STRUCTURE-FUNCTION PARAMETERS IN THE CONVECTIVE BOUNDARY-LAYER FROM LARGE-EDDY SIMULATION

The conventional theory of the scattering of transmitted acoustic, mic rowave, and optical radiation by turbulence uses the refractive index structure-function parameter C(N)2. The authors calculate the vertical profiles of the structure-function parameters on which C(N)2 depends, those for temperature and humidity, C(T)2 and C(Q)2, and their joint structure-function parameter, C-TQ, from large-eddy simulation (LES) d ata for convective boundary layers. The results agree well with experi mental measurements. Modern views of wave propagation through turbulen ce with substantial intermittency, such as that found in the high Reyn olds number flows in geophysics, suggest that the structure-function p arameter be interpreted as a local flow variable rather than the tradi tional ensemble average. Through the refined Kolmogorov-Obukhov simila rity hypotheses, a set of local structure-function parameters is defin ed that depends on locally averaged values of the molecular destructio n rates of velocity and scalar variances. Through analysis of the loca lly averaged variance budgets, the coupling between the resolvable-sca le fields in LES and these local destruction rates are outlined, with the focus on scalars. Using data from direct numerical simulation, we test two models of the locally averaged destruction rate of scalar var iance. Each emulates its approximately lognormal statistics and can be used with LES codes, enabling predictions of local structure-function parameter fields.