Large-Eddy Simulation (LES)

Instantaneous snapshot of 3D plumes from an LES of free convection.

The planetary boundary layer (PBL) is a critically important region in atmospheric and oceanic flows. Turbulence and in particular coherent structures embedded in PBL turbulence determine important fluxes of momentum, heat, and scalars at the surface and entrainment zone of the PBLs which in turn impact larger scale motions. We are (and have been) studying three-dimensional, time-dependent, PBL turbulence using turbulence-resolving numerical simulations and in particular Large-Eddy Simulation (LES) for a wide variety of geophysical flows.

The methodology of LES is increasingly used in PBL research. LES is formulated by specifying the large-scale environment and calculating the fluid dynamics on scales ranging from approximately the PBL thickness down to a grid scale d limited by the feasible size of the computation. LES explicitly captures large turbulent eddies, which contain most of the turbulent kinetic energy and carry most of turbulent transport, and approximates the effects of small turbulent eddies in its subgrid-scale model. This technique was first developed at NCAR in the late 1960s by Jim Deardorff and Doug Lilly, and is now widely used as a major tool for investigations of turbulence in engineering and geophysical fields.

The current NCAR LES code was first built in 1984 by Moeng (1984) to study clear convective PBLs and since then has continuously evolved to include a variety of physical processes, eg, clouds, chemistry, shear and stable stratification, vegetative surface canopies, and Langmuir cells and wave breaking in the ocean mixed layer (see a partial citation list below). The basic numerical algorithm is a mixed pseudo-spectral finite difference code with third-order Runge-Kutta time stepping utilizing a staggered vertical grid with options for variable spacing. This base algorithm has been extended to a cell centered co-located grid architecture that allows for the resolution of time varying sinusoidal waveforms. The code is written to run on massively parallel computer architectures using the Message-Passing Interface (MPI) and OpenMP programming models. Work in the vertical (z-) direction is partitioned across compute nodes using MPI with horizontal (x-y) work split across multiple OpenMP threads.

The NCAR LES code is freely available to the outside community. Over the years it has been adopted by outside researchers to study a variety of geophysical flows (see citations). The
NCAR LES code will continue to evolve to take advantage of new computer hardware and algorithm developments as well as advances in subgrid-scale parameterizations. Requests for the code can be made to Peter Sullivan (pps@ucar.edu), Chin-Hoh Moeng (moeng@ucar.edu), Edward Patton
(patton@ucar.edu).

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Algorithmic descriptions:

Moeng, C-H, 1984: A Large-Eddy Simulation Model for the Study of Planetary Boundary-Layer turbulence. J. Atmos. Sci., 41, 2052-2062.

Moeng, C-H., and Wyngaard, J.C. 1988: Spectral Analysis of Large-Eddy Simulations of the Convective Boundary layer. J. Atmos. Sci., 45, 3573-3587.

Sullivan, P. P., J. C. McWilliams, and C.-H. Moeng, 1996: A grid nesting method for large-eddy simulation of planetary boundary layer flows. Boundary-Layer Meteorology, 80, 167-202.

Moeng, C-H. and P.P. Sullivan, 2002: Large Eddy Simulation. In: Encyclopedia of Atmospheric Sciences, 1140-1150.

Clear PBLs:

Moeng, C.-H., J.C. McWilliams, R. Rotunno, P.P. Sullivan & J.C. Weil, 2003: Investigating 2D modelling of atmospheric convection in the PBL. J. Atmospheric Sciences, 61, 889--903.

Moeng, C.-H., and P. P. Sullivan, 1994: A comparison of shear and buoyancy driven planetary-boundary-layer flows. Journal of the Atmospheric Sciences, 51,999-1022.

Lin, C.-L., J.C. McWilliams, C.-H. Moeng, and P. P. Sullivan, 1996: Coherent structures and dynamics in a neutrally stratified planetary boundary layer flow. Physics of Fluids, 8, 2626-2639.

Nieuwstadt, F.T.M., Mason, P.J., Moeng, C-H. and Schumann, U. 1991: Large-Eddy Simulation of the Convective Boundary layer: A Comparison of Four Computer Codes. 8th Symposium on Turbulent Shear Flows, Springer-Verlag.

Saiki, E. M., C-H. Moeng, and P. P. Sullivan, 2000: Large eddy simulation of the stably stratified planetary boundary layer. Boundary-Layer Meteorology, 95, 1-30.

Sullivan, P. P., C.-H. Moeng, B. Stevens, D. H. Lenschow, and S. D. Mayor, 1998: Structure of the entrainment zone capping the convective atmospheric boundary layer. Journal of the Atmospheric Sciences, 55, 3042-3064.

Ayotte, K. W., P. P. Sullivan, A. Andren, S. C. Doney, A. Holtslag, W. G. Large, J. C. McWilliams, C.-H. Moeng, M. Otte, J. Tribbia, J. C. Wyngaard, 1996: An evaluation of neutral and convective planetary boundary layer parameterizations relative to large eddy simulation. Boundary-Layer Meteorology, 79, 131-175.

Cloudy PBLs:

Moeng, C-H., B. Stevens, & P.P. Sullivan, 2004: Large-eddy simulation of cloud-topped mixed layers. In Atmospheric Turbulence and Mesoscale Meteorology, Eds. E. Federovich, R. Rotunno, & B. Stevens, Cambridge University Press.

Moeng, C.-H., P. P. Sullivan, and B. Stevens, 1999: Including radiative effects in an entrainment-rate formula for buoyancy-driven PBLs. Journal of Atmospheric Sciences, 56, 1031--1049.

Moeng, C.-H., 1986: Large-eddy simulation of a stratus-topped boundary layer. Part I: Structure and budgets. Journal of the Atmospheric Sciences, 43, 2886-2900.

Moeng, C.-H., 1987: Large-eddy simulation of a stratus-topped boundary layer. Part II: Implications for mixed-layer modeling. Journal of the Atmospheric Sciences, 44, 1605-1614.

Schumann, U., and C.-H. Moeng, 1991: Plume fluxes in clear and cloudy convective boundary layers. Journal of the Atmospheric Sciences, 48, 1746--1757.

Schumann, U., and C.-H. Moeng, 1991: Plume budgets in clear and cloudy convective boundary layers. Journal of the Atmospheric Sciences, 48, 1758--1770.

Moeng, C.-H., and U. Schumann, 1991: Composite structure of plumes in stratus-topped boundary layers. Journal of the Atmospheric Sciences, 48, 2280--2291.

Randall, D. A., Q. Shao, and C.-H. Moeng, 1992: A second-order bulk boundary-layer model.
Journal of the Atmospheric Sciences, 49, 1903--1923.

Moeng, C.-H., S. Shen, and D. A. Randall, 1992: Physical processes within the nocturnal stratus-topped boundary layer. Journal of the Atmospheric Sciences, 49, 2384--2401.

Moeng, C.-H., W. R. Cotton, C. Bretherton, A. Chlond, M. Khairoutdinov, S. Krueger, W. S. Lewellen, M. K. MacVean, J.R.M. Pasquier, H. A. Rand, A. P. Siebesma, R. I. Sykes, B. Stevens, 1996: Simulation of a stratocumulus-topped PBL: Intercomparison among different numerical codes. Bulletin of the Amer. Meteor. Soc., 77, 261--278.

Land-surface interactions, Canopies, Chemistry and Dispersion in the PBL:

Weil, J.C., P.P. Sullivan & C-H. Moeng, 2004: On the use of large-eddy simulations in lagrangian particle dispersion models. J. Atmospheric Sciences, accepted.

Patton, E.G., P.P. Sullivan, & C-H. Moeng, 2003: Influence of idealized heterogeneity on wet and dry planetary boundary layers coupled to the land surface. I: Instantaneous fields and statistics. J. Atmospheric Sciences, submitted.

Patton, E.G, P.P. Sullivan, and K.J. Davis, 2003: The influence of a forest canopy on top-down and bottom-up diffusion in the planetary boundary layer. Q.J. Royal Meteorological Society, 129,
1415-1434.

Patton, E.G., K.J. Davis, M.C. Barth, & P.P. Sullivan, 2001: Decaying scalars emitted by a forest canopy: A numerical study. Boundary-Layer Meteorology, 100, 91-129.

Subgrid-scale Investigations:

Sullivan, P.P, T.W. Horst, D.H. Lenschow, C-H. Moeng, & J.C. Weil, 2003: Structure of subfilter-scale fluxes in the atmospheric surface layer with application to large-eddy simulation modeling. J. Fluid Mech., 482, 101-139.

Horst, T.W., J. Kleissl, D.H. Lenschow, C. Meneveau, C-H. Moeng, M.B. Parlange, P.P. Sullivan, & J.C. Weil, 2003: HATS: Field observations to obtain spatially-filtered turbulence fields from crosswind arrays of sonic anemometers in the atmospheric surface layer. J. Atmos. Sci., accepted.

Dubrulle, B., J-P. Laval, P.P. Sullivan, & J. Werne, 2002: A new dynamical subgrid model for the planetary surface layer. I. The model and a priori tests. Journal of the Atmospheric Sciences, 59, 861-876.

Sullivan, P. P., J. C. McWilliams, and C.-H. Moeng, 1994: A subgrid-scale model for large-eddy simulation of planetary boundary-layer flows. Boundary-Layer Meteorology, 71, 247-276.

Ocean mixed layers:

McWilliams, J.C. and P.P. Sullivan, 2000: Vertical mixing by Langmuir circulations. Spill Science and Technology Bulletin, 6, 225-237.

Sullivan, P.P., J.C. McWilliams, & W.K. Melville, 2003: The oceanic boundary layer driven by wave breaking with stochastic variability. I: Direct numerical simulations. J. Fluid Mechanics,
accepted in press.

McWilliams, J.C. and P.P. Sullivan, 2001: Surface-wave effects on marine boundary layers. In: Fluid Mechanics and the Environment: Dynamical Approaches, J. Lumley, ed., Springer-Verlag, pp 412.

McWilliams, J. C., P. P. Sullivan, and C.-H. Moeng, 1997: Langmuir turbulence in the ocean. Journal of Fluid Mechanics, 334, 1-30.

McWilliams, J.C., C-H. Moeng, and P. P. Sullivan, 1999: Turbulent fluxes and coherent structures in marine boundary layers: Investigations by large-eddy simulation. In: Air-Sea Exchange: Physics, Chemistry, Dynamics, and Statistics, G. Geernaert, ed., Kluwer Publishers.

McWilliams, J. C., P. C. Gallacher, C-H. Moeng and J. C. Wyngaard, 1993: Modeling the oceanic planetary boundary layer. In Large-Eddy Simulations of Complex Engineering and Geophysical Flows, Eds. B. Galperin and S. A. Orszag, Cambridge University Press, 441-454.

Research Topics Include: Marine stratocumulus regime, Clear-air boundary layers, and Cold-air outbreak convection

Large-Eddy Simulation (LES)

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