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

Yuanping He,Renjian Zhang,Weizhen Lu,Qingxiang Shui,Daizhou Zhang,Chuck Wah Yu,Zhaolin Gu,Botao Liu

doi:10.3390/atmos11040321

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.

Highlights

Local strong sandstorms (LSSs) in China, similar to haboobs in Sudan and America, usually with a maximum instantaneous wind speed > 25 m/s and visibility < 50 m, are generally characteristic of a sudden disastrous weather in arid and semiarid regions [1]
The evolution process is associated with the interaction between a mesoscale cold pool and small-scale convective cells and the microscopic mechanism of wind–sand movement in swirling wind fields
This study used the Reynolds averaged Navier–Stokes (RANS) method to simulate the small-scale process of an LSS, and explained the formation of “lobes” at the leading edge of an LSS

Summary

Introduction

Local strong sandstorms (LSSs) in China, similar to haboobs in Sudan and America, usually with a maximum instantaneous wind speed > 25 m/s and visibility < 50 m, are generally characteristic of a sudden disastrous weather in arid and semiarid regions [1]. Huang et al [33] simulated the cold pool outflow in a desert region by adding an idealized cooling in a two-dimension model, considering the local features of haboobs Their simulation by considering dust as a passive tracer in their shorten duration of simulation runs, illustrated the dust uplift would largely occur at the “head” of the density current, and this was consistent with some existing observations reported in the literature [34]. This study, a high-resolution numerical model for the accurate simulation of small-scale processes in the evolution of LSS was established by considering a cold pool with a vertical helicity (the vorticity), that could be formed due to the interaction of a mesoscale cold pool with small-scale thermal convective cells. A description of the meso- and small-scale coupling evolution process of LSS has been introduced to strengthen our understanding of the formation of LSS

Physical Modeling

Governing Equations

Results and Discussion

The Initial Stage of LSS Development

Effects

Summary and Conclusions