Low Level Jet properties and the rotor layer wind shear from offshore lidar measurements

The Low Level Jet (LLJ), an atmospheric flow phenomena well known as an important source for wind power production in Great Plains, is not well characterized or understood in the offshore regions considered for wind-farm development, due to the lack of measurements with needed precision and vertical resolution at rotor heights. To begin to fill this knowledge gap, in this paper, we have analyzed ship-borne Doppler lidar measurements in the Gulf of Maine in 09 July-12 August 2004. 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. The LLJ properties were analyzed for areas close to the shore and farther out to sea, and for periods of strong and weak winds. Similar to inland LLJs, a high frequency of LLJs below 200 m was evident, especially at night time, except for some episodes of strong winds (> 15 m/s) when jet maxima were found at 500-600 m. The existence of LLJs can significantly modify wind profiles producing vertical wind-shear of 0.03 s-1 or more across the layer of 50-150 m. While shear was positive during nighttime hours (3-13 UTC), some cases of negative shear were observed during evening (0-2 UTC) and late morning (16-19 UTC) transitional periods. In both cases the shear may be strong enough to influence operations of turbine blades. The paper also shows that estimates of power based on the hub height winds only lead to significant error compare to estimates based on the equivalent winds.