Comparisons of “Gray-Zone” Simulations with One-and Three-Dimensional Planetary Boundary Layer Parameterizations and Upscaled Large-Eddy Simulations over Coastal Central California

Weather Research and Forecasting (WRF) Model simulations at a “gray-zone” horizontal grid spacing of Dx 5 200 m are compared with an upscaled large-eddy simulation (LES) with the FastEddy model and observations over coastal central California to understand and characterize high-resolution turbulent flows for offshore wind resource assess-ment. A case study of a strong coastal low-level jet is examined, resulting from the typical spring and summertime conditions of a broad subtropical ridge in the northeastern Pacific Ocean. The WRF Model simulations use three different planetary boundary layer (PBL) parameterizations: two one-dimensional (1D) parameterizations and a three-dimensional (3D) parameterization, ideally suited for simulating flows at gray-zone horizontal resolutions. Similarities and differences between the WRF Model simulations and upscaled LES are assessed. Overall, the WRF Model simulations compare rea-sonably well with the upscaled LES in simulating the coastal flows, and both the upscaled LES and WRF Model simulations compare favorably with observations. The WRF 3D PBL simulation compares most favorably with the upscaled LES, in particular in simulating distributions of vertical velocity over the ocean and land and in capturing energetic small-scale structures, due to its more realistic treatment of horizontal mixing. SIGNIFICANCE STATEMENT: The coastal environment in central California is complex, with a stable marine boundary layer interacting with a boundary layer over steep, heterogeneous terrain, which can be rapidly destabilized with daytime heating. It is not well observed or simulated with current computer models that use grid spacings on the order of 1 km. Motivated by the need to understand the small-scale heterogeneous flow features relevant to the evaluation of offshore wind resources in this region, a detailed computer modeling study is carried out. A computer model simulation that explicitly resolves turbulence from large eddies is used as a reference in which to compare simulations that do not or only partially resolve this turbulence. The main finding is that a computer model simulation with more realistic treatment of horizontal mixing most closely matches the simulation that explicitly resolves turbulence, when compared at equivalent resolutions.