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Recent increases in computing power mean that atmospheric models for numerical weather prediction are now able to operate at grid spacings of the order of a few hundred meters, comparable to the dominant turbulence length scales in the atmospheric boundary layer. As a result, models are starting to partially resolve the coherent overturning structures in the boundary layer. In this resolution regime, the so‐called boundary layer “gray zone,” neither the techniques of high‐resolution atmospheric modeling (a few tens of meters resolution) nor those of traditional meteorological models (a few kilometers resolution) are appropriate because fundamental assumptions behind the parameterizations are violated. Nonetheless, model simulations in this regime may remain highly useful. In this paper, a newly formed gray zone boundary layer community lays the basis for parameterizing gray zone turbulence, identifies the challenges in high‐resolution atmospheric modeling and presents different gray zone boundary layer models. We discuss both the successful applications and the limitations of current parameterization approaches, and consider various issues in extending promising research approaches into use for numerical weather prediction. The ultimate goal of the research is the development of unified boundary layer parameterizations valid across all scales.

International audienceIn this review, we address the use of numerical computations called Large-Eddy Simulations (LES) to study dust devils, and the more general class of atmospheric phenomena they belong to (convec-tive vortices). We describe the main elements of the LES methodology. We review the properties, statistics, and variability of dust devils and convective vortices resolved by LES in both terrestrial and Martian environments. The current challenges faced by modelers using LES for dust devils are also discussed i

Taking advantage of the huge computational power of a massive parallel supercomputer (K-supercomputer), this study conducts large eddy simulations of entire tropical cyclones by employing a numerical weather prediction model, and explores near-surface coherent structures. The maximum of the near-surface wind changes little from that simulated based on coarse-resolution runs. Three kinds of coherent structures appeared inside the boundary layer. The first is a Type-A roll, which is caused by an inflection-point instability of the radial flow and prevails outside the radius of maximum wind. The second is a Type-B roll that also appears to be caused by an inflection-point instability but of both radial and tangential winds. Its roll axis is almost orthogonal to the Type-A roll. The third is a Type-C roll, which occurs inside the radius of maximum wind and only near the surface. It transports horizontal momentum in an up-gradient sense and causes the largest gusts.

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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.

Dust devils are small-scale vertical vortices often observed over deserts or bare land during the daytime under fair weather conditions. Previous numerical studies have demonstrated that dust devil–like vertical vortices can be simulated in idealized convective mixed layers in the absence of background winds or environmental shear. Their formation mechanism, however, has not been completely clarified. In this paper, the authors attempt to clarify the vorticity source of a dust devil–like vortex by means of a large-eddy simulation, in which a material surface initially placed in the vortex is tracked backward and the circulation on the material surface is examined. The material surface is found to originate from downdrafts, which already have sufficient circulation. As the material surface converges toward the vortex, the vorticity is increased because of conservation of circulation. It is shown that a convective mixed layer is inherently accompanied by circulation, which is scaled by a product of the convective velocity scale and the depth of the convective mixed layer. This circulation is considered to be originally generated by tilting of baroclinically generated horizontal vorticity principally at middepths of the convective mixed layer.

Observations show that optical depth over desert increase during daytime when a convective mixed layer develops under a light general wind condition. This implies that dust suspension by horizontal winds associated with convective motions occur even in the absence of general winds. In the present paper, a large eddy simulation is performed to study how much dust is suspended in a convective mixed layer without a general wind. The results show that dust particle concentration in the convective mixed layer can reach on the order of 10 µg m −3, which is in reasonable agreement with observations. Tiny dust particles that have small terminal velocities are easily brought up by convective winds during daytime and remain in the atmosphere throughout night. If a similar weather continues for several days, dust particles concentration on the evening of the second day can reach 1.8 times as large as that on the evening of the first day, accordingly.