Applying a Surface Heat Flux with Temporal Gradient to Large-Eddy Simulations of the Stable Boundary Layer

Abstract

The impact of the lower boundary condition for potential temperature is investigated in idealized large-eddy simulations of the stable boundary layer. The numerical simulations are conducted by the flow solver EULAG (Eulerian/semi-Lagrangian fluid solver) using a non-linear forward-in-time advection scheme. While focusing on the moderately stable inter-comparison case GABLS1 of the GEWEX Atmospheric Boundary Layer Study initiative as a benchmark, the simulations are performed with a lower potential-temperature boundary condition prescribed by a surface heat flux rather than the more commonly applied time-dependent surface temperature. A negative surface heat flux increasing in magnitude with time successfully reproduces the benchmark results, whereas, a constant surface heat flux leads to temperature decoupling, a shallower boundary layer and a weaker low-level jet. In addition, the sensitivity of the results towards three subgrid-scale models (Deardorff-Schumann, Anisotropy, Nonlinear Backscatter) and various grid resolutions (12.5 m - 2 m) is analyzed. A crucial factor is the amount of the vertical turbulent transport in the numerical model, which is significantly influenced by the applied advection scheme and the SGS model. Further, the selected SGS model influences the energy spectrum by affecting the spatial range of eddy sizes.