Title: Estimating aerodynamic resistance of rough surfaces from angular reflectance Authors
Submitted to: Remote Sensing of Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 30, 2010
Publication Date: July 12, 2010
Citation: Chappell, A., Van Pelt, R.S., Zobeck, T.M., Dong, Z. 2010. Estimating aerodynamic resistance of rough surfaces from angular reflectance. Remote Sensing of Environment. 114(7):1462-1470. Interpretive Summary: In spite of recent advances in process-based wind erosion modeling, surface roughness elements and the effects they have on wind resistance and surface protection are largely ignored. This results in potential overestimation of horizontal sand flux and fugitive dust emissions. Surfaces are routinely sensed from remote locations and the potential of remotely sensed surface characteristics offers great opportunity to easily incorporate surface characteristics into wind erosion models. We simulated angular reflectance of roughness elements from existing digital elevation models and successfully matched wind flow and erosion data from field and wind tunnel studies. The technique offers great promise for temporally refining wind erosion prediction technology using remotely sensed surface roughness induced shadows.
Technical Abstract: Current wind erosion and dust emission models neglect the heterogeneous nature of surface roughness and its geometric anisotropic effect on aerodynamic resistance, and over-estimate the erodible area by assuming it is not covered by roughness elements. We address these shortfalls with a new model which estimates aerodynamic roughness length (z0) using angular reflectance of a rough surface. The new model is proportional to the frontal area index, directional, and represents the geometric anisotropy of z0. The model explained most of the variation in two sets of wind tunnel measurements of aerodynamic roughness lengths (z0). Field estimates of z0 for varying wind directions were similar to predictions made by the new model. The model was used to estimate the erodible area exposed to abrasion by saltating particles. Vertically integrated horizontal flux (Fh) was calculated using the area not covered by non-erodible hemispheres; the approach embodied in dust emission models. Under the same model conditions, Fh estimated using the new model was up to 85% smaller than that using the conventional area not covered. These Fh simulations imply that wind erosion and dust emission models without geometric anisotropic sheltering of the surface, may considerably over-estimate Fh and hence the amount of dust emission. The new model provides a straightforward method to estimate aerodynamic resistance with the potential to improve the accuracy of wind erosion and dust emission models, a measure that can be retrieved using bi-directional reflectance models from angular satellite sensors, and an alternative to notoriously unreliable field estimates of z0 and their extrapolations across landform scales.