Direct evaluation of scalar and velocity structure functions in the atmospheric convective boundary layer from large eddy simulation output

Second-order structure functions of potential temperature, specific humidity, and flow velocity components in the atmospheric convective boundary layer (CBL) are directly evaluated from large eddy simulation (LES) output. Structure functions and associated structure-function parameters are often used for quantification of the spatial variability of meteorological fields. Particularly, spatial variability of temperature and humidity can strongly affect the propagation of acoustic and electromagnetic waves in the atmosphere. This variability can also significantly modify signals measured by remote sensing devices such as radars and sodars that are used to investigate atmospheric flow structure.

Since LES provides values of the considered meteorological variables in every point in the simulation domain at every time step, it appears attractive to use LES output data for structure-function parameter calculations. Previous studies estimated structure-function parameters from LES of CBL flow fields using supplementary relations between these parameters and turbulence dissipation/destruction rates. In our study, structure functions are calculated directly from the simulated flow fields.

Calculated structure functions plotted versus separation distance exhibit a 2/3 power slope over an extended range of separations. It is for this range that structure-function parameter definition makes sense. The lower limit of the 2/3 slope range depends on simulation spatial resolution and the subfilter-scale model employed in LES. The behavior of structure-function parameters evaluated from LES in the lower portion of the CBL generally agrees with Monin-Obukhov similarity theory predictions. In the mixed portion of the layer, the temperature structure-function parameter nondimensionalized using mixed-layer scales is found to follow the -4/3 law. The magnitude of the proportionality coefficient is, however, smaller than its previous estimates in the literature. As in previous numerical and observational studies, the nondimensionalized mixed-layer structure-function parameter for humidity is strongly influenced by entrainment. For structure functions of horizontal velocity components, the ratio of transverse and longitudinal structure-function parameters is approximately 4/3. This ratio tends to be larger than 4/3 for structure functions of the vertical velocity component. Shear-induced anisotropy of turbulence structure in the CBL is investigated through comparison of structure functions calculated from sheared and idealized shear-free simulations.