Evaluation of Reanalysis Models in Complex Terrain: Comparing ICON-DREAM and ERA5 Against Met-Mast Observations at the WINSENT Test Site

    Accurately representing wind in complex terrain remains challenging for atmospheric reanalyses, but is an important prerequisite for estimating wind energy potential. In this study, wind measurements from two meteorological masts at the WINSENT  ("Wind Science and Engineering Test Site in Complex Terrain") on the Swabian Alb are analyzed with respect to orographic effects and local surface influences. The site is located on top of the Swabian Alb plateau, in the vicinity of the forested escarpment, creating a heterogeneous environment. The performance of the high-resolution reanalysis ICON-DREAM ("ICON-Dual resolution Reanalysis for Emulators, Applications and Monitoring", operated by Germany's national meteorological service DWD (Deutscher Wetterdienst)) and the global reanalysis ERA5 ("ECMWF ReAnalysis 5", operated by European Centre for Medium-Range Weather Forecasts (ECMWF)) is evaluated for the location of WINSENT by comparing the reanalysis data with continuous mast observations for 2019-2024 at hub-height (100 m above ground level).
    
    The comparison of the measurements from the two masts, although only about 200 m apart, shows pronounced differences in wind characteristics caused by small-scale orographic effects such as speed-up over exposed terrain as well as reduced wind speeds due to the influence of the nearby forest. These small-scale features are not resolved by reanalysis datasets and therefore cannot be reproduced adequately. 
    
    Both ICON-DREAM and ERA5 show a systematic underestimation of wind speeds compared to the observations. The magnitude of the bias varies over the course of the day, with larger underestimations occurring during nighttime conditions for both reanalysis datasets. Despite its higher spatial resolution, ICON-DREAM shows particular limitations in reproducing the observed diurnal cycle of wind speed, with especially low wind speeds at night. While observations show the occurrence of a nocturnal low-level jet at 100 m, ICON-DREAM only shows this behaviour for higher model levels. This behaviour may indicate reduced model performance of ICON-DREAM under stable atmospheric conditions.