EVIDENCE OF PRESSURE-FORCED TURBULENT-FLOW IN A FOREST

Lagged cross-correlation analyses between streamwise velocity at several heights within and above a forest, and between streamwise velocity and surface pressure, provide evidence that turbulence in the sub-crown region of the forest is to a large extent driven by pressure perturbations. The analyses support earlier results based on examination of coherent structures observed in the same forest. The phase of the streamwise velocity signal exhibits an increasing delay with decreasing height, indicative of a downwind tilted structure, until the upper region of the forest is reached, at which point the effect is reversed. It is suggested that positive pressure perturbations ahead of advancing microfronts induce streamwise accelerations in the trunk space. This link between the pressure pattern and the wind field explains why velocity spectra in the trunk space are depleted in the higher frequencies, relative to levels above.