Offshore Low-Level Jet properties from offshore lidar measurements in the Gulf of Maine
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The Low Level Jet (LLJ), an atmospheric flow phenomenon well known as an important source for wind power production in the U.S. Great Plains, is not well characterized or understood in offshore regions considered for wind-farm development, due to a lack of wind measurements having needed precision and vertical resolution at turbine rotor heights. These measurements are also needed to verify whether numerical weather prediction (NWP) forecast models are able to predict offshore LLJ speed, height, direction, and other properties, such as shear. To begin to fill this knowledge gap, in this paper, we have analyzed ship-borne Doppler lidar measurements taken in the Gulf of Maine from 09 July to 12 August 2004. The lidar observing system, NOAA /ESRL's High Resolution Doppler Lidar (HRDL), features high-precision and high-resolution wind measurements and a motion compensation system to provide accurate wind data with a static pointing-angle precision of 0.15°, and dynamic precision < 0.5° despite ship and wave motions (Pichugina et al. 2012, J. Appl. Meteor., 51, 327-349). Fine-resolution wind profiles (15-min time resolution and 15-m vertical resolution) obtained from the water surface up to 1 km, were used to statistically characterize LLJ events, including frequency of occurrence, jet speed maxima, and the height of these maxima. LLJ structure was evident in the lidar-measured wind profiles, often during nighttime and transitional periods, but also during the day on occasion. The LLJ strength for the entire experiment ranged from 5 to 20 m s-1 with a mean value of 9.4 m s-1 and a median value of 8.7 m s-1. A high frequency (about 48%) of jet maxima was observed below 200 m above sea level (ASL) as shown in Figure 1, although for some episodes of strong winds (> 15 m s-1), jet maxima were found at 500-600 m ASL. The existence of LLJs can significantly modify wind profiles, producing vertical wind-shears of 0.03 s-1 or more across the rotor layer. The strong speed and directional shear through the rotor layer during LLJ events, as well as the enhanced winds and turbulence, may all act to increase turbine loads, since the top and bottom tips of the blades would operate in different wind regimes. Additionally, the deviation of LLJ-shape profiles from standardized profiles often leads to significant errors in rotor-layer wind speeds and the calculated wind resource power assessments, such as by using the power-law relation, as will be shown by examples. The shear exponent computed from lidar-measured wind speeds at 10 m and 100 m ASL showed diurnal and other nonsystematic variations ranging from 0.09 to 0.24, with mean value of 0.16. The paper will also present statistics and distributions of wind flow parameters aloft in the turbine rotor layer of the marine boundary layer over a range of heights and under various atmospheric conditions, and show diurnal variations of many flow-related quantities critical to wind energy. It will discuss discrepancies in power estimates using measured hub-height and rotor equivalent winds. Such results cannot be captured by surface measurements or by remote sensing instruments with a coarse vertical resolution of data.