Offshore Wind Farms Modify Low-Level Jets

Offshore wind farms are scheduled to be constructed along the east coast of the United States in the coming years to take advantage of the strong and consistent winds, often driven by low-level jets (LLJ) (Debnath et al. 2021). Because LLJs are critical to the wind resource in this area, as well as influencing bird migration, moisture transport, and pollution transport, it is important to understand how LLJs might change with the construction of these offshore wind farms.

We compare one year of WRF simulations with and without wind farms incorporated into the model (Rosencrans et al. 2023), focusing on five locations chosen by their proximity to future wind development areas. We quantify the heights, wind speeds, shear, diurnal cycle, and annual cycle of LLJs following the methodology of Vanderwende et al. (2015) with modifications for the offshore environment. We also distinguish “very low low-level jets”, with nose heights within turbine rotor regions (lowest 260 m) from LLJs.

Offshore LLJs in this region occur most frequently at night (Figure 1), in the spring and summer months (Figure 2) , in stably stratified conditions, and when a southwesterly wind is blowing. LLJ nose wind speeds range from 10 m/s to over 40 m/s. Wind farms erode LLJs, as fewer LLJs occur in the wind farm simulations than in the no-wind-farm simulation. Wind farms have a larger effect on LLJs with lower wind speeds than LLJs with higher wind speeds. When LLJs do occur in the simulation with wind farms, their noses are higher than in the NWF simulation: the LLJ “nose”, or wind speed maximum, has a mean altitude near 300 m for the “no wind farm” jets, but that nose height moves higher in the presence of wind farms, to a mean altitude near 400m. Similar changes in the rotor region (30m-250m) wind shear and wind veer occur, with lower values across almost all months of the year in the wind farm simulations (Figure 3).

Figure 1: The number of LLJs in each hour of the day is shown for the Vineyard Wind centroid (left), and southern lease area centroid (right). The bottom x-axis time is in UTC, and the top x-axis is in local time (EST). The NWF results are marked by the solid blue line and the wind farm results are in the dashed blue line. LLJs with nose heights 260 m or below are in black, where the solid line is for the no wind farm simulation and the dashed line is for the wind farm simulation.

Figure 2: The number of LLJs in each month of the year is shown for the Vineyard Wind centroid (left), and the southern lease area centroid (right). The no wind farm (NWF) results are marked by a solid blue line, and the wind farm results are marked by a dashed blue line. LLJs with nose heights 260 m or below are in black, where the solid line is for the NWF simulation and the dashed line is for the wind farm simulation.

Figure 3: Rotor region mean veer (column 1) and shear (column 2) by month at the Vineyard Wind centroid (row 1) and the southern lease area centroid (row 2). No-wind-farm simulations are in solid blue, and the wind-farm simulations are in dashed orange. The number of data points for each month corresponds to the right y-axis and is shown in grey.

References

Debnath, M., Doubrawa, P., Optis, M., Hawbecker, P., and Bodini, N.: Extreme wind shear events in US offshore wind energy areas and the role of induced stratification, Wind Energ. Sci., 6, 1043–1059, https://doi.org/10.5194/wes-6-1043-2021, 2021.

Rosencrans, D., Lundquist, J. K., Optis, M., Rybchuk, A., Bodini, N., and Rossol, M.: Annual Variability of Wake Impacts on Mid-Atlantic Offshore Wind Plant Deployments, Wind Energ. Sci. Discuss. [preprint], https://doi.org/10.5194/wes-2023-38, in review, 2023.

Vanderwende, B. J., Lundquist, J. K., Rhodes, M. E., Takle, E. S., and Irvin, S. L.: Observing and Simulating the Summertime Low-Level Jet in Central Iowa, Monthly Weather Review, 143, 2319–2336, https://doi.org/10.1175/MWR-D-14-00325.1, 2015.