Properties Of The Offshore Low Level Jet And Rotor Layer Wind Shear As Measured By Scanning Doppler Lidar

The atmospheric flow phenomenon known as the Low Level Jet (LLJ) is an important source of wind power production in the Great Plains. However, due to the lack of measurements with the precision and vertical resolution needed, particularly at rotor heights, it is not well-characterized or understood in offshore regions being considered for wind-farm development. The present paper describes the properties of LLJs and wind shear through the rotor layer of a hypothetical wind turbine, as measured from a ship-borne Doppler lidar in the Gulf of Maine in July-August 2004. LLJs, frequently observed below 600m, were mostly during nighttime and transitional periods, but they were also were seen during some daytime hours. The presence of a LLJ significantly modified wind profiles producing vertical wind speed shear. When the wind shear was strong, the estimates of wind power based upon wind speeds measured at hub-height could have significant errors. Additionally, the inference of hub-height winds from near-surface measurements may introduce further error in the wind power estimate. The lidar dataset was used to investigate the uncertainty of the simplified power-law relation that is often employed in engineering approaches for the extrapolation of surface winds to higher elevations. The results show diurnal and spatial variations of the shear exponent empirically found from surface and hub-height measurements. Finally, the discrepancies between wind power estimates using lidar-measured hub-height winds and rotor equivalent winds are discussed. Copyright (c) 2016 John Wiley & Sons, Ltd.