May 23 - July 29
Testing the Accuracy of Different Lidar Scanning Strategies with Large-Eddy Simulations
Maggie McMahon (University of Georgia), Dr. Joshua Gebauer (OU/CIWRO & NOAA/NSSL), and Dr. Jeremy Gibbs (NOAA/NSSL)
What is already known:
- Lidars only measure radial wind velocities and can make errors when calculating 3-D wind components
- The accuracy of winds measured by a lidar are impacted by the type of and the elevation angle of the scan
- Lidar scans tend to underestimate vertical wind variance
What this study adds:
- Lidar scans at elevation angles of 50° and 60° consistently get the most accurate wind measurements in environments of low to average vertical wind shear
- Scans that include vertical beams get much more accurate vertical turbulence retrievals
- Velocity-azimuth display scans generally get better wind velocity and variance measurements than Doppler beam swinging scans
- The six-beam strategy got decently accurate wind variance retrievals, but also calculated some negative and thus unreal variance values
Abstract:
Lidar scans can obtain data with high temporal resolution but they can only measure radial velocities, which can lead to inaccurate measurements for wind velocities and turbulence. To improve the performance of future lidar scans in various environments, this study collected data on the accuracy of different scanning strategies’ retrievals under particular wind conditions. A virtual lidar took measurements from the output of large-eddy simulations with different initial vertical wind shears, using a variety of scanning techniques whose results were compared. The retrievals from each scan were used to calculate the component winds and variances, which were then analyzed on how closely they matched the true wind values of the simulations. The scanning strategies tested were different Doppler beam swinging (DBS) and velocity-azimuth display (VAD) scans with modified elevation angles and numbers of beams, as well as the six-beam method. The scans were judged based on the mean and instantaneous root-mean-square errors (RMSEs) between their observations and the actual winds. While there is no single scanning strategy that always got the best results for every wind component, the 8-point VAD with a vertical beam method got the best average wind velocity and turbulence results, and the VAD scans overall did a better job than the DBS scans. Additionally, techniques that were tested with multiple elevation angles got the most consistently accurate wind observations from scans done at 60° and 50°. The findings also show that it is difficult to measure vertical velocity variance accurately unless the lidar scan includes a 90° beam.