Abstract
In numerical weather prediction (NWP) models, at mesoscale, the subgrid convective boundary-layer turbulence is dominated by the uni-directional (1D) vertical thermal production. In Large-Eddy Simulations (LES), the thermal plumes are resolved and the residual subgrid turbulent motions are homogeneous and isotropic, thus three-dimensional (3D), resulting from the dynamical production. This article sets the critical horizontal resolution for which the usually 1D turbulence schemes of NWP models must be replaced by 3D turbulence schemes. LES from five dry and cumulus-topped free convective boundary layers and one forced convective boundary layer are performed. From these LES data, the thermal production and vertical and horizontal dynamical productions are calculated at several resolutions from LES to mesoscale. It appears that the production terms of both dry and cumulus-topped free convective boundary layers have the same behavior. A pattern emerges whenever data are ranked by the resolution scaled by the size of thermal plumes, (h + hc , where h is the boundary-layer height and hc is the depth of the cloud layer). In free onvective boundary layers, the critical horizontal resolution for which the horizontal motions must be represented is 0.5(h + hc ). However, the critical horizontal resolution in the forced convective boundary layer case is 3(h + hc ).
Highlights
Wyngaard (2004) first described the gray zone of turbulence, which he calls terra incognita
The authors showed that the gray zone of turbulence ranges from 0.2 to 2(h+hc), where h is the boundary-layer height and hc is the depth of the cloud layer and they proposed recommendations of the sub-grid/resolved partitioning of the turbulence
This article provides recommendations about the dimensionality of a turbulence scheme when used in convective cases in the gray zone of turbulence
Summary
Wyngaard (2004) first described the gray zone of turbulence, which he calls terra incognita. BL thermals were thoroughly investigated in the latter two studies At large scale, these structures produce unidimensional (1D) thermal turbulence (André et al, 1978), on the other hand, in LES, the thermal production is resolved and the turbulence is homogeneous, isotropic and produced by the threedimensional (3D) motions of eddies resulting from dynamical processes. These structures produce unidimensional (1D) thermal turbulence (André et al, 1978), on the other hand, in LES, the thermal production is resolved and the turbulence is homogeneous, isotropic and produced by the threedimensional (3D) motions of eddies resulting from dynamical processes It follows that the usually 1D turbulence schemes must be converted into 3D ones in the gray zone of turbulence. How does this transition proceed on production terms? When is the horizontal grid spacing no longer suitable for a 1D scheme? This article addresses these questions for free and forced CBLs
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