Sediment transport equations used in wind erosion and dust emission models generally incorporate a threshold for particle motion (u*t) with a correction function to account for roughness-induced momentum reduction and aerodynamic sheltering. The prevailing approach is to adjust u*t by the drag partition R, estimated as the ratio of the bare soil threshold (u*ts) to that of the surface in the presence of roughness elements (u*tR). Here, we show that application of R to adjust only the entrainment threshold (u*t = u*ts/R) is physically inconsistent with the effect of roughness on the momentum partition as represented in models and produces overestimates of the sediment flux density (Q). Equations for Q typically include a wind friction velocity scaling term (u*n). As Q scales with wind friction velocity at the soil surface (uS*), rather than total wind friction velocity uT* (implicitly u* in models), u*n must be also adjusted for roughness effects. We further note that the practice of reducing Q by the vegetation cover fraction to account for the physically-protected surface area constitutes double accounting of the surface protection when R is represented through the basal-to-frontal area ratio of roughness elements (s) and roughness density ('). If the drag partition is implemented fully, additional tuning for surface protection is unnecessary to produce more accurate aeolian transport estimates. These findings apply equally to models of the vertical dust flux.

CN - 367635 VL - 42 UR - files/bibliography/20-003.pdf CP - 100560 ER - TY - JOUR T1 - A tribute to Michael R. Raupach for contributions to aeolian fluid dynamics JF - Aeolian Research Y1 - 2015 A1 - Shao, Yaping A1 - Nickling, William A1 - Bergametti, Gilles A1 - Butler, Harry A1 - Chappel, Adrian A1 - Findlater, Paul A1 - Gillies, John A1 - Ishizuka, Masahide A1 - Klose, Martina A1 - Kok, Jasper F. A1 - Leys, John A1 - Lu, Hau A1 - Marticorena, Béatrice A1 - McTainsh, Grant A1 - McKenna-Neuman, Cheryl A1 - Gregory S. Okin A1 - Strong, Craig A1 - Webb, Nicholas KW - carbon cycle KW - Drag partition KW - dust emission KW - Michael R. Raupach KW - roughness KW - threshold friction velocity AB -Since the early work of Bagnold in the 1940s, aeolian research has grown to become a major integral part of earth-system studies. Many individuals have contributed to this development, and Dr. Michael R. Raupach (MR2, 1950 – 2015) was one of the most outstanding. MR2 worked for about a decade (1985 – 1995) intensively on wind erosion problems, but he did so brilliantly by relating aeolian problems to his deep knowledge of turbulence, and made profound contributions to the field with far-reaching influences and a lasting legacy. The beauty of MR2s work is crystal clear conceptual thinking, reducing problems to their essentials and expressing that essence with elegance yet simplicity. The results of his work are robust and practically applicable. In this review we reflect on MR2’s contribution to a number of important aeolian research themes, summarize the developments since his inspirational work and place MR2’s effort in the context of aeolian science. We also demonstrate how MR2’s work provided the foundations for many of the new developments in aeolian new research. In this tribute, we concentrate on MR2’s work in five areas: (1) drag partition theory; (2) saltation roughness length; (3) saltation bombardment; (4) threshold friction velocity; and (5) carbon cycle.

CN - 319934 VL - 19 UR - /files/bibliography/15-018.pdf ER - TY - JOUR T1 - Relationship between the aerodynamic roughness length and the roughness density for low roughness density JF - Environmental Fluid Mechanics Y1 - 2003 A1 - Minivielle, F. A1 - Marticorena, G. A1 - Gillette, D. A. A1 - Lawson, R. E. A1 - Thompson, R. A1 - Bergametti, G. KW - aeolian transport KW - article KW - eolian processes, vegetated surfaces KW - journal KW - roughness KW - wind tunnel AB -This paper presents measurements of roughness length performed in a wind tunnel for low roughness density. The experiments were performed with both compact and porous obstacles (clusters), in order to simulate the behavior of sparsely vegetated surfaces. The experimental results have been used to investigate the relationship between the ratio z/h and the roughness density, and the influence of an obstacle’s porosity on this relationship. The experiments performed for four configurations of compact obstacles provide measurements of roughness length z for roughness densities ë between 10^{-3} and 10^{-2} which are in good agreement with the only data set available until now for this range of low roughness densities. The results obtained with artificial porous obstacles suggests that the aerodynamic behavior of such roughness elements can be represented by the relationship established for compact obstacles, provided a porosity index has been used to determine the efficient roughness density (the fraction of the silhouette area actually sheltered by solid elements) rather than counting the porous object as solid. However, the experiments have been performed with relatively low porosity indices (maximum =25%) for which the porosity has a negligible influence. In this range of porosity index, representing the aerodynamic behavior of porous obstacles using the relationship established for compact obstacles, should not lead to a significant error. However, the influence of the porosity may be important for porosity indices larger than 30%.