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Dust Devil Steady-State Structure from a Fluid Dynamics Perspective

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Abstract

Simple analytical models for the flow structure of dust devils in steady state, and a “thermophysical” scaling theory that explains how these flow structures are maintained are reviewed. Then, results from high-resolution numerical simulations are used to provide insights into the structure of dust-devil-like vortices and study the impact of surface roughness on them. The article concludes with an overview of the influence of lofted dust on the flow structure of dust devils and a discussion of open questions.

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Notes

  1. The individual dust devil path may deviate from the straight line parallel to the wind direction, as, e.g., evident by dust-devil cycloidal tracks (Reiss et al. 2013).

  2. According to Balme et al. (2012), the difference between the direction of the 2-h averaged dust devil ground velocity and the direction of the 10-m ambient wind is less than 90° in all instances, and only 9 out of 52 measurements show a difference of more than 30°.

  3. The vortex Rossby number (also named the helical parameter in Kurgansky (2005)) has an intrinsic (internal) character and essentially differs from the reciprocal of swirl ratio \(S\) in Davies-Jones (1973), the latter being an external non-dimensional parameter defined in terms of input variables defining the “vortex simulator” experiment (e.g. the imposed volumetric flow rate \(\hat{Q}\) through the convection zone of the radius \(R\) in the vortex chamber, and the imposed ambient circulation \(\hat{\varGamma}\); so that \(S = R \hat{\varGamma} / 2\hat{Q}\), see e.g. Rotunno (1984), Bluestein (2013), etc.).

  4. Figure 4 from Balme and Greeley (2006) (image credit S. Metzger, Planetary Science Institute, Tuscon, USA) shows “typical” dust devil morphology ranging from (a) a narrow, tightly defined column (\(\sim1~\mbox{m}\) in diameter) through (b) an inverted V-shaped dust cloud with a less well defined column (\(\sim10~\mbox{m}\) in diameter) to (c) a poorly defined inverted V shape with no visible internal column (\(\sim50~\mbox{m}\) in diameter at ground). In this paper, two morphological types of dust devils are considered, corresponding to (a) and \(\mbox{(b)}+\mbox{(c)}\) types in Balme and Greeley (2006); cf. Schwiesow (1981).

  5. The corner flow domain is the part of the vortex flow, which is immediately below the vortex core and is adjacent to the ground. In the corner flow domain, the main vortex updraft adjusts to the inward air motion within in the surface boundary layer beneath the peripheral vortex flow.

  6. More detailed and comprehensive accounts of the classical analytical models can be found in reviews within the tornado literature (e.g. Lewellen 1976, 1993), but in this chapter we are focusing more on possible interests and demands of practitioners in dust devil research.

  7. In his study of steady-state hurricanes, Riehl (1963) assumed conservation of potential vorticity in the vortex inflow region, i.e. vanishing of the azimuthal component of the frictional force, and on this basis concluded that \(r \tau_{\theta z} = \mbox{constant}\), where \(\tau_{\theta z}\) is the shearing stress in the \(\theta - z\) plane, taken on the ground surface. Application of the bulk formula then yields \(v \propto r^{- 1/2}\).

  8. These extrema are about an order of magnitude greater than those in Kanak et al. (2000) where 35-m horizontal grid spacing is used.

  9. Sinclair (1966, 1973) notes that the Rankine-combined model (1) is capable of explaining approximately 75 % of the total mean pressure drop from the environment to the dust devil center (see also Leverson et al. 1977). It follows from Table 1 that an application of the Gaussian and/or Vatistas et al. (especially for \(n = 2\)) vortex model gives better results, in this respect.

  10. Regarding the possibility that the tallest dust-devil vortices form in the vertices of Rayleigh–Bénard convective cells within the convective boundary layer see, e.g., Willis and Deardorff (1979); Hess and Spillane (1990); Kanak et al. (2000).

  11. The first option is more relevant to mechanically driven vortices in vortex chambers (e.g. Ward 1972; Davies-Jones 1973, 1986; Rotunno 1984; and references therein). The second option refers more directly to the atmospheric dust devil.

  12. Where the radius is small compared with the atmospheric scale height, and in fact (for dust devils) with the CBL depth.

  13. However, differently from Dessens (1972) and Wilkins et al. (1975), the vortex core diameter was found to decrease after introduction of surface roughness in the laboratory study of Diamond and Wilkins (1984). Also, PIV measurements by Zhang and Sarkar (2008) show that the magnitudes of maximum tangential velocity and vortex radius both reduce with increasing surface roughness. These and some similar LES results (e.g. Liu and Ishihara 2016, and references therein) show that there is not full consensus as to how the roughness affects the flow near the ground and the net effect may be sensitive to details of the laboratory and/or numerical setup.

  14. It has been observed that typical diameters and near-surface shapes of dust devils may depend strongly on the geographical position of an observational site (Balme and Greeley 2006).

  15. Lewellen et al. (1997) have performed the LES of turbulent transport in the tornado vortex for a particular set of physical boundary conditions that lead to several features resembling those ones found in radar observations by Wurman et al. (1996) and showed that for sufficiently fine grid resolution the simulation results are relatively independent of grid resolution and subgrid model modifications.

  16. The values of \(V_{m}\), \(W\) and \(\Delta p\) in Table 2 approach the values observed by Sinclair (1973). Experiment F is an exception. Owing to high SMIH, the low (reduced) momentum layer near the ground might be unrealistically thick, which leads to simulation results far from Rennó et al. (2004) and the observations. However, the goal was to demonstrate the influence of SMIH.

  17. In this respect dust devils are more similar to hurricanes than tornadoes, the source of energy for the dust devil being the thermal disequilibrium between the surface and the overlying air, which is the same as that in hurricanes (Emanuel 1988; see also Kanak 1999). Emanuel (1988) suggests that dust devils may be maintained in a similar fashion as the hurricanes; that is, by a feedback process between the vortex circulation and surface heat fluxes. Sinclair (1969) came to a similar conclusion by suggesting that vortex translation permits the dust devil to continuously acquire new potential energy sources. Sinclair (1969) explains the observed decrease in dust devil activity directly following a maximum in occurrence, as due to depletion of the surface-adjacent superadiabatic layer, which must recover before dust devil frequency can again increase.

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Acknowledgements

We are thankful to two anonymous reviewers whose critical review and useful suggestions contributed tremendously to improvements in the content and in the style of this article.

Michael Kurgansky is grateful to Aldo Montecinos, Victor Villagran and Stephen M. Metzger with whom he carried out field studies of dust devils in Chile’s Atacama Desert in 2008–2009, where the photos of Figs. 1 and 4 were taken. His work on this review was supported by the Presidium of the Russian Academy of Sciences, program no. 9 (Experimental and Theoretical Studies of Solar System Objects and Stellar Planetary Systems) and the Russian Foundation for Basic Research, projects 13-05-01025-a and 14-05-00847-a. Ralph Lorenz acknowledges the support of the NASA Mars Fundamental Research Program grant NNX12AI04G.

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Correspondence to Michael V. Kurgansky.

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Kurgansky, M.V., Lorenz, R.D., Renno, N.O. et al. Dust Devil Steady-State Structure from a Fluid Dynamics Perspective. Space Sci Rev 203, 209–244 (2016). https://doi.org/10.1007/s11214-016-0281-0

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