2-POINT COHERENCE IN THE ATMOSPHERIC SURFACE-LAYER

Two-point, one-dimensional coherence in horizontally homogeneous atmos pheric turbulence is studied, both by experiment and analysis. Measure ments are carried out using horizontally spaced sensors with the separ ation perpendicular to the mean velocity. Two-dimensional spectral mod els and three-dimensional inertial-range spectral tensors are used in the coherence calculations. The one-dimensional coherence for both vel ocity and scalar fluctuations is found to roll off at a wavenumber muc h smaller than we would expect from the classical notion of eddy corre lation. This is a consequence of the cancellation of Fourier component s aliased from the direction of the sensor separation into the streamw ise direction. However, the coherence for the three velocity component s behaves somewhat differently reflecting the relative orientations of the velocity component, sensor separation and the mean velocity These features are well predicted by the calculation. The analysis is also extended to calculate the two-point scalar-vertical velocity cospectru m and the results are in good agreement with our experimental data. Th e ratio of two- to one-point cospectra decreases at slightly larger wa venumber than the two-point scalar coherence does.