Formation Mechanism of Dust Devil–Like Vortices in Idealized Convective Mixed Layers

Abstract

Dust devils are small-scale vertical vortices often observed over deserts or bare land during the daytime under fair weather conditions. Previous numerical studies have demonstrated that dust devil–like vertical vortices can be simulated in idealized convective mixed layers in the absence of background winds or environmental shear. Their formation mechanism, however, has not been completely clarified. In this paper, the authors attempt to clarify the vorticity source of a dust devil–like vortex by means of a large-eddy simulation, in which a material surface initially placed in the vortex is tracked backward and the circulation on the material surface is examined. The material surface is found to originate from downdrafts, which already have sufficient circulation. As the material surface converges toward the vortex, the vorticity is increased because of conservation of circulation. It is shown that a convective mixed layer is inherently accompanied by circulation, which is scaled by a product of the convective velocity scale and the depth of the convective mixed layer. This circulation is considered to be originally generated by tilting of baroclinically generated horizontal vorticity principally at middepths of the convective mixed layer.

Particle Image Velocimetry of a Dust Devil Observed in a Desert

“…Ito et al 2013). The estimated environmental wind speed U ~ 2.6 m s −1 seems reasonable: dust devils are known to favor such a weak environmental winds (Sinclair 1969;Ito et al 2010;Raasch and Franke 2011).…”

“…The dust devil investigated in this paper was not remarkably strong among those observed in the same day. This suggests that dust devils in the real atmosphere have much faster tangential and vertical velocities than those reproduced in numerical simulations (e.g Kanak 2005; Gheynani and Taylor 2010; Raasch and Franke 2011;Ito et al 2013) except for an experiment in which ambient rotation is forced at the lateral boundary (Gu et al 2006). …”

“…However, most numerical studies suffer from underestimates of wind speeds even if very high resolution is used (e.g. Kanak 2005;Raasch and Franke 2011;Ito et al 2013). The reason for the underestimate is still uncertain, and further observations of the wind speeds and turbulence characteristics in dust devils are desired.…”

Wind-Speed—Surface-Heat-Flux Feedback in Dust Devils

“…Kanak et al 2000;Ito et al 2013). Here, we conduct both C-LES and LES using the quasibulk method (hereafter referred to as QB-LES) that considers the dependence of the surface heat fluxes on the near-surface wind speed in the way described below.…”

“…Kanak et al 2000). In such idealized environments, circulations about the vertical axis are produced by the turbulent convection in the CBL and updrafts accelerated by warm buoyant air stretch the vertical vorticity associated with the circulations (Ito et al 2011(Ito et al , 2013.…”