Large eddy simulation of dust devils in a diurnally-evolving convective mixed layer

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*, where w* 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.