Downscaling of Historical Wind Fields over Switzerland using Generative Adversarial Networks

Near-surface wind is difficult to estimate using global numerical weather and climate models, as airflow is strongly modified by underlying topography, especially that of a country such as Switzerland. In this article, we use a statistical approach based on deep learning and a high-resolution Digital Elevation Model to spatially downscale hourly near-surface wind fields at coarse resolution from ERA5 reanalysis from their original 25 km to a 1.1 km grid. A 1.1 km resolution wind dataset for 2016–2020 from the operational numerical weather prediction model COSMO-1 of the national weather service, MeteoSwiss, is used to train and validate our model, a generative adversarial network (GAN) with gradient penalized Wasserstein loss aided by transfer learning. The results are realistic-looking high-resolution historical maps of gridded hourly wind fields over Switzerland and very good and robust predictions of the aggregated wind speed distribution. Regionally averaged image-specific metrics show a clear improvement in prediction compared to ERA5, with skill measures generally better for locations over the flatter Swiss Plateau than for Alpine regions. The downscaled wind fields demonstrate higher-resolution, physically plausible orographic effects, such as ridge acceleration and sheltering, which are not resolved in the original ERA5 fields.