WIND CHARACTERISTICS IN THE SURFACE-LAYER OVER HETEROGENEOUS TERRAIN

The flow in the atmospheric surface layer is summarized with emphasis on those properties which are of primary importance for the prediction of wind pressures on low-rise structures from wind tunnel simulation experiments. For neutral flow near the surface over flat, smooth and uniform (FSU) terrain, flow models derived from similarity analysis match the available observations quite well. However as the upwind terrain becomes increasingly perturbed with scattered obstacles such as buildings, trees etc. or with topographic features, the observed flow characteristics deviate increasingly from those associated with the FSU model, and parameters derived from mean wind profiles cannot be used to predict turbulence intensities. Over complex terrain the large-scale horizontal velocity fluctuations bear the characteristics of the upwind terrain and their intensities can only be predicted with the use of a "regional" roughness length. Based on the variation and scale of the upwind terrain roughness and topography, four aerodynamic terrain categories each with its own flow parameters are recognized. A general model for the velocity spectra has been presented, for which the coefficients and exponents have been adapted to each of the four terrain categories. Predicted spectral shapes are compared with existing models and existing observations.