RANS Simulation of Local Strong Sandstorms Induced by a Cold Pool with Vorticity

Abstract

Local strong sandstorms (LSSs) in northwestern China often occur suddenly in tens of minutes during the late afternoon and by dusk. Observations and theoretical studies have shown the trigger role of cold-air pools over desert areas for the occurrence of LSS. In this study, a numerical heat convection model was established to simulate an LSS that was induced by a single cold pool with vertical helicity to study the evolution process. The Reynolds averaged Navier–Stokes (RANS) method was used for the numerical calculation to illustrate different stages of the evolution process of an LSS. Results show that after the intrusion of a cold pool into the upper region of the surface convective mixing layer, descending of the cold air would lead to the downward transport of vorticity, enabling thermal convection cells in the mixing layer to become swirling convection cells. After LSS is fully developed, there occurs many subvortices (secondary vortices) in the convection field. The velocity at different altitudes over selected positions in the calculation domain is consistent with the "lobe" shape of an LSS. The secondary vortices cause quick and huge energy dissipation and the decay of the LSS. These results are consistent with observations and indicate the crucial effect of convection cells structure in the mixing layer and the cold pool in the upper layer on the formation of LSS.