The Atmospheric Boundary Layer and the “Gray Zone” of Turbulence: A Critical Review

Abstract

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.

Resolution Dependence of Turbulent Structures in Convective Boundary Layer Simulations

“…However, the use of backscatter or of changing the mixing length throughout the domain does not seem to provide suitable remedies within the grey zone. A variety of grey zone parameterizations that seek to modify mixing in a more targeted manner [8] are being actively developed and investigated. We believe that analyses such as that presented here, focussed on the properties and behaviour of the dominant resolved eddies as a function of scale, should form an important part of such investigations in order to build confidence that improved results can be produced for sound physical reasons.…”

“…Column-based parametrization schemes for boundary layer turbulence were designed for relatively low resolution NWP simulations and assume that the horizontal grid spacing is much larger than l. This approximation is no longer valid for the simulation of the CBL with the latest NWP models [3], which operate with ∆ on the order of 1 km or less (e.g., [4][5][6]). Grid lengths of order of hundreds of metres mean that the turbulence is partially resolved, being in the regime of "coarse-grid" large eddy simulations (LES); the coarser end of this regime is sometimes called the "grey zone" or "terra incognita" [7][8][9]. Modellers need to address two key questions: (i) How physically realistic is the resolved turbulence?…”

“…In a review paper, Honnert et al [8] described different resolution regimes as being LES [11], near grey zone [26], grey zone [3] and mesoscale [12], with increasing grid length. Honnert et al [3] defined the CBL grey zone as ranging between filter scales of 0.2z i to 2z i .…”

“…Enhancements that have been proposed to the Smagorinsky-Lilly scheme include the introduction of backscatter to account for the transfer of energy from the smaller eddies to large eddies (e.g., [34,35]) or making the scheme dynamic by calculating the Smagorinsky "constant" explicitly as the simulation progresses (e.g., [29,36,37]). It is likely that more sophisticated schemes like these, perhaps with suitable modifications [8], may offer better prospects at low resolutions. Doubrawa and Muñoz-Esparza [6] compared several schemes and found that a scale-aware scheme performed better in simulating real grey zone boundary layers during highly convective days; however, the overall error metrics were similar to other schemes, and turbulent energy was overpredicted.…”

“…We study in particular the effect of grid spacing on each quadrant's contribution to the heat flux, which offers a useful diagnosis to inform the development of sub-filter schemes for coarser resolutions. This may be particularly valuable for further developing those parameterization approaches that distinguish explicitly between local and non-local contributions to turbulent fluxes (e.g., [8,28]). The rest of the paper is organised as follows.…”