Validation of the 2D-VAR lidar retrieval algorithm for non-homogeneous
wind fields using FINO1 and SCADA data
Abstract
A scanning lidar measures a projection of the actual wind speed to the
line-of-sight. Reconstruction of the original field is possible via
various retrieval algorithms, especially if several lidars are deployed
in the area. However, this may not be the case due to financial
constraints. Additionally, conventional algorithms like Volume Velocity
Processing (VVP) do not perform well for non-homogeneous flow fields,
e.g., smooth wind turbine wakes in a retrieved field. A 2D-VAR algorithm
using VVP as an intermediate step was suggested allowing the retrieval
for consecutive scans at a low elevation angle. We validate this
algorithm for scans containing prominent wakes under elevation angles
within 5° using FINO1 mast and supervisory control and data acquisition
(SCADA) data. We show that prominent wakes scanned at the hub height
introduce substantial disturbances in the VVP retrieval output and
affect the retrieved field. The unwanted disturbance may be overcome by
masking the wakes. In point measurements, the wind speed magnitude is
retrieved with an acceptable accuracy, but the wind direction appears to
be sensitive to the weights and initial guess chosen. According to our
sensitivity analysis, the weight assigned to the radial velocity
residuals affects the outcome most, while other weights act as
corrections.