Preliminary Results from Numerical Integrations of the Unstable Planetary Boundary Layer
01 Nov 1970-Journal of the Atmospheric Sciences (American Meteorological Society)-Vol. 27, Iss: 8, pp 1209-1211
About: This article is published in Journal of the Atmospheric Sciences.The article was published on 1970-11-01. It has received 156 citations till now. The article focuses on the topics: Planetary boundary layer.
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TL;DR: In this paper, the authors provide a clear theoretical framework for the simulation of turbulent flows, and the source of the subgrid terms in the Navier-Stokes equation is clarified.
Abstract: The spatial resolution appropriate for the simulation of deep moist convection is addressed from a turbulence perspective. To provide a clear theoretical framework for the problem, techniques for simulating turbulent flows are reviewed, and the source of the subgrid terms in the Navier‐Stokes equation is clarified. For decades, cloud-resolving models have used large-eddy simulation (LES) techniques to parameterize the subgrid terms. A literature review suggests that the appropriateness of using traditional LES closures for this purpose has never been established. Furthermore, examination of the assumptions inherent in these closures suggests that grid spacing on the order of 100 m may be required for the performance of cloud models to be consistent with their design. Based on these arguments, numerical simulations of squall lines were conducted with grid spacings between 1 km and 125 m. The results reveal that simulations with 1-km grid spacing do not produce equivalent squallline structure and evolution as compared to the higher-resolution simulations. Details of the simulated squall lines that change as resolution is increased include precipitation amount, system phase speed, cloud depth, static stability values, the size of thunderstorm cells, and the organizational mode of convective overturning (e.g., upright towers versus sloped plumes). It is argued that the ability of the higher-resolution runs to become turbulent leads directly to the differences in evolution. There appear to be no systematic trends in specific fields as resolution is increased. For example, mean vertical velocity and rainwater values increase in magnitude with increasing resolution in some environments, but decrease with increasing resolution in other environments. The statistical properties of the simulated squall lines are still not converged between the 250- and 125-m runs. Several possible explanations for the lack of convergence are offered. Nevertheless, it is clear that simulations with O(1 km) grid spacing should not be used as benchmark or control solutions for resolution sensitivity studies. The simulations also support the contention that a minimum grid spacing of O(100 m) is required for traditional LES closures to perform appropriately for their design. Specifically, only simulations with 250- and 125-m grid spacing resolve an inertial subrange. In contrast, the 1-km simulations do not even reproduce the correct magnitude or scale of the spectral kinetic energy maximum. Furthermore, the 1-km simulations contain an unacceptably large amount of subgrid turbulence kinetic energy, and do not adequately resolve turbulent fluxes of total water. A guide to resolution requirements for the operational and research communities is proposed. The proposal is based primarily on the intended use of the model output. Even though simulations with O(1 km) grid spacing display behavior that is unacceptable for the model design, it is argued that these simulations can still provide valuable information to operational forecasters. For the research community, O(100 m) grid spacing is recommended for most applications, because a modeling system that is well founded should be desired for most purposes.
762 citations
Cites methods from "Preliminary Results from Numerical ..."
...Deardorff (1970a) was the first to use this approach....
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TL;DR: In this article, the surface stress and fluxes of heat and moisture are parameterized for use in numerical models of the general circulation of the atmosphere, and the roughness length, Zo, is incorporated in the combination of h/zo.
Abstract: The surface stress and fluxes of heat and moisture are parameterized for use in numerical models of the general circulation of the atmosphere. The parameterization is designed to be consistent with recent advances in knowledge of both the planetary boundary layer and the surface layer. A key quantity throughout is the height, h, of the planetary boundary layer, which appears in the governing stability parameter, a bulk Richardson number. With upward heat flux, a time-dependent prediction equation is proposed for h that incorporates penetrative convection and vertical motion. Under stable conditions, h is assumed to depart from the neutral value and to become nearly proportional to the Monin-Obukhov length. The roughness length, Zo, is incorporated in the combination h/zo, and the parameterization is consistent with h/zo affecting only the wind component in the direction of the surface velocity. The direction of the surface wind and stress is derived in a manner consistent with the known value of ...
522 citations
TL;DR: Why RANS is still frequently used and whether this is justified or not is illustrated by examples for five application areas in building simulation: pedestrian-level wind comfort, near-field pollutant dispersion, urban thermal environment, natural ventilation of buildings and indoor airflow.
Abstract: Large Eddy Simulation (LES) undeniably has the potential to provide more accurate and more reliable results than simulations based on the Reynolds-averaged Navier-Stokes (RANS) approach. However, LES entails a higher simulation complexity and a much higher computational cost. In spite of some claims made in the past decades that LES would render RANS obsolete, RANS remains widely used in both research and engineering practice. This paper attempts to answer the questions why this is the case and whether this is justified, from the viewpoint of building simulation, both for outdoor and indoor applications. First, the governing equations and a brief overview of the history of LES and RANS are presented. Next, relevant highlights from some previous position papers on LES versus RANS are provided. Given their importance, the availability or unavailability of best practice guidelines is outlined. Subsequently, why RANS is still frequently used and whether this is justified or not is illustrated by examples for five application areas in building simulation: pedestrian-level wind comfort, near-field pollutant dispersion, urban thermal environment, natural ventilation of buildings and indoor airflow. It is shown that the answers vary depending on the application area but also depending on other—less obvious—parameters such as the building configuration under study. Finally, a discussion and conclusions including perspectives on the future of LES and RANS in building simulation are provided.
278 citations
Cites background or methods from "Preliminary Results from Numerical ..."
...What is now called LES emerged from the early days of NWP by the efforts of Smagorinsky, Lilly, Deardorff and others that followed in Richardson’s footsteps (e.g. Smagorinsky 1963; Lilly 1962, 1964; Deardorff 1970a,b,c,d)....
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...Later, he focused on idealized neutral (Deardorff 1970b) and unstable (Deardorff 1970c, 1972) planetary boundary layers....
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...In 1970, he published a first simulation of turbulent channel flow in a domain with 24 × 14 horizontal and 20 vertical grid points (Deardorff 1970a)....
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TL;DR: Turbulent Boussinesq convection under the influence of rapid rotation was studied in this paper, where the transition to turbulence proceeds through a relatively simple bifurcation sequence, starting with unstable convection rolls at moderate Rayleigh (Ra) and Taylor numbers (Ta), and culminating in a state dominated by coherent plume structures at high Ra and Ta.
Abstract: Turbulent Boussinesq convection under the influence of rapid rotation (ie with comparable characteristic rotation and convection timescales) is studied The transition to turbulence proceeds through a relatively simple bifurcation sequence, starting with unstable convection rolls at moderate Rayleigh (Ra) and Taylor numbers (Ta) and culminating in a state dominated by coherent plume structures at high Ra and Ta Like non-rotating turbulent convection, the rapidly rotating state exhibits a simple power-law dependence on Ra for all statistical properties of the flow When the fluid layer is bounded by no-slip surfaces, the convective heat transport (Nu − 1, where Nu is the Nusselt number) exhibits scaling with Ra2/7 similar to non-rotating laboratory experiments When the boundaries are stress free, the heat transport obeys ‘classical’ scaling (Ra1/3) for a limited range in Ra, then appears to undergo a transition to a different law at Ra ≈ 4 × 107 Important dynamical differences between rotating and non-rotating convection are observed: aside from the (expected) differences in the boundary layers due to Ekman pumping effects, angular momentum conservation forces all plume structures created at flow-convergent sites of the heated and cooled boundaries to spin-up cyclonically; the resulting plume/cyclones undergo strong vortex-vortex interactions which dramatically alter the mean state of the flow and result in a finite background temperature gradient as Ra → ∞, holding Ra/Ta fixed
252 citations
TL;DR: In this article, a large eddy simulation (LES) code was developed and applied to study the effect of spatially variable surface properties on the areally averaged surface shear stress at the land-atmosphere interface.
Abstract: A large eddy simulation (LES) code of the atmospheric boundary layer (ABL) has been developed and applied to study the effect of spatially variable surface properties on the areally averaged surface shear stress at the land-atmosphere interface. The LES code simulates the space and time evolution of the large-scale turbulent eddies and their transport effects in the ABL. We report here on simulations of flow over spatially variable roughness fields. The dynamics are simulated, and the resulting space-time fields are averaged to explore the effects of the surface variability length scales on the average surface shear stress, as used in large-scale models to estimate scalar fluxes, such as evaporation. We observe asymmetrical response of the smooth-to-rough and rough-to- smooth transitions, such that the effects of the transitions accumulate rather than cancel. It is shown that the presence of abrupt changes in surface roughness and the atmosphere's response to these patches create a marked dependence of the statistical structure of surface shear stress on the length scale of the surface patches. An increase in regionally averaged surface stress for decreasing horizontal patch length scale is found.
251 citations
Cites methods from "Preliminary Results from Numerical ..."
...The LES technique has proven useful for simulating boundary layer dynamics for both neutrally and unstably stratified flows [e.g., Deardorff, 1970a, b; Moeng, 1984; Schmidt and Schumann, 1989]....
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