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 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 high wind ocean boundary layers with Langmuir cells and wave breaking (partial citation list). 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 (hills and time varying surface wave fields). The code is written to run on massively parallel computer architectures using the Message-Passing Interface (MPI).

Older well tested versions of the NCAR LES code are 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).

Subsection of an image from a Large Eddy Simulation.

 

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LES-related publications by topic

MMM Research on the Planetary Boundary Layer