Large-Eddy Simulation: How Large is Large Enough?
- 1 February 2004
- journal article
- Published by American Meteorological Society in Journal of the Atmospheric Sciences
- Vol. 61 (4), 403-421
- https://doi.org/10.1175/1520-0469(2004)061<0403:lshlil>2.0.co;2
Abstract
The length scale evolution of various quantities in a clear convective boundary layer (CBL), a stratocumulus- topped boundary layer, and three radiatively cooled (''smoke cloud'') convective boundary layers are studied by means of large-eddy simulations on a large horizontal domain (25.6 3 25.6 km2). In the CBL the virtual potential temperature and the vertical velocity fields are dominated by horizontal scales on the order of the boundary layer depth. In contrast, the potential temperature and the specific humidity fields become gradually dominated by mesoscale fluctuations. However, at the mesoscales their effects on the virtual potential temperature fluctuations nearly compensate. It is found that mesoscale fluctuations are negligibly small only for conserved variables that have an entrainment to surface flux ratio close to 20.25, which is about the flux ratio for the buoyancy. In the CBL the moisture and potential temperature flux ratios can have values that significantly deviate from this number. The geometry of the buoyancy flux was manipulated by cooling the clear convective boundary layer from the top, in addition to a positive buoyancy flux at the surface. For these radiatively cooled cases it is found that both the vertical velocity as well as the virtual potential temperature spectra tend to broaden. The role of the buoyancy flux in their respective prognostic variance equations is discussed. It is argued that in the upper part of the clear CBL, where the mean vertical stratification is stable, vertical velocity variance and virtual potential temperature variance cannot be produced simultaneously. For the stratocumulus case, in which latent heat release effects in the cloud layer play an important role in its dynamics, the field of any quantity, except for the vertical velocity, becomes dominated by mesoscale fluctuations. In general, the location of the spectral peak of any quantity becoming constrained by the domain size should be avoided. The answer to the question of how large the LES horizontal domain size should be in order to include mesoscale fluctuations will, on the one hand, depend on the type of convection to be simulated and the kind of physical question one aims to address, and, on the other hand, the time duration of the simulation. Only if one aims to study the dynamics of a dry CBL that excludes moisture, a rather small domain size suffices. In case one aims to examine either the spatial evolution of the fields of any arbitrary conserved scalar in the CBL, or any quantity in stratocumulus clouds except for the vertical velocity, a larger domain size that allows the development of mesoscale fluctuations will be necessary.Keywords
This publication has 37 references indexed in Scilit:
- Surface energy balance and turbulence characteristics observed at the SHEBA Ice Camp during FIRE IIIJournal of Geophysical Research: Atmospheres, 2001
- Comparison of the ECMWF Reanalysis with FIRE I Observations: Diurnal Variation of Marine StratocumulusJournal of Climate, 2001
- Turbulent length-scales in the marine atmospheric mixed layerQuarterly Journal of the Royal Meteorological Society, 2000
- An intercomparison of radiatively driven entrainment and turbulence in a smoke cloud, as simulated by different numerical modelsQuarterly Journal of the Royal Meteorological Society, 1999
- Broadening of convective cellsQuarterly Journal of the Royal Meteorological Society, 1997
- Mesoscale shallow convection in the atmosphereReviews of Geophysics, 1996
- Scale Invariance of Liquid Water Distributions in Marine Stratocumulus. Part I: Spectral Properties and Stationarity IssuesJournal of the Atmospheric Sciences, 1996
- Observations of Marine Stratocumulus Clouds During FIREBulletin of the American Meteorological Society, 1988
- Further results from a laboratory model of the convective planetary boundary layerBoundary-Layer Meteorology, 1985
- Observations from Space and Thermal Convection: A Historical PerspectiveBulletin of the American Meteorological Society, 1984