Scale Invariance of Albedo‐Based Wind Friction Velocity

Abstract

Obtaining reliable estimates of aerodynamic roughness is necessary to interpret and accurately predict aeolian sediment transport dynamics. However, inherent uncertainties in field measurements and models of surface aerodynamic properties continue to undermine 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 across scales from albedo data. Here, we compare estimates of wind friction velocity (u*) derived from traditional methods (wind speed profiles) with those derived from the albedo model at two separate scales using bare soil patch (via net radiometers) and landscape (via Moderate Resolution Imaging Spectroradiometer [MODIS] 500 m) data sets. Results show that profile‐derived estimates of u* are highly variable in anisotropic surface roughness due to changes in wind direction and fetch. Wind speed profiles poorly estimate soil surface (bed) wind friction velocities necessary for aeolian sediment transport research and modeling. Albedo‐based estimates of u* at both scales have small variability because the estimate is integrated over a defined, fixed area and resolves the partition of wind momentum between roughness elements and the soil surface. We demonstrate that the wind tunnel‐based calibration of albedo for predicting wind friction velocities at the soil surface (us*) is applicable across scales. The albedo‐based approach enables consistent and reliable drag partition correction across scales for model and field estimates of us* necessary for wind erosion and dust emission modeling.

Weaknesses in dust emission modelling hidden by tuning to dust in the atmosphere

“…In our new formulation called AEM for Albedo-based dust Emission Model, the spatio-temporal variation in 𝑢 𝑠 * is simulated using the concept that aerodynamic sheltering of vegetation is proportional to its shadow (1-albedo) (Chappell et al, 2010;Chappell and Webb, 2016). This albedo-based approximation of the drag partition was investigated and tested to provide an area-weighted value, shown to be scale invariant (Chappell et al, 2018;Chappell et al, 2019;Ziegler et al, 2020). This approach enables direct calculation of 𝑢 𝑠 * given measurements of albedo from satellites, and the correct ) + 0.007.…”

“…Modelled U10 may be too large. However, the scaleinvariant albedo-based approach (Ziegler et al, 2020) able to operate over large grid boxes should eradicate scale differences. The grid-boxes approach used here overcome concerns about precision in the location of dust points.…”

“…However, by comparison with dust in the atmosphere represented by DOD, the use of DPS frequency is up to two orders of magnitude smaller. There is a small, perhaps lower-order likelihood that the original calibration of the albedo-based approach is not representative and universal, despite recent support for the approach (Ziegler et al, 2020).…”

Implementation of an albedo-based drag partition into the WRF-Chem v4.1 AFWA dust emission module

“…Essentially, their technique assumes shadows cast by roughness elements can serve as a proxy for the sheltered-area extent (Figure 1). As described in Ziegler et al (2020),…”

“…To quantify areal estimates of uns* for use in WRF-Chem, we calculated surface shadowing from the 500 m Moderate Resolution Imaging Spectroradiometer (MODIS) Bidirectional Reflectance Distribution Function (BRDF) albedo product: Collection 6, MCD43A1 (Schaaf and Wang 2015). Following Chappell and Webb (2016) and Ziegler et al (2020), we determined the normalized proportion of shadow, ωn, via…”