TURBULENT CHARACTERISTICS OVER A ROUGH NATURAL SURFACE .1. TURBULENT STRUCTURES

A comprehensive experiment for turbulence and profiles has been carried out over a very rough natural mallee bushland. In Part I, the large inherent structures of the turbulence are presented. Short-period space-time correlations of wind and temperature indicate that periodic fluctuations pervade the lower surface layer above the canopy. These are associated with the appearance of large turbulent structures. Prominent peaks in the spectra are identified with the arrival of these structures, which show that they have a relatively large length scale over rough surfaces. These findings suggest a different mechanism for the enhancement of turbulent diffusivity over rough surfaces from that by the simple generation of turbulence by plant wakes. Analysis via conditional sampling of the turbulent components has revealed that 'bursts' make almost the same contributions as 'gusts' (at the measurement height of 8.4 m above ground) for either momentum or temperature under neutral conditions; for unstable conditions and high 'hole' size H (see Equation (3)), gusts dominate for momentum while bursts dominate for heat transfer. This implies that the mechanisms for momentum and heat transfer are different. © 1990 Kluwer Academic Publishers.