New DOE Lidar Buoys: Evaluation of lidar wind profiles obtained during initial offshore performance tests

In 2014 the U.S. DOE procured two WindSentinel buoys from AXYS Technologies, Inc. to support research and development activities associated with the development of offshore wind plants in U.S. waters. Each buoy is equipped with a Vindicator Doppler lidar (manufactured by OADS inc), and a variety of other meteorological and oceanographic sensors. The lidars provide measurements of wind speed and wind direction at six height levels between 30 and 200 m above the ocean surface. As such, the buoy systems help fill a critical observational gap in offshore wind resource characterization by providing routine, long-term, high-resolution data of low-level winds. The accuracy of the buoy-based lidar wind measurements is affected by factors not typically encountered with land-based lidar systems. Platform motion represents one such challenge. The Vindicator lidar is particularly well-suited for operation on a moving platform, because the design incorporates three lasers with fixed pointing directions that pulse simultaneously at 1kHz. The returns from each line-of-sight are averaged down to one second and converted to wind speed and direction while correcting for the attitude and motion of the platform. Since the line-of-sight of each laser is fixed, no scanner is necessary, and no errors are introduced as a result of non-simultaneity of the radial velocity data. Following the initial acceptance tests, the WindSentinel buoys were deployed in the waters off Washington's Olympic Peninsula in September and October of 2014 in order to evaluate the performance of the sensors and assess the accuracy of the lidar wind profiles. The two systems were deployed less than 1 km apart at distances of approximately 500 and 1000m from shore in water depths of approximately 80 m. At the conclusion of the test, wind profile data from the two buoys were compared by compiling statistics of wind speed and wind direction differences at each of the six measurement heights. In this presentation the results of the comparison are summarized. Differences in the lidar profiles are analyzed in the context of differences in surface wind measurements at the buoy locations and at a nearby land-based station in order to account for differences due to spatial variability of the flow.