LiDAR-mast deviations in complex terrain and their simulation using CFD

LiDARs (Light Detection and Ranging) are becoming important tools for wind resource assessments in all kinds of terrain. Compared to mast measurements, mobility and flexibility are their greatest benefits. However, care needs to be taken when setting up a measurement campaign. The influence of complex terrain on the wind leads to inhomogeneous flow. This can cause considerable errors in ground based monostatic LiDAR measurements due to their measurement principle and simplifying assumptions.Within this work, wind measurements from Fraunhofer IWES's 200 m research mast in complex terrain at Rodeser Berg in Kassel, Germany, and a pulsed Doppler LiDAR (Leosphere windcube), located at the mast, are compared. The relative deviation between the measurements of the horizontal wind speed by the LiDAR and the mast (LiDAR-mast deviations) varies with wind direction and height. It ranges from about 4% underestimation to +2.5 % overestimation by the LiDAR - for heights between 120 and 200 m. Two steadystate Reynolds-Averaged-Navier-Stokes (RANS) Computational Fluid Dynamics (CFD)-models and a model based on linearized Navier-Stokes Equations were used to estimate the LiDAR error from a flow simulation. Model results were evaluated depending on model parameterisation such as forest height and density. Given the right parameterisations especially for the forest model the CFD-models showed a good performance when compared to the observed LiDAR-mast deviations. These simulations can thus be used to correct the LiDAR error induced by the complex flow. To demonstrate variations of LiDAR errors due to choice of measurement location, one of the models was run to calculate the wind flow in an area of 2 x 2 km2 around the 200 m measurement mast. This allows the visualization of the estimated LiDAR errors to characterize measurement locations. Results showed the significant variation of measurement errors due to the location.