Variability of Local Structure Parameters in the Convective Boundary Layer

The propagation of optical and acoustical waves is affected by the atmospheric turbulence through the local instantaneous refractive index structure parameter in a volume of characteristic size r, denoted C-n,r(2). Often r is well within the inertial-convective range. In this study, a large-eddy simulation (LES) of spatial resolution Delta is used to analyze the distribution of C-n,Delta(2) in the convective boundary layer. The local formulation used to calculate C-n,Delta(2) is described and is found to back up the subgrid parameterization of atmospheric LES. The mean vertical profile behaves according to the mixed layer similarity theory. The spatial organization relates to the presence of buoyant ascending plumes. This structure is associated with some bimodal probability density functions of the inertial-convective range variables, in agreement with experimental results. The standard model of jointly lognormal statistics is challenged. The intermittency of these variables is characterized, and its inertial-convective range component quantitatively agrees with experimental estimates. The present LES results are used to document the bias produced by averaging the dissipation rates of TKE and temperature variance to estimate the average structure parameters. Comparing with nonconvective turbulence measurements, the bias for C-T,r(2) is found to depend on the stability. A physical explanation is offered that emphasizes the role of the large-scale turbulence under convective conditions.