Examining turbulent structures within canopy sublayer

The proper orthogonal decomposition (POD) technique is applied to the turbulent flow below and just above canopy to extract coherent structures by decomposing the velocity field into a series of eigenmodes. The coherent structure projected onto the cross-stream plane is found to be composed of a strong sweep motion encompassed by a pair of counter-rotating vortices, consistently with precedent findings from a wind tunnel experiment. The effects of vegetation density on coherent structures was also investigated. One dimensional analysis reveals that the energy percentage of coherent structure increases as the vegetation goes from sparse to dense. The pattern of coherent structure resembles that in a rough boundary layer for the sparse case, and for the dense case agrees well with that obtained from the wind tunnel experiment. The evolution of coherent structure across a forest-grass discontinuity was also explored and it was found that the contribution of coherent structure to the TKE decreases sharply while experiencing the transition from forest to grass, and then increase gradually to match its value in rough boundary layer. The counter-rotating vortex pair also moves downward and then upward as it goes downstream from forest to grass.