RESOLVABLE-SCALE AND SUBGRID-SCALE MEASUREMENT IN THE ATMOSPHERIC SURFACE-LAYER - TECHNIQUE AND ISSUES

A new technique for the measurement of two-dimensionally filtered reso lvable- and subgrid-scale (SGS) turbulence in the atmospheric surface layer is studied. The technique uses an array of sensors to do spatial filtering in the direction transverse to the mean flow. Taylor's hypo thesis is used to approximate streamwise filtering with time filtering . The performance of this two-dimensional surrogate filter is evaluate d with data from a high-resolution large-eddy simulation of the atmosp heric boundary layer. In general, both resolvable- and subgrid-scale v elocity and temperature fields obtained from a two-dimensional spectra l filter and the surrogate filter exhibit high cross correlation (>0.8 5-0.95). The correlation between the true and the surrogate SGS stress and temperature flux is somewhat lower than that for the velocities. A detailed analysis of the applicability of Taylor's hypothesis to the energy-containing scales of vertical velocity shows that among the me chanisms that could limit its fidelity, only the effect of fluctuating convection velocity is nonnegligible, and its aliasing effects are mo re significant for stress and scalar-flux fluctuations than for veloci ty fluctuations. The authors suggest this is why the correlations were lower for stress and flux than velocities. The results suggest that t he sensor array is a feasible technique for SGS measurement in the atm ospheric surface layer.