Turbulent fluxes, stability and shear in the offshore environment: Mesoscale modelling and field observations at FINO1

Abstract

This paper is focused on the evaluation of five planetary boundary layer (PBL) schemes in the Weather Research and Forecasting model for offshore wind energy purposes. One first order scheme: Yonsey University and four one-and-a-half order schemes: Mellor-Yamada-Janić, Quasi-Normal Scale Elimination, Mellor-Yamada-Nakanishi-Niino, and Bougeault-Lacarrère, are considered. Turbulent flux measurements from the FINO1 platform in the North Sea are used to estimate the Obukhov length, allowing the sorting of the data into different stability classes. In addition, wind LiDAR measurements are used to analyze wind profiles up to 251.5 m, encompassing the heights where today's wind turbines operate. The ability of the different PBL schemes to forecast turbulent fluxes of heat and momentum and surface stability is evaluated. Obukhov length results show that in general, PBL schemes forecast more moderated stable stratifications and a reinforcement of the instability for neutral and convective conditions, compared to FINO1 observations. The vertical structure of the wind speed profile is thoroughly analyzed for stable, near-neutral, unstable, and very unstable conditions by using total shear stresses, eddy diffusivities, and wind speed shears. The Mellor-Yamada-Nakanishi-Niino scheme presents the best agreement with measurements considering the different atmospheric stabilities analyzed. Stable conditions are the most complicated scenario for the PBL schemes to reproduce due to their overdiffusive formulations, which effect is to lower the vertical wind shear. Under such conditions, Quasi-Normal Scale Elimination and Yonsey University outperform the rest of the PBL schemes, the latest using a revised diffusion formulation.