Schmidt, H. and Schumann, U. (1989) Coherent Structure of the Convective Boundary Layer Derived from LargeEddy Simulations. Journal of Fluid Mechanics, 200, pp. 511562. DOI: 10.1017/S0022112089000753.

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Abstract
Turbulence in the convective boundary layer (CBL) uniformly heated from below and topped by a layer of uniformly stratified fluid is investigated for zero mean horizontal flow using largeeddy simulations (LES). The Rayleigh number is effectively infinite, the Froude number of the stable layer is 0.09 and the surface roughness height relative to the height of the convective layer is varied between 10<sup>6</sup> and 10<sup>2</sup>. The LES uses a finitedifference method to integrate the threedimensional gridvolumeaveraged Navier–Stokes equations for a Boussinesq fluid. Subgridscale (SGS) fluxes are determined from algebraically approximated secondorder closure (SOC) transport equations for which all essential coefficients are determined from the inertialrange theory. The surface boundary condition uses the Monin–Obukhov relationships. A radiation boundary condition at the top of the computational domain prevents spurious reflections of gravity waves. The simulation uses 160 × 160 × 48 grid cells. In the asymptotic state, the results in terms of vertical mean profiles of turbulence statistics generally agree very well with results available from laboratory and atmospheric field experiments. We found less agreement with respect to horizontal velocity fluctuations, pressure fluctuations and dissipation rates, which previous investigations tend to overestimate. Horizontal spectra exhibit an inertial subrange. The entrainment heat flux at the top of the CBL is carried by cold updraughts and warm downdraughts in the form of wisps at scales comparable with the height of the boundary layer. Plots of instantaneous flow fields show a spoke pattern in the lower quarter of the CBL which feeds largescale updraughts penetrating into the stable layer aloft. The spoke pattern has also been found in a few previous investigations. Smallscale plumes near the surface and remote from strong updraughts do not merge together but decay while rising through largescale downdraughts. The structure of updraughts and downdraughts is identified by threedimensional correlation functions and conditionally averaged fields. The mean circulation extends vertically over the whole boundary layer. We find that updraughts are composed of quasisteady largescale plumes together with transient rising thermals which grow in size by lateral entrainment. The skewness of the vertical velocity fluctuations is generally positive but becomes negative in the lowest mesh cells when the dissipation rate exceeds the production rate due to buoyancy near the surface, as is the case for very rough surfaces. The LES results are used to determine the rootmeansquare value of the surface friction velocity and the mean temperature difference between the surface and the mixed layer as a function of the roughness height. The results corroborate a simple model of the heat transfer in the surface layer.
Document Type:  Article  

Title:  Coherent Structure of the Convective Boundary Layer Derived from LargeEddy Simulations  
Authors: 
 
Date:  1989  
Journal or Publication Title:  Journal of Fluid Mechanics  
Volume:  200  
DOI:  10.1017/S0022112089000753  
Page Range:  pp. 511562  
Status:  Published  
Keywords:  LES, SGS, SOC, CBL  
HGF  Research field:  Aeronautics, Space and Transport (old)  
HGF  Program:  Space (old)  
HGF  Program Themes:  W EO  Erdbeobachtung  
DLR  Research area:  Space  
DLR  Program:  W EO  Erdbeobachtung  
DLR  Research theme (Project):  W  Vorhaben Atmosphären und Klimaforschung (old)  
Location:  Oberpfaffenhofen  
Institutes and Institutions:  Institute of Atmospheric Physics  
Deposited By:  Jana Freund  
Deposited On:  15 Feb 2008  
Last Modified:  12 Dec 2013 20:29 
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