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Boundary-layer rolls have an impact on bushfires, pollution dispersion, the triggering of extreme convective weather events, the air-sea interaction and intensification of tropical cyclones and momentum, particle and gas fluxes. Previous numerical modelling studies that investigate the effects of boundary-layer rolls and the processes that control their dynamics have relied upon high-resolution, idealised simulations. Recently, however, numerical weather prediction (NWP) systems have increased in horizontal resolution to the point at which they are capable of explicitly resolving shallow convective circulations. Therefore, there is a need to assess the ability of NWP to accurately forecast the formation, development and breakup of boundary-layer rolls. Here, we assess the ability of the UK Met Office Unified Model (UM), the NWP component of the Australian Community Climate and Earth-System Simulator (ACCESS), to simulate the life cycle of boundary-layer rolls. A suite of simulations is performed, over a range of horizontal resolutions, of boundary-layer rolls observed across Victoria, Australia, on Black Saturday, 7 February 2009. In this case, it is found that a horizontal grid spacing of less than 0.6 km is required to adequately reproduce the scale and evolution of the observed rolls. We note that the boundary layer was particularly deep on this day and as a consequence the boundary-layer rolls were wider than may be typically observed, meaning the grid spacing specified here lies at the upper end of what would be required with a shallower boundary layer. The model output is used to assess how the boundary-layer rolls contribute to the high fire danger on Black Saturday. It is suggested that boundary-layer rolls may increase fire danger by (i) causing wind-direction variability at the surface, which increases the rate of spread of fires; and (ii) enhancing the process of long-range spotting by augmenting the vertical lofting of embers. [ABSTRACT FROM AUTHOR]

Operational high-resolution numerical weather prediction models are now able to partially resolve turbulent motions due to increased computing power. The partitioning of resolved and parametrized fluxes becomes important in the representation of turbulent transfer that determines the state of the atmospheric boundary layer. In this study, successive simulations of a convective boundary layer using the Met Office Large Eddy Model from the large-eddy simulation to the mesoscale limit are compared with the corresponding coarse-grained fields to examine model behaviour over the grey zone. The differences in the turbulent kinetic energy partitioning between coarse-grained (reference) and actual fields are identified and used to quantify sub-grid diffusion in the grey zone under different atmospheric forcings and surface heat flux. It is shown that the excessive mixing due to the large values of mixing length at coarse resolutions results in the cut-off of resolved turbulence in the grey zone. The damping of resolved motions comes earlier for wind shear runs. In contrast, coarse-grained fields exhibit a smooth transition of the resolved turbulent kinetic energy across the scales. Decreasing numerical dissipation through the sub-grid scheme leads to the increase of resolved turbulence, but fails to reproduce the reference transition pattern that imposes some physical limitations to the partially resolved turbulence simulations. Pragmatic blending of mixing length values maintains a more realistic turbulent kinetic energy transition from fine to coarse resolutions and potential temperature profiles in the grey zone. Bounding vertical diffusion to its effective values, an approach based on maintaining inherent properties of the flow across the scales, is able to match the coarse-grained fields from the highly resolved to the almost unresolved state, regardless of the forcing. Finally, the complexity of modelling in the grey zone is exhibited in the dependence of turbulence onset on time and vertical resolution. [ABSTRACT FROM AUTHOR]