The Cell Perturbation Method for Turbulence Generation in Nested Large-Eddy Simulations for the Perdigão Field Campaign.

Atmospheric boundary layer turbulence in a traditional numerical weather prediction model is parameterized by planetary boundary layer (PBL) schemes appropriate for meso- and larger-scale phenomena. As computational power grows, it has become common to nest finer resolved models within mesoscale models to explicitly resolve turbulent motions, often using a large-eddy simulation (LES) closure instead of a PBL scheme. Initially, no turbulence is resolved near the inflow lateral boundary which is forced by the mesoscale flow. While turbulence does eventually appear in the nested large-eddy simulation, it takes many grid points from the inflow boundary for realistic turbulence to develop. Perturbations in potential temperature applied at the inflow boundary have been shown to significantly reduce the fetch required for turbulence to develop in idealized large-eddy simulations nested within mesoscale models with smooth terrain. This cell perturbation method (CPM) is examined here in a real nested simulation over complex terrain. Specifically, we explore the utility of the CPM in the context of simulations for two parallel ridges in Portugal, the site of the 2017 Perdigão field campaign. We find the CPM can greatly reduce this fetch requirement in some circumstances even over complex terrain. While the presence of complex terrain itself accelerates the development of resolved turbulence, using the CPM may be more practical. Results from Weather Research and Forecasting (WRF) simulations with different configurations at the Perdigão field site are examined to evaluate turbulence spectra in different nested domains.