Large Eddy Simulation of Dust Devils in a Diurnally-Evolving Convective Mixed Layer

Abstract

Formation of dust devils in diurnally-evolving convective mixed layers is studied by means of a large eddy simulation. It is found that a weaker general wind and a stronger surface heat flux for which cellular convection rather than roll convection prevails are favorable for the formation of dust devils. The simulation results show that when the general wind is weak, the maximum vertical vorticity in the convective mixed layer is a monotonically increasing function of w à , where w à is the convective velocity scale for a convective mixed layer. Therefore, dust devils occur most frequently in the early afternoon when the heat flux is large and the convective mixed layer grows to a significant height.The simulated dust devils are found to have a horizontal length scale comparable with observed larger dust devils. They have either one-celled or two-celled structure. Some of them have a one-celled structure initially, but later evolve into a two-celled structure.

Observed Effect of Mesoscale Vertical Vorticity on Rotation Sense of Dust Devil-Like Vortices in an Urban Area

“…2a) was similar to the cellular convective ("fish net") pattern (see Stull 1988;Fujiwara et al 2011). This feature is similar to previous observations (Fujiwara et al 2011) and LES results (e.g., Kanak et al 2000;Kanak 2005;Kanak 2008;Ohno and Takemi 2010;Ito et al 2010;Ito et al 2011;Raasch and Franke 2011). However, the wind velocity in the eastern area, about 4 m s −1 , was stronger than that in all other areas.…”

Effects of Ambient Rotation on Dust Devils

“…As they are visualized by lifted dust particles, many observational studies have examined their structure and characteristics (e.g., Sinclair 1969;Hess and Spillane 1990;Snow and McClelland 1990;Bluestein et al 2004;Oke et al 2007), revealing that dust devils usually have no preferred direction of rotation and implying that the earth's rotation has a negligible effect on the vortex dynamics (Sinclair 1965;Carroll and Ryan 1970). In the past decade, dust devils have been also studied by means of large eddy simulations (LESs) (Kanak et al 2000;Toigo et al 2003;Kanak 2005;Ito et al 2010;Ohno and Takemi 2010a;Ohno and Takemi 2010b;Gheynani and Taylor 2010). These numerical studies have shown that turbulent convective motions in a convective mixed layer, without the earth's rotation, produce dust devils with no preferred direction of rotation, consistent with the observations.…”

Dust Devil Sediment Transport: From Lab to Field to Global Impact

“…To date, most LES studies related to terrestrial dust devils have focused on vortex formation conditions and/or their physical characteristics (Kanak et al 2000;Kanak 2005;Zhao et al 2004;Gu et al 2008;Ito et al 2010b;Ohno and Takemi 2010;Raasch and Franke 2011;, with only a few such studies examining the fluxes of sand or dust. Zhao et al (2004) injected sand particles into their terrestrial LES to study particle trajectories in a dust devil.…”