Assessment Wind Turbine Repowering on Radar Data Quality

Energy production from wind turbines is an important contribution to the overall energy production in Germany. Adequate sites for wind energy production are limited and there is a site competition between radar sites
and wind turbines. The German Meteorological Service attempts to keep a 15 km zone around a weather radar site free of wind turbines. In law suits against DWD, this is often challenged successfully by wind turbine operators. Though it is generally accepted that the quality of radar data is significantly degradated by wind turbines, the negative influence on warnings in terms of increased false alarm rates is difficult to prove. The effect of a wind turbine is not only confined to the range bin where the turbine is located. The influence is seen in larger lateral and vertical extent through side lobes effects (see e.g. Norin, 2015). Overall, the influence on radar data is pronounced because classic Doppler filters do not work when a wind turbine is operating.

Aside from installing wind turbines at new site, more and more wind turbines have reached an age where they need to be replaced. This is commonly called as a repowering, where the existing turbine is replaced
by more efficient technology and typically significantly taller towers and systems with longer rotor blades. This in turn increases the backscatter cross section which is expected have an influence on radar data.

The effect of repowering is investigated at the radar site Prötzel, where new installations on exisiting sites are planned Easter 2017. In order to assess the affect on radar quality we have set up a so called clutter target scan in February 2017 as part of the operational scanning. Once an hour the anntenna is pointing at a turbine clutter target (in this case to the wind turbine area, where the repowering is taking place) and
data is acquired for a about 2 seconds. The target is at a 13.85 km range. We also extract the data for this range from the operational volume scans including the QPE estimated and the hydrometeor classification.

We show how the wind turbines affect the quality of the radar reflectivity Z, the Doppler velocity v, and the dualpol moments. In particular we assess the effect of the repowering by comparing the
the data quality before and after the installation of new wind turbines. The data from the turbine clutter target scan are compared to data from clutter target scan of a tower at a range of 16.8 km.