Extreme wind shear events in US offshore wind energy areas and the role of induced stratification

Abstract

Abstract. As the offshore wind industry emerges on the US East Coast, a comprehensive understanding of the wind resource – particularly extreme events – is vital to the industry's success. Such understanding has been hindered by a lack of publicly available wind profile observations in offshore wind energy areas. However, the New York State Energy Research and Development Authority recently funded the deployment of two floating lidars within two current lease areas off the coast of New Jersey. These floating lidars provide publicly available wind speed data from 20 to 200 m height with a 20 m vertical resolution. In this study, we leverage a year of these lidar data to quantify and characterize the frequent occurrence of high-wind-shear and low-level-jet events, both of which will have a considerable impact on turbine operation. In designing a detection algorithm for these events, we find that the typical, non-dimensional power-law-based wind shear exponent is insufficient to identify many of these extreme, high-wind-speed events. Rather, we find that the simple vertical gradient of wind speed better captures the events. Based on this detection method, we find that almost 100 independent events occur throughout the year with mean wind speed at 100 m height and wind speed gradient of 16 m s−1 and 0.05 s−1, respectively. The events have strong seasonal variability, with the highest number of events in summer and the lowest in winter. A detailed analysis reveals that these events are enabled by an induced stable stratification when warmer air from the south flows over the colder mid-Atlantic waters, leading to a positive air–sea temperature difference.

Structure of Offshore Low-Level Jet Turbulence and Implications to Mesoscale-to-Microscale Coupling

“…We study 2020-05-15, a day in which the NYSERDA floating lidar [9] observed an LLJ in the New York Bight that persisted for approximately 9 hours, starting from 15:00 UTC. Selection of this case day was based on the jet-identification criteria of [18] and motivated by the excellent agreement between WRF mesoscale model predictions and the observations. Mesoscale data were provided by the same model setup as used by other researchers that recently studied the same offshore region [19].…”

Classification and properties of non-idealized coastal wind profiles – an observational study

“…Similarly, frictional decoupling can also occur when warmer air is advected over a cooler surface, typically during spring or early summer when the wind is directed from land towards a water surface and the water is still cold after the winter (e.g., Smedman et al, 1993;Smedman et al, 1997;Debnath et al, 2021) or during winter when air is advected over an ice sheet (Vihma and Brümmer, 2002). As a result of the uneven response to daytime warming of a land surface compared to the water surface, a sea-breeze circulation can form, and this alteration of the wind profile can in turn create an LLJ (e.g., Fisher, 1960; see also Aird et al, 2022).…”

Classification and properties of coastal wind profiles with negative gradients – an observational study

“…Similarly, frictional decoupling can also occur when warmer air is advected over a cooler surface, typically during spring or early summer when the wind is directed from land towards a water surface and the water is still cold after the winter (e.g. Smedman et al 1993, Smedman et al 1997and Debnath et al 2021 or during winter when air is advected over an ice sheet (Vihma and Brümmer, 2002). As an effect of the uneven response to daytime warming of a land surface compared to a water surface, a sea-breeze circulation can form, and this alteration of the wind profile can in turn create an LLJ (e.g., Fisher, 1960).…”