Large-Eddy Simulation: How Large is Large Enough?

The length scale evolution of various quantities in a clear convective boundary layer (CBL), a stratocumulustopped boundary layer, and three radiatively cooled (''smoke cloud'') convective boundary layers are studied by means of large-eddy simulations on a large horizontal domain (25.6 25.6 km 2 ). In the CBL the virtual potential temperature and the vertical velocity fields are dominated by horizontal scales on the order of the boundary layer depth. In contrast, the potential temperature and the specific humidity fields become gradually dominated by mesoscale fluctuations. However, at the mesoscales their effects on the virtual potential temperature fluctuations nearly compensate. It is found that mesoscale fluctuations are negligibly small only for conserved variables that have an entrainment to surface flux ratio close to 0.25, which is about the flux ratio for the buoyancy. In the CBL the moisture and potential temperature flux ratios can have values that significantly deviate from this number.