Scale Invariance of Albedo‐Based Wind Friction Velocity

Nancy P Ziegler,Sandra L Legrand,Nicholas P Webb,Adrian Chappell

doi:10.1029/2019jd031978

Abstract

AbstractObtaining 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.

Highlights

Wind erosion and dust emission exhibit large spatiotemporal variability in response to land surface‐atmosphere interactions within the planetary boundary layer (Bergametti & Gillette, 2010; Sweeney et al, 2011)
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
Consistent with understanding of momentum partitioning over vegetated surfaces, these results show that estimates of z0 and u*/Uh obtained from wind speed profiles described the total site roughness and u* but poorly described us* that drives aeolian sediment transport

Summary

Introduction

Wind erosion and dust emission exhibit large spatiotemporal variability in response to land surface‐atmosphere interactions within the planetary boundary layer (Bergametti & Gillette, 2010; Sweeney et al, 2011). These interactions are controlled by soil properties, atmospheric turbulence, and drag partitioning between roughness elements at different scales that affect the aerodynamic roughness length (z0), total wind friction velocity (u*), and wind friction velocity at the soil surface or bed (us*) (Raupach et al, 1993). Measuring the spatiotemporal variability of z0 to parameterize areal dust models would require multiple samples in space

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