Zone” of Turbulence: A critical review

28 Recent increases in computing power mean that atmospheric models for numerical weather 29 prediction are now able to operate at grid spacings of the order of a few hundred me30 ters, comparable to the dominant turbulence length scales in the atmospheric bound31 ary layer. As a result, models are starting to partially resolve the coherent overturning 32 structures in the boundary layer. In this resolution regime, the so-called boundary-layer 33 ”gray zone”, neither the techniques of high-resolution atmospheric modeling (a few tens 34 of meters resolution) nor those of traditional meteorological models (a few kilometers 35 resolution) are appropriate because fundamental assumptions behind the parameteriza36 tions are violated. Nonetheless, model simulations in this regime may remain highly use37 ful. In this paper, a newly-formed gray-zone boundary-layer community lays the basis 38 for parameterizing gray-zone turbulence, identifies the challenges in high-resolution at39 mospheric modeling and presents different gray-zone boundary-layer models. We discuss 40 both the successful applications and the limitations of current parameterization approaches, 41 and consider various issues in extending promising research approaches into use for nu42 merical weather prediction. The ultimate goal of the research is the development of uni43 fied boundary-layer parameterizations valid across all scales. 44