Estimating aerodynamic roughness over complex surface terrain

Nield, Joanna M., King, James, Wiggs, Giles F.S., Leyland, Julian, Bryant, Robert G., Chiverrell, Richard C., Darby, Stephen E., Eckardt, Frank D., Thomas, David S.G., Vircavs, Larisa H. and Washington, Richard (2013) Estimating aerodynamic roughness over complex surface terrain. Journal of Geophysical Research: Atmospheres, 118, 12948-12961. (doi:10.1002/2013JD020632).

Abstract

Surface roughness plays a key role in determining aerodynamic roughness length (zo) and shear velocity, both of which are fundamental for determining wind erosion threshold and potential. Whilst zo can be quantified from wind measurements, large proportions of wind erosion prone surfaces remain too remote for this to be a viable approach. Alternative approaches therefore seek to relate zo to morphological roughness metrics. However, dust emitting landscapes typically consist of complex small-scale surface roughness patterns and few metrics exist for these surfaces which can be used to predict zo for modelling wind erosion potential. In this study terrestrial laser scanning was used to characterise the roughness of typical dust emitting surfaces (playa and sandar) where element protrusion heights ranged from 1 to 199?mm, over which vertical wind velocity profiles were collected to enable estimation of zo. Our data suggest that, although a reasonable relationship (R^2?>?0.79) is apparent between 3D roughness density and zo, the spacing of morphological elements is far less powerful in explaining variations in zo than metrics based on surface roughness height (R^2?>?0.92). This finding is in juxtaposition to wind erosion models that assume the spacing of larger-scale isolated roughness elements is most important in determining zo. Rather, our data show that any metric based on element protrusion height has a higher likelihood of successfully predicting zo. This finding has important implications for the development of wind erosion and dust emission models that seek to predict the efficiency of aeolian processes in remote terrestrial and planetary environments

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Contributors

Author: Joanna M. Nield

Author: Julian Leyland

Author: Stephen E. Darby

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