ANALYSIS OF MONIN-OBUKHOV SIMILARITY FROM LARGE-EDDY SIMULATION

A detailed analysis of the predictability of observed Monin-Obukhov (M O) similarity within the near-ground region of near-neutral to moderat ely convective atmospheric boundary layers (ABL) from large-eddy simul ation (LES) fields is reported in this work. High-resolution LES predi ctions of means, variances, budgets of turbulent kinetic energy and te mperature variance, and the velocity and temperature spectra from thre e ABL states (-z(i)/L = 0.44, 3 and 8) are analysed under MO scaling. The resolution in the near-ground region is increased by using 'nested meshes.' For the close-to-neutral case (-z(i)/L = 0.44) the relative roles of grid resolution and subgrid-scale (SGS) parameterization on t he predictability of MO-similarity are also studied. The simulated tem perature field is found to satisfy the MO hypothesis and agree well wi th observations. The simulated velocity field, on the other hand, show s significant departures. Except for the horizontal variance, MO scale s are the appropriate normalizing scales for the near-ground-layer sta tistics. However, the LES suggest that the boundary layer depth z(i) h as an 'indirect' influence on all near-ground-layer variables except t emperature, and the LES-predicted MO-scaled variables exhibit a functi onal dependence on both z/L and z/z(i). The simulated two-dimensional spectra of velocity and temperature fluctuations, however, suggest tha t while large scales deviate from MO-similarity, inertial subrange sca les are MO-similar. Discrepancies with field observations raise import ant questions of the non-dimensional depth z/z(i) over which MO-simila rity holds for a particular variable. Surface-layer field studies gene rally do not document z(i). It is also not clear to what extent these discrepancies are due to approximations made in LES. Measurements are needed designed specifically for comparing with LES predictions.