Evaluation of 25-Year ERA5-driven Convection-permitting Climate Simulations over the Black Sea Basin

The Black Sea Basin (BSB) is a climate change hotspot where complex orography and air–sea interactions determine regional climate. Since these intricate dynamics are not fully represented in coarse-resolution models, we used the WRF model to perform ERA5-driven, 3 km convection-permitting climate simulations for the 2000–2025 period over the BSB. In the study, the model's performance is assessed by comparing simulation outputs with an ensemble of gridded observations and reanalyses, as well as daily station data from the GHCN. To this end, we utilized ERA5-Land and TerraClimate datasets to evaluate temperature patterns, while ERA5-Land, CHIRPS, and MSWEP were employed to analyze precipitation regimes. Additionally, we used ESACCI and CCMP satellite-based gridded observations for soil moisture and marine wind evaluations. The results indicate that the WRF model successfully captures the monthly cycles of maximum and minimum temperatures, with a warm bias in the range of 1–3°C. We found that a significant portion of the temperature bias stems from differences in model and station elevations. Applying a lapse-rate correction substantially reduces these discrepancies for both ERA5 and WRF outputs. Specifically, we identified a warm bias of 2°C in minimum temperatures at stations between 1000 and 2000 m in winter, which can be linked to a positive soil moisture bias of about 0.15 m³/m³. This bias is attributed to the model's tendency to produce precipitation amounts 2–3 mm/day higher than observations in winter in high-altitude regions, driven by enhanced orographic forcing at the convection-permitting scale. Notably, the results demonstrate the added value of WRF simulations over ERA5 in simulating extreme precipitation events and capturing the diurnal cycle of precipitation. The model successfully reproduces daily extreme precipitation thresholds exceeding 100 mm, specifically at the 99th and 99.9th percentiles, and peak afternoon precipitation when compared to observations. Furthermore, the model exhibits high skill in reproducing both the magnitude and direction of offshore wind over Black Sea, the Marmara Sea, and the northern Aegean Sea with a mean bias of 0.5–1 m/s. These results validate the performance of the high-resolution WRF configuration, which explicitly resolves fine-scale processes, positioning it as a reliable atmospheric driver for future studies focusing on climate extremes and coupled atmosphere-ocean modeling in the BSB.

Acknowledgment: The numerical calculations reported in this paper were fully performed using the EuroHPC Joint Undertaking (EuroHPC JU) supercomputer MareNostrum 5, hosted by the Barcelona Supercomputing Center (BSC). Access to MareNostrum 5 was provided through a national access call coordinated by the Scientific and Technological Research Council of Turkey (TÜBİTAK). We gratefully acknowledge BSC, TÜBİTAK, and the EuroHPC JU for providing access to these resources and supporting this research.