Integral length scales in a low-roughness atmospheric boundary layer

Abstract

This paper discusses the integral length scales in a low-roughness atmospheric boundary layer (ABL), based on the high-fidelity measurements of wind velocity. Results from the analysis shows that longitudinal integral length scales follow a linear relationship with height in a low-roughness ABL that deviates significantly from semi-empirical Engineering Sciences Data Unit (ESDU) 85020 model derived for open country and urban terrains with larger surface roughness heights. Although the model accurately predicts the integral length scales non-dimensionalised relative to the boundary layer thickness for the majority of the profile, they are over-predicted by more than double in the lowest 10% of the ABL, corresponding to the atmospheric surface layer (ASL). The analysis shows that the largest eddies at lower heights in the ASL over a very low roughness desert terrain have length scales similar to the characteristic lengths of physical structures positioned on the ground, which corresponds to the maximum wind loads for buildings. Hence, it is recommended that the integral length scales in the ASL are characterised over an estimated range at each of the four terrain categories in AS/NZS 1170.2 to ensure that buildings and other large physical structures can be optimised in terms of their size and location.