Sensitivity of Coupled Atmosphere / Ocean Simulations for U.S. West Coast Offshore Wind Resource Applications

The installation of wind turbines in the U.S. offshore environment is underway with operational wind farms off the U.S. East Coast, and the first wind energy lease sales being held off the U.S. West Coast in 2022. Despite the technical challenges, these deployments aim to take advantage of a wind resource that is typically stronger and less variable than what exists over land. While deployments of DOE Lidar Buoys near the Humboldt and Morro Bay lease areas have verified these general characteristics of the resource, modeling hub-height winds at these locations is complicated through the effects of multiple processes. These complicating factors include topographic and channel-type flows, seasonal upwelling and its thermal effect on the boundary layer, frequent warm-season stratiform cloudiness, occasional cold-season atmospheric river events, and the roughness caused by frequent multimodal sea states. Many of these processes involve mutual interactions of the atmosphere, wave state, and ocean at various space- and timescales, which are seldom all captured in a single numerical model. Furthermore, the scarcity of offshore observations around wind turbine hub height (100 -200 m) make it difficult to properly initialize such models, and to evaluate their forecast skill.

Observationally Driven Resource Assessment with Coupled Models (ORACLE) is a multi-institutional project that aims to increase the trust in novel observational algorithms and ocean-wave-atmosphere coupled modeling techniques to quantify and reduce wind resource uncertainty along the U.S. West Coast. For this work we use the Coupled-Ocean-Atmosphere-Wave-Sediment-Transport (COAWST) system, a framework that couples atmospheric (WRF), wave (WAVEWATCH III) and oceanic (ROMS) models in various user-specified configurations. Model skill is assessed by evaluating against a set of NDBC, CDIP, and lidar buoy observations as well as existing analysis products across all seasons over the annual cycle. Sensitivity tests of the fully three-way online system, with stand-alone simulations of the different components of the system, provide insights into the modes of coupling most relevant to offshore wind forecasting interests. Evaluation of different physics specifications and methods of performing the simulations over extended time periods show that these strongly affect the coupling behavior observed.