The physics of wind-blown sand and dust

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2012 Rep. Prog. Phys. 75 106901

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Rep. Prog. Phys. 75 (2012) 106901 (72pp) doi:10.1088/0034-4885/75/10/106901

The physics of wind-blown sand and dustJasper F Kok1, Eric J R Parteli2,3, Timothy I Michaels4 and Diana Bou Karam51 Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, NY, USA2 Departamento de Fsica, Universidade Federal do Ceara, Fortaleza, Ceara, Brazil3 Institute for Multiscale Simulation, Universitat Erlangen-Nurnberg, Erlangen, Germany4 Southwest Research Institute, Boulder, CO, USA5 LATMOS, IPSL, Universite Pierre et Marie Curie, CNRS, Paris, France

E-mail: jasperkok@cornell.edu

Received 5 January 2012, in final form 4 July 2012Published 14 September 2012Online at stacks.iop.org/RoPP/75/106901

AbstractThe transport of sand and dust by wind is a potent erosional force, creates sand dunes and ripples, andloads the atmosphere with suspended dust aerosols. This paper presents an extensive review of thephysics of wind-blown sand and dust on Earth and Mars. Specifically, we review the physics of aeoliansaltation, the formation and development of sand dunes and ripples, the physics of dust aerosol emission,the weather phenomena that trigger dust storms, and the lifting of dust by dust devils and other small-scalevortices. We also discuss the physics of wind-blown sand and dune formation on Venus and Titan.

(Some figures may appear in colour only in the online journal)This article was invited by G Gillies.

Contents

1. Introduction 11.1. Modes of wind-blown particle transport 21.2. Importance of wind-blown sand and dust to

the Earth and planetary sciences 21.3. Scope and organization of this review 4

2. The physics of wind-blown sand (saltation) 52.1. The four main physical processes of aeolian

saltation 52.2. The path of saltation to steady state 132.3. Steady-state saltation 142.4. Saltation on Mars, Venus and Titan 20

3. Sand dunes and ripples 233.1. The physics of sand dunes and ripples 233.2. Numerical modeling 283.3. Dunes and ripples on Mars and other

planetary bodies 35

4. The physics of dust emission 394.1. The physics of dust emission on Earth 394.2. The physics of dust emission on Mars 46

5. Atmospheric dust-entrainment phenomena 475.1. Dust storms on Earth 475.2. Dust storms on Mars 515.3. Dust entrainment by small-scale vertical

vortices on Earth and Mars 546. Conclusions and remaining questions 55

6.1. Important remaining questions regarding thephysics of wind-blown sand 56

6.2. Important remaining questions regarding thephysics of wind-blown dust 56

Acknowledgments 57References 57

1. Introduction

The wind-driven emission, transport, and deposition of sandand dust by wind are termed aeolian processes, after the Greekgod Aeolus, the keeper of the winds. Aeolian processes occurwherever there is a supply of granular material and atmosphericwinds of sufficient strength to move them. On Earth, thisoccurs mainly in deserts, on beaches, and in other sparselyvegetated areas, such as dry lake beds. The blowing of sandand dust in these regions helps shape the surface through theformation of sand dunes and ripples, the erosion of rocks, and

the creation and transport of soil particles. Moreover, airbornedust particles can be transported thousands of kilometers fromtheir source region, thereby affecting weather and climate,ecosystem productivity, the hydrological cycle, and variousother components of the Earth system.

But aeolian processes are not confined to Earth, and alsooccur on Mars, Venus and the Saturnian moon Titan (Greeleyand Iversen 1985). On Mars, dust storms occasionally obscurethe Sun over entire regions of the planet for days at a time,while their smaller cousins, dust devils, punctuate the mostlyclear daytime skies elsewhere (Balme and Greeley 2006).

0034-4885/12/106901+72$88.00 1 2012 IOP Publishing Ltd Printed in the UK & the USA

http://dx.doi.org/10.1088/0034-4885/75/10/106901mailto: jasperkok@cornell.eduhttp://stacks.iop.org/RoPP/75/106901

Rep. Prog. Phys. 75 (2012) 106901 J F Kok et al

Figure 1. Schematic of the different modes of aeolian transport.Reproduced with permission from Nickling and McKenna Neuman(2009). Copyright 2009 Springer.

The surface of Mars also hosts extensive fields of barchan,transverse, longitudinal and star-like dunes, as well as otherexotic dune shapes that have not been documented on Earth(Bourke et al 2010). On Venus, transverse dunes have beenidentified by the Magellan orbiter (Weitz et al 1994), while theCassini orbiter has documented extensive longitudinal sanddunes on Titan (Lorenz et al 2006).

The terms dust and sand usually refer to solid inorganicparticles that are derived from the weathering of rocks. Inthe geological sciences, sand is defined as mineral (i.e. rock-derived) particles with diameters between 62.5 and 2000 m,whereas dust is defined as particles with diameters smaller than62.5 m (note that the boundary of 62.5 m differs somewhatbetween particle size classification schemes, see Shao 2008,p 119). In the atmospheric sciences, dust is usually definedas the material that can be readily suspended by wind (e.g.Shao 2008), whereas sand is rarely suspended and can thusform sand dunes and ripples, which are collectively termedbedforms.

1.1. Modes of wind-blown particle transport

The transport of particles by wind can occur in severalmodes, which depend predominantly on particle size and windspeed (figure 1). As wind speed increases, sand particles of100 m diameter are the first to be moved by fluid drag(see section 2.1.1). After lifting, these particles hop along thesurface in a process known as saltation (Bagnold 1941, Shao2008), from the Latin salto, which means to leap or spring.The impact of these saltators on the soil surface can mobilizeparticles of a wide range of sizes. Indeed, dust particles arenot normally directly lifted by wind because their interparticlecohesive forces are large compared to aerodynamic forces (seesection 2.1.1). Instead, these small particles are predominantlyejected from the soil by the impacts of saltating particles(Gillette et al 1974, Shao et al 1993a). Following ejection,dust particles are susceptible to turbulent fluctuations andthus usually enter short-term (2070 m diameter) or long-term (20 m diameter) suspension (figure 1). Long-termsuspended dust can remain in the atmosphere up to severalweeks and can thus be transported thousands of kilometersfrom source regions (Gillette and Walker 1977, Zender et al2003a, Miller et al 2006). As outlined in the next section, these

dust aerosols affect the Earth and Mars systems through a widevariety of interactions.

The impacts of saltating particles can also mobilize largerparticles. However, the acceleration of particles with diametersin excess of 500 m is strongly limited by their large inertia,and these particles generally do not saltate (Shao 2008).Instead, they usually settle back to the soil after a short hopof generally less than a centimeter, in a mode of transportknown as reptation (Ungar and Haff 1987). Alternatively,larger particles can roll or slide along the surface, driven byimpacts of saltating particles and wind drag forces in a mode oftransport known as creep (Bagnold 1937). Creep and reptationcan account for a substantial fraction of the total wind-blownsand flux (Bagnold 1937, Namikas 2003).

The transport of soil particles by wind can thus becrudely separated into several physical regimes: long-termsuspension (20 m diameter), short-term suspension (2070 m), saltation (70500 m), and reptation and creep(500 m) (figure 1). Note that these four transport modes arenot discrete: each mode morphs continuously into the next withchanging wind speed, particle size and soil size distribution.The divisions based on particle size between these regimes arethus merely approximate.

1.2. Importance of wind-blown sand and dust to the Earthand planetary sciences

Wind-blown sand has shaped a substantial portion of theEarths surface by creating sand dunes and ripples in bothcoastal and arid regions (Bagnold 1941, Pye and Tsoar 1990),and by weathering rocks (Greeley and Iversen 1985), whichcontributes to the creation of soils over long time periods (Pye1987). Since aeolian processes arise from the interaction ofwind with the surface, the study of aeolian bedforms (such asdunes) and aeolian sediments (such as loess soils or aeolianmarine sediments) can provide information on the past stateof both the atmosphere and the surface (Greeley and Iversen1985, Pye and Tsoar 1990, Rea 1994). For instance, importantconstraints on both the ancient and contemporary history ofMars are provided by the inference of formative winds andclimate from the morphology and observed time evolution ofaeolian surface features (Greeley et al 1992a). Finally, asdiscussed above, wind-blown sand is also the