Evaluation of Mesoscale Model Simulations with 1D and 3D Planetary Boundary Layer Parameterizations over California’s Central Coast Using Large Eddy Simulations and Observations

A main thrust of the U.S. Dept. of Energy Observational Driven Resource Assessment with Coupled Models (ORACLE) project is to characterize and assess the atmospheric and oceanic conditions along the U.S. west coast, where future floating offshore wind farms may be deployed. To that end, it is critical to understand the veracity of planetary boundary layer (PBL) winds (including hub-height winds) predicted by mesoscale numerical model simulations in this environment.

Using FastEddy® large-eddy simulations (LES) as proxy for the truth, as well as available observations, Weather Research and Forecasting (WRF) model simulations using three different PBL parameterizations are evaluated during a strong coastal jet case of May 8, 2021: two 1D PBL parameterizations with stark differences in the treatment of vertical mixing [the first-order, nonlocal, K-profile closure Yonsei University (YSU) and 1.5 order, local, prognostic turbulence kinetic energy (TKE)-based closure Mellor-Yamada (MY) parameterizations] and a 3D PBL parameterization based on the full MY level 2.5 model designed for the “gray zone” (i.e., horizontal grid spacings less than ~1 km). Unlike 1D PBL parameterizations, the 3D PBL parameterization does not use the deformation-based 2D Smagorinsky parameterization in the horizontal direction, which is not as appropriate for horizontal mixing at gray zone grid spacings. All turbulent stress and flux terms are parameterized using TKE, mean gradients, and lengths scales in the MY model in the 3D PBL parameterization, yielding a more complete representation of 3D subgrid-scale mixing.

Using observations and upscaled LES to match the effective resolutions of the WRF model simulations, the strengths and weaknesses of the 1D and 3D PBL parameterizations over the ocean and land are characterized for the following phenomena: the 3D structure of the coastal jet, land-sea breezes, terrain-induced mesoscale circulations, coherent meso-gamma-scale PBL structures over the ocean and land, and the spatiotemporal variability of hub height winds.