Direct Evaluation of Refractive-Index Structure Functions from Large-Eddy Simulation Output for Atmospheric Convective Boundary Layers

Fluctuations of the refractive index associated with atmospheric turbulence affect the propagation of electromagnetic and acoustic waves in the atmosphere. In the reported study, the effects of turbulence on wave propagation in the atmospheric convective boundary layer (CBL) are considered in terms of the second-order refractive index structure function and related to its structure-function parameter C-n(2). Two structure-function evaluation methods are compared. The direct evaluation method involves calculating the refractive index at each point in the simulation domain with subsequent calculation of the structure function. The second method is based on a parameterized linear relationship between the refractive-index structure function and temperature/humidity structure functions. For each evaluation method, vertical profiles of C-n(2) computed for separations along the three coordinate directions collapse together over a significant portion of the CBL. Near-surface divergence of C-n(2) values along the horizontal directions was noted and attributed to the influence of surface wind shear on the turbulent fluctuations of temperature and humidity. The behavior of C-n(2) near the surface was shown to agree favorably with similarity-theory predictions.