarctic stations are analysed to provide the first large scale characterization of the fine scale vertical structure of the low troposphere up to 3 km of altitude over the coastal margins of East Antarctica. Radiosonde data show a large spatial variability of wind, temperature and humidity profiles, with different features between stations in katabatic regions (e.g., Dumont d'Urville and Mawson stations), stations over two ice shelves (Neumayer and Halley stations) and regions with complex orography (e.g., Mc Murdo). At Dumont d'Urville, Mawson and Davis stations, the yearly median wind speed profiles exhibit a clear low-level katabatic jet. During precipitation events, the low-level flow generally remains of continental origin and its speed is even reinforced due to the increase in the continent- ocean pressure gradient. Meanwhile, the relative humidity profiles show a dry low troposphere, suggesting the occurence of low-level sublimation of precipitation in katabatic regions but such a phenomenon does not appreciably occur over the ice-shelves near Halley and Neumayer. Although ERA-Interim and ERA5 reanalyses assimilate radiosoundings at most stations considered here, substantial – and sometimes large – low-level wind and humidity biases are revealed but ERA5 shows overall better performances. A free simulation with the regional model Polar WRF (at a 35-km resolution) over the entire continent shows too strong and too shallow near-surface jets in katabatic regions especially in winter. This may be a consequence of an understimated coastal cold air bump and associated sea-continent pressure gradient force due to the coarse 35 km resolution of the Polar WRF simulation. Beyond documenting the vertical structure of the low troposphere over coastal East-Antarctica, this study gives insights into the reliability and accuracy of two major reanalysis products in this region on the Earth and it raises the difficulty of modeling the low-level flow over the margins of the ice sheet with a state-of-the-art climate model.

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Effect of Turbulence Models and Spatial Resolution on Resolved Velocity Structure and Momentum Fluxes in Large-Eddy Simulations of Neutral Boundary Layer Flow

Journal of Applied Meteorology and Climatology ◽

10.1175/2008jamc2021.1 ◽

2009 ◽

Vol 48 (6) ◽

pp. 1161-1180 ◽

Cited By ~ 18

Author(s):

Francis L. Ludwig ◽

Fotini Katopodes Chow ◽

Robert L. Street

Keyword(s):

Dynamic Models ◽

Turbulence Models ◽

Coarse Grid ◽

Large Eddy Simulations ◽

Near Surface ◽

Velocity Statistics ◽

Neutral Boundary Layer ◽

Regional Prediction ◽

Large Eddy ◽

Flow Features

Abstract This paper demonstrates the importance of high-quality subfilter-scale turbulence models in large-eddy simulations by evaluating the resolved-scale flow features that result from various closure models. The Advanced Regional Prediction System (ARPS) model was used to simulate neutral flow over a 1.2-km square, flat, rough surface with seven subfilter turbulence models [Smagorinsky, turbulent kinetic energy (TKE)-1.5, and five dynamic reconstruction combinations]. These turbulence models were previously compared with similarity theory. Here, the differences are evaluated using mean velocity statistics and the spatial structure of the flow field. Streamwise velocity averages generally differ among models by less than 0.5 m s−1, but those differences are often significant at a 95% confidence level. Flow features vary considerably among models. As measured by spatial correlation, resolved flow features grow larger and less elongated with height for a given model and resolution. The largest differences are between dynamic models that allow energy backscatter from small to large scales and the simple eddy-viscosity closures. At low altitudes, the linear extent of Smagorinsky and TKE-1.5 structures exceeds those of dynamic models, but the relationship reverses at higher altitudes. Ejection, sweep, and upward momentum flux features differ among models and from observed neutral atmospheric flows, especially for Smagorinsky and TKE-1.5 coarse-grid simulations. Near-surface isopleths separating upward fluxes from downward are shortest for the Smagorinsky and TKE-1.5 coarse-grid simulations, indicating less convoluted turbulent interfaces; at higher altitudes they are longest. Large-eddy simulation (LES) is a powerful simulation tool, but choices of grid resolution and subfilter model can affect results significantly. Physically realistic dynamic mixed models, such as those presented here, are essential when using LES to study atmospheric processes such as transport and dispersion—in particular at coarse resolutions.

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A wall-layer model for large-eddy simulations of turbulent flows with/out pressure gradient

Physics of Fluids ◽

10.1063/1.3529358 ◽

2011 ◽

Vol 23 (1) ◽

pp. 015101 ◽

Cited By ~ 53

Author(s):

C. Duprat ◽

G. Balarac ◽

O. Métais ◽

P. M. Congedo ◽

O. Brugière

Keyword(s):

Pressure Gradient ◽

Turbulent Flows ◽

Wall Layer ◽

Large Eddy Simulations ◽

Layer Model ◽

Large Eddy

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On the Equivalence of Two Schemes for Convective Momentum Transport

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-12-068.1 ◽

2012 ◽

Vol 69 (12) ◽

pp. 3491-3500 ◽

Cited By ~ 13

Author(s):

David M. Romps

Keyword(s):

Large Eddy Simulation ◽

Pressure Gradient ◽

Mass Flux ◽

Deep Convection ◽

Momentum Transport ◽

Gradient Force ◽

Pressure Gradient Force ◽

Eddy Simulation ◽

Large Eddy ◽

Convective Momentum Transport

Abstract The Gregory–Kershaw–Inness (GKI) parameterization of convective momentum transport, which has a tunable parameter C, is shown to be identical to a parameterization with no pressure gradient force and a mass flux smaller by a factor of 1 − C. Using cloud-resolving simulations, the transilient matrix for momentum is diagnosed for deep convection in radiative–convective equilibrium. Using this transilient matrix, it is shown that the GKI scheme underestimates the compensating subsidence of momentum by a factor of 1 − C, as predicted. This result is confirmed using a large-eddy simulation.

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Large-Eddy Simulations and Damped-Oscillator Models of the Unsteady Ekman Boundary Layer*

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-15-0038.1 ◽

2015 ◽

Vol 73 (1) ◽

pp. 25-40 ◽

Cited By ~ 11

Author(s):

Mostafa Momen ◽

Elie Bou-Zeid

Keyword(s):

Boundary Layer ◽

Large Eddy Simulations ◽

Reduced Model ◽

Gradient Force ◽

Pressure Gradient Force ◽

Ekman Boundary Layer ◽

Wide Range ◽

Large Eddy ◽

Mass Spring ◽

Damped Oscillator

Abstract The Ekman boundary layer (EBL) is a central problem in geophysical fluid dynamics that emerges when the pressure gradient force, the Coriolis force, and the frictional force interact in a flow. The unsteady version of the problem, which occurs when these forces are not in equilibrium, is solvable analytically only for a limited set of forcing variability regimes, and the resulting solutions are intricate and not always easy to interpret. In this paper, large-eddy simulations (LESs) of neutral atmospheric EBLs are conducted under various unsteady forcings to reveal the range of physical characteristics of the flow. Subsequently, it is demonstrated that the dynamics of the unsteady EBL can be reduced to a second-order ordinary differential equation that is very similar to the dynamical equation of a damped oscillator, such as a mass–spring–damper system. The validation of the proposed reduced model is performed by comparing its analytical solutions to LES results, revealing very good agreement. The reduced model can be solved for a wide range of variable forcing conditions, and this feature is exploited in the paper to elucidate the physical origin of the inertia (mass), energy storage (spring), and energy dissipation (damper) attributes of Ekman flows.

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On the fine vertical structure of the low troposphere over the coastal margins of East Antarctica

Atmospheric Chemistry and Physics ◽

10.5194/acp-19-4659-2019 ◽

2019 ◽

Vol 19 (7) ◽

pp. 4659-4683 ◽

Cited By ~ 11

Author(s):

Étienne Vignon ◽

Olivier Traullé ◽

Alexis Berne

Keyword(s):

Pressure Gradient ◽

Vertical Structure ◽

East Antarctica ◽

Radiosonde Data ◽

Gradient Force ◽

Pressure Gradient Force ◽

Near Surface ◽

Ice Shelves ◽

Low Level ◽

Humidity Profiles

Abstract. In this study, 8 years of high-resolution radiosonde data at nine Antarctic stations are analysed to provide the first large-scale characterization of the fine vertical structure of the low troposphere up to 3 km altitude over the coastal margins of East Antarctica. Radiosonde data show a large spatial variability of wind, temperature and humidity profiles, with different features between stations in katabatic regions (e.g., Dumont d'Urville and Mawson stations), stations over two ice shelves (Neumayer and Halley stations) and regions with complex orography (e.g., McMurdo). At the Dumont d'Urville, Mawson and Davis stations, the yearly median wind speed profiles exhibit a clear low-level katabatic jet. During precipitation events, the low-level flow generally remains of continental origin and its speed is even reinforced due to the increase in the continent–ocean pressure gradient. Meanwhile, the relative humidity profiles show a dry low troposphere, suggesting the occurrence of low-level sublimation of precipitation in katabatic regions but such a phenomenon does not appreciably occur over the ice shelves near Halley and Neumayer. Although ERA-Interim and ERA5 reanalyses assimilate radiosoundings at most stations considered here, substantial – and sometimes large – low-level wind and humidity biases are revealed but ERA5 shows overall better performance. A free simulation with the regional polar version of the Weather Research and Forecasting model (Polar WRF) (at a 35 km resolution) over the entire continent shows too-strong and too-shallow near-surface jets in katabatic regions especially in winter. This may be a consequence of an underestimated coastal cold air bump and associated sea–continent pressure gradient force due to the coarse 35 km resolution of the Polar WRF simulation. Beyond documenting the vertical structure of the low troposphere over coastal East Antarctica, this study gives insights into the reliability and accuracy of two major reanalysis products in this region on the Earth. The paper further underlines the difficulty of modeling the low-level flow over the margins of the ice sheet with a state-of-the-art atmospheric model.

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