From the perspective of models with horizontal grid sizes on the order of kilometers and perhaps 5–10 vertical layers in the lowest kilometer, both typical of weather prediction models, it is essential to represent these sub-grid effects well in order to get a realistic boundary layer growth and mean structure. Features of these eddies have scales ranging from an order of magnitude less than the PBL depth to a similar scale. In the real atmosphere this is accomplished by large surface-based eddies or thermals with horizontal scales comparable to vertical scales and structures that depend on the heat flux forcing and the shear. The role of planetary boundary layer (PBL) schemes in numerical weather prediction models is to transfer heat and moisture fluxes from the surface through the growing boundary layer along with the accompanying surface stress effects on the momentum. Some of these schemes also show sensitivity in the range from about 3 km to 1 km grid sizes as they start to permit resolved scale cells only as the grid size reduces which may make their results grid-size dependent. study ( Figure 1) and as some schemes suppress near-grid-scale cells while others allow them even if they are larger than they should be, and poorly resolved. Even at grid sizes near one kilometer, boundary-layer parameterization schemes show a variety of behavior as noted by the Ching et al. The challenge at these grid scales is that typical convective boundary layer eddies are themselves of scales comparable to the grid and can be considered neither fully resolved nor fully sub-grid, and this puts them in the so-called grey zone or terra incognita. Introduction of moisture with condensation in the eddies expands this problem to that of handling shallow convection, but similarities between dry and cloud-topped convective boundary layers can lead to some unified views of the processes that need to be represented in convective boundary-layers which will be briefly addressed here.Īs computational power increases, we are at a time when real-time forecast models for local and regional scales (100–1000 km) are becoming feasible with sub-kilometer grid sizes given that such models have to run at least ten times as fast as real time in operational use. This article also reviews various approaches that have been taken to span this gap in the proper representation of eddy transports in the sub-kilometer grid range using scale-aware approaches. Between these scales, at hundreds of meters grid size, is a so-called grey zone in which the primary transport is neither entirely sub-grid nor resolved, where explicit large-eddy models and sub-grid boundary-layer parameterization models fail in different ways that are outlined in this review article. At tens of meters grid scales, large-eddy simulation models, explicitly resolve all the primary three-dimensional eddies associated with boundary-layer transport from the surface and entrainment at the top. At multi-kilometer grid scales, numerical weather prediction models represent surface-based convective eddies as a completely sub-grid one-dimensional vertical mixing and transport process.
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