Evaluation framework for regional reanalyses at DWD: results and outlook

The development and provision of regional reanalyses requires a coordinated quality assessment targeted to user needs. Regional reanalyses with higher temporal and spatial resolution are of particular importance for applications in the energy sector. Detailed information is required on meteorological parameters such as wind speed and direction at hub heights of modern wind turbines (offshore and onshore), global radiation and near-surface temperature. In addition, regional reanalyses are of interest for climate applications dealing with climate extremes such as strong wind, heat waves, heavy precipitation and droughts.

The regional reanalysis COSMO-R6G2 is currently produced at DWD as a successor of COSMO-REA6. It is based on a newer version of the COSMO model and ERA5 boundary conditions, which enables continuous updating. The nudging technique (Newtonian relaxation) is used for the assimilation of conventional observations stemming from radiosondes, SYNOP stations, ships, buoys or aircrafts. No satellite data is assimilated.

The quality is assessed using in-situ measurements of offshore FINO research platforms, tower and lidar measurements over Germany (companion paper by Bär et al., EMS 2024), DWD’s station network for Germany and satellite-based surface irradiance data of CM SAF as reference. Uncertainties are addressed by a comprehensive comparison to other state-of-the-art reanalyses and derived products.

The temperature and radiation biases seen in COSMO-REA6 are still similarly present in COSMO-R6G2. However, there is evidence that the representation of reduced shortwave transmissivity related to the effect of optically thick clouds is more realistic in COSMO-R6G2 compared to COSMO-REA6 where post-processing methods have already been proven for bias adjustment of non-cloudy cases.