TY - JOUR

T1 - Structure and formation of dust devil-like vortices in the atmospheric boundary layer

T2 - A high-resolution numerical study

AU - Raasch, S.

AU - Franke, T.

N1 - The authors would like to thank three anon-ymous reviewers for their useful comments. All simulations have been car-ried out on IBM‐Regatta and SGI‐ICE systems of the North‐GermanSupercomputing Alliance (HLRN). This work was partly funded from grantRA 617/19‐1 of the German Research Foundation (DFG). The first draft ofthis paper was written during a stay of the first author as a VisitingResearcher at the Research Institute for Applied Mechanics, Kyushu Uni-versity, Fukuoka, Japan.

PY - 2011/8/26

Y1 - 2011/8/26

N2 - The development of dust devil-like vortices in the atmospheric convective boundary layer (CBL) is studied using large-eddy simulation (LES). Special focus is placed on the analysis of the spatial structure of the vortices, the vorticity-generating mechanisms, and how the vortices depend on the larger-scale coherent near-surface flow pattern of the CBL. Vortex centers are automatically detected during the simulation, and a tracking method is developed, which allows us to determine the temporally averaged structures of selected vortices. Also, various vorticity budget terms are calculated. A reference study with high resolution (2 m) and large model domain (2000 × 2000 × 500 grid points) is carried out to account for the dependency of vortex generation on the larger-scale CBL flow pattern, i.e., the near-surface hexagonal cells. Vortices predominantly appear within the vertices of the cells. Their vorticity is maintained by a combination of divergence and twisting effects. Flow visualizations by tracers show that the vortices have an inverted cone-like shape, similar to observed dust devils. Simulated vortex characteristics like tangential velocity or vorticity are at the lower limit of observed values. Strength and number of vortices heavily depend on the background wind. A small background wind enhances vortices, but for a mean wind speed of 4.4 m s -1, vortex generation is significantly reduced, mainly because the near-surface flow changes from a cellular to a more band-like pattern. A new mechanism is suggested, which relates the initial vortex generation to the cellular flow pattern.

AB - The development of dust devil-like vortices in the atmospheric convective boundary layer (CBL) is studied using large-eddy simulation (LES). Special focus is placed on the analysis of the spatial structure of the vortices, the vorticity-generating mechanisms, and how the vortices depend on the larger-scale coherent near-surface flow pattern of the CBL. Vortex centers are automatically detected during the simulation, and a tracking method is developed, which allows us to determine the temporally averaged structures of selected vortices. Also, various vorticity budget terms are calculated. A reference study with high resolution (2 m) and large model domain (2000 × 2000 × 500 grid points) is carried out to account for the dependency of vortex generation on the larger-scale CBL flow pattern, i.e., the near-surface hexagonal cells. Vortices predominantly appear within the vertices of the cells. Their vorticity is maintained by a combination of divergence and twisting effects. Flow visualizations by tracers show that the vortices have an inverted cone-like shape, similar to observed dust devils. Simulated vortex characteristics like tangential velocity or vorticity are at the lower limit of observed values. Strength and number of vortices heavily depend on the background wind. A small background wind enhances vortices, but for a mean wind speed of 4.4 m s -1, vortex generation is significantly reduced, mainly because the near-surface flow changes from a cellular to a more band-like pattern. A new mechanism is suggested, which relates the initial vortex generation to the cellular flow pattern.

UR - http://www.scopus.com/inward/record.url?scp=80052339217&partnerID=8YFLogxK

U2 - 10.1029/2011JD016010

DO - 10.1029/2011JD016010

M3 - Article

AN - SCOPUS:80052339217

VL - 116

JO - Journal of Geophysical Research Atmospheres

JF - Journal of Geophysical Research Atmospheres

SN - 0148-0227

IS - 16

M1 - D16120

ER -