Intercomparing the quality of recent reanalyses for offshore wind farm planning

The German energy transition demands the increasing usage of renewable energy resources. Following current plans of the German government, the proportion of renewable energy in total electricity needs shall rise to more than 80 percent until 2030 and reach 100 percent in 2035. One important contribution to reach these goals is the increased setting up of wind farms within the German Exclusive Economic Zone (EEZ). Here, the effective usage of the available space and the economic and environmental planning require knowledge on the meteorological and climatological conditions. A crucial factor is the spatial and temporal variability of the wind over the North Sea and Baltic Sea. Additional insight is gained by the representation of the atmospheric boundary layer.

Global and regional atmospheric reanalyses provide detailed information of the wind speed and wind direction at hub heights of modern wind turbines. In order to facilitate offshore wind farm tenders, Deutscher Wetterdienst (DWD, Germany’s national meteorological service) provides reanalysis data and quality assessments to Bundesamt für Seeschifffahrt und Hydrographie (BSH, Federal Maritime and Hydrographic Agency). Currently, the regional reanalysis COSMO-REA6, maintained by DWD, is used besides the global reanalysis ERA5, produced by the European Centre for Medium-Range Weather Forecasts (ECMWF).

At present, new reanalyses and derived products become available. One example is the Copernicus European Regional Re-Analysis (deterministic system, CERRA-DET and ensemble of data assimilations, CERRA-EDA). Moreover, a successor of COSMO-REA6, R6G2 (COSMO-REA6 Generation 2), is currently produced by DWD. Furthermore, a convection-resolving climate simulation for Germany with COSMO-CLM, as a regional downscaling of ERA5, was recently produced by DWD.

The quality of the reanalysis data for offshore wind energy application is assessed on the basis of a consistent evaluation approach. Different types of observations are used as reference. In-situ measurements of the wind speed and wind direction at heights between 30 m and 100 m are available from FINO (Research platforms in the North Sea and Baltic Sea, https://www.fino-offshore.de/en/index.html). Moreover, near surface wind speed and wind direction, retrieved from ASCAT scatterometers onboard METOP-A and METOP-B satellites, are used (satellite data provided by CMEMS, Copernicus Marine Environment Monitoring Service). Results are presented for the 100 m wind speed and wind direction at FINO1 and near surface wind fields over the North Sea. Uncertainties of the different products are addressed by carefully intercomparing the reanalyses.