Scale invariance of albedo-based wind friction velocity

Authors: ZIEGLER, NANCY - Us Army Engineer Research And Dvelopment Center; WEBB, NICHOLAS - New Mexico State University; CHAPPELL, ADRIAN - Cardiff University; LEGRAND, SANDRA - Us Army Engineer Research And Dvelopment Center

Submitted to: Journal of Geophysical Research Atmospheres
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/1/2020
Publication Date: 7/26/2020
Citation: Ziegler, N., Webb, N., Chappell, A., LeGrand, S.L. 2020. Scale invariance of albedo-based wind friction velocity. Journal of Geophysical Research Atmospheres. 125(16):e2019JD031978. https://doi.org/10.1029/2019JD031978.
DOI: https://doi.org/10.1029/2019JD031978

Interpretive Summary: Approaches are needed to characterize soil and vegetation (roughness) properties that affect wind erosion to improve accuracy of prediction models. However, uncertainties in field measurements and models of roughness properties continue to impact wind erosion research, monitoring, and modeling. A new model based on land surface albedo has been established from wind tunnel experiments that enables estimates of wind erosion to be obtained from albedo data. Here, we compare estimates of wind erosivity derived from traditional methods (wind speed profiles) with those derived from the new model using bare soil patch (via net radiometers) and landscape (via MODIS 500 m) datasets. Results show that estimates of wind erosivity from wind speed profiles are highly variable and therefore uncertain as they change in response to wind speed, direction, turbulence scales, and heterogeneity of the surface roughness. Albedo (shadow) based estimates of wind erosivity at both scales have small variability because they are integrated over the measurement area and resolve the partition of wind energy between roughness elements and the soil surface. We confirm the need for wind speed profiles over a rough surface to be corrected for energy partitioning if erosivity of the wind at the soil surface is of interest, rather than erosivity over roughness canopy. In the absence of a reliable drag partition correction in the field, measurements of albedo will reduce uncertainty in field estimates of surface wind erosivity for wind erosion research and modeling.

Technical Abstract: Obtaining reliable estimates of aerodynamic roughness is necessary to interpret and accurately predict aeolian sediment transport dynamics. However, inherent uncertainties in traditional field measurements and models of surface aerodynamic properties continue to impact basic aeolian research, monitoring, and dust modeling. A new relation between aerodynamic shelter and land surface shadow has been established at the wind tunnel scale, enabling the potential for estimates of wind erosion and dust emission to be obtained from albedo data. Here, we compare estimates of wind friction velocity (u*) derived from traditional methods (wind speed profiles) with those derived from the shadow model at two separate scales using bare soil patch (via net radiometers) and landscape (via MODIS 500 m) datasets. Results show that estimates of u* from wind speed profiles are highly variable and therefore uncertain as they change in response to wind speed, direction, turbulence scales, and heterogeneity of the surface roughness. Shadow-based estimates of u* at both scales have small variability because they are integrated over the measurement area and resolve the partition of wind momentum between roughness elements and the soil surface. We demonstrate that the wind tunnel-based relation for predicting wind friction velocities at the soil surface (uS*) is scale invariant. We confirm the need for all wind speed profiles over a rough surface to be corrected for the drag partition if uS* is of interest. In the absence of a reliable drag partition correction in the field, measurements of albedo will reduce uncertainty in field estimates of uS* for wind erosion and dust emission modeling.