Quantifying the influence of complex surface roughness on aerodynamic roughness using terrestrial laser scanning to model wind erosion potential

Surface roughness plays a key role in determining the aerodynamic roughness of a wind profile above a surface making it important to quantify its spatial variation when considering environments where aeolian processes dominate. Whilst aerodynamic roughness can be quantified using vertical wind profiles or sonic anemometry, large proportions of terrestrial areas and most planetary surfaces that are prone to wind erosion remain too remote for an aerodynamic measurement-based approach. Surface roughness metrics have been measured for some time in geomorphic research, using high resolution datasets of varying spatial scales, however few metrics for surfaces with complex small-scale roughness have been used to model aeolian processes and these tend to be based on a transect approach. Most previous studies assessing element configuration find that roughness element density and height, rather than the element configuration appear to have the greatest influence on wind erosion potential. However, these studies were designed to consider larger, discrete roughness elements such as vegetation and their performance for surfaces with continuous micro-topography are less well quantified. For planetary surfaces, differences in atmospheric properties scale the effects of larger elements to those measured on much smaller surface roughness on Earth. In this study, terrestrial laser scanning (TLS, ground-based LiDAR) was used on complex playa surfaces in Botswana ranging over one order of magnitude of element physical height (7 to 70 mm). Aerodynamic height measured by vertical wind profiles at sites over a two to four week period were constrained by wind direction, stability parameters, and boundary-layer scaling, to compare with TLS-based metrics on upwind surfaces. A thorough comparison of existing metrics and new metrics resulted in the aerodynamic roughness being best correlated to element mean height based on a zero-up crossing method. Although there is also a strong relationship between 2D or 3D roughness density and aerodynamic roughness, the spacing of the elements is less important than their height. This is in contrast to more recent wind erosion models that consider the spacing of larger-scale isolated roughness elements to be more important, and instead we suggest that the majority of dust emitting areas are mainly controlled by the height of the local roughness. Planform elevation for eight surfaces measured in this study using TLS at Makgadikgadi Pan,Botswana