Turbulence Characteristics of the Shear-Free Convective Boundary Layer Driven by Heterogeneous Surface Heating |
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| Authors: | Gang Liu Jianning Sun Lei Yin |
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| Institution: | (1) Department of Biological and Agricultural Engineering, The University of Georgia, 1109 Experiment st., Griffin, GA 30223, USA;(2) Lab for Environmental Physics, The University of Georgia, 1109 Experiment st., Griffin, GA 30223, USA;(3) Department of Geography, University of Florida, Gainsville, FL 32611, USA |
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| Abstract: | Large-eddy simulations (LESs) are employed to investigate the turbulence characteristics in the shear-free convective boundary
layer (CBL) driven by heterogeneous surface heating. The patterns of surface heating are arranged as a chessboard with two
different surface heat fluxes in the neighbouring patches, and the heterogeneity scale Λ in four different cases is taken as 1.2, 2.5, 5.0 and 10.0 km, respectively. The results are compared with those for the
homogeneous case. The impact of the heterogeneity scale on the domain-averaged CBL characteristics, such as the profiles of
the potential temperature and the heat flux, is not significant. However, different turbulence characteristics are induced
by different heterogeneous surface heating. The greatest turbulent kinetic energy (TKE) is produced in the case with the largest
heterogeneity scale, whilst the TKE in the other heterogeneous cases is close to that for the homogeneous case. This result
indicates that the TKE is not enhanced unless the scale of the heterogeneous surface heating is large enough. The potential
temperature variance is enhanced more significantly by a larger surface heterogeneity scale. But this effect diminishes with
increasing CBL height, which implies that the turbulent eddy structures are changed during the CBL development. Analyses show
that there are two types of organized turbulent eddies: one relates to the thermal circulations induced by the heterogeneous
surface heating, whilst the other identifies with the inherent turbulent eddies (large eddies) induced by the free convection.
At the early stage of the CBL development, the dominant scale of the organized turbulent eddies is controlled by the scale
of the surface heterogeneity. With time increasing, the original pattern breaks up, and the vertical velocity eventually displays
horizontal structures similar to those for the homogeneous heating case. It is found that after this transition, the values
of λ/z
i
(λ is the dominant horizontal scale of the turbulent eddies, z
i
is the boundary-layer height) ≈1.6, which is just the aspect ratio of large eddies in the CBL. |
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