Modeling the convective boundary layer in the Terra Incognita: Evaluation of different strategies with real-case simulations

Modeling atmospheric turbulence in the convective boundary layer is challenging at kilometer and sub-kilometer resolutions, as the horizontal grid spacing approaches the size of the most energetic turbulent eddies. In this range of resolutions, termed Terra Incognita or gray zone, partially resolved convective structures are grid-dependent and neither traditional 1D mesoscale parameterizations nor 3D Large Eddy Simulations closures are theoretically appropriate. Leveraging on a new set of one-way nested, full-physics multiscale numerical experiments, we quantify the magnitude of the errors introduced at gray zone resolutions in a real-case application and we provide new perspectives on recently proposed modeling approaches. The new set of experiments is forced by real time-varying boundary conditions, spans a wide range of scales and includes traditional 1D schemes, 3D closures, scale-aware parameterizations and strategies to suppress resolved convection at gray zone resolutions. The study area is Riyadh (Saudi Arabia), where deep CBLs develop owing to strong convective conditions. Detailed analyses of our experiments, including validation with radiosonde data, calculations of spectral features and partitioning of turbulent fluxes between resolved and subgrid scales, show that (i) grid-dependent convective structures entail minor impacts on the first order characteristics of the fully developed boundary layer due to some degree of implicit scale-awareness of 1D parameterizations and (ii) 3D closures and scale-aware schemes outperform traditional 1D schemes especially in the surface layer, among other findings. The new suite of experiments provides a benchmark of real simulations that can be extended to assess how new turbulence closures perform at gray zone resolutions.