During the period from December 2014 to June 2016 a DOE WindSentinel buoy was deployed off the Virginia coast in support of an offshore wind energy demonstration project funded by the DOE Office of Energy Efficiency and Renewable Energy (EERE). The buoy was equipped with a Vindicator III Doppler lidar, manufactured by Optical Air Data Systems (OADS), and a host of other meteorological and oceanographic instruments. Initial analyses of the 10-minute average wind speeds from the lidar shows a consistent negative shear in the upper range gates of the lidar, resulting in an apparent low-level jet (LLJ) feature with a wind speed maximum near the 90 m level.
The apparent maximum in the wind speed profile is believe to be the result of slow biases in the upper range gates due to weak backscatter signals. In this study, we attempt to address this issue by first examining the OADS wind retrieval algorthim used to generate the 1Hz Vindicator data, from which 10-minute average wind speed and direction profiles are computed. We find that the OADS approach is fundamentally sound, but the quality of the 10-minute averages could be improved by applying a higher signal strength threshold to the 1Hz data.
We show that the overall effect of the signal strength threshold is to decrease the magnitude of the shear, both above and below the apparent maxima in the wind speed profile. The effect is most pronounced in the highest range gates where the return signal strength is generally weakest. An undesired side effect is a substantial reduction in the data recovery rate. For a signal strength threshold of 60, the annually averaged data recovery rate was about 75% for the best case (range gate 3), and about 15 to 20% for the worst case (range gate 6). Data recovery rates are higher during the warm season and during daytime periods.
Revised: September 22, 2016 |
Published: June 30, 2016