The use of a compact set of climate change indexes enhances our understanding of the combined impacts of extreme climatic conditions. In this study, we developed the modified Climate Extremes Index (mCEI) to obtain unified information about different types of extremes. For this purpose, we calculated 10 different climate change indexes considering the temperature extremes, extreme precipitation, and moisture surplus and drought over the Europe–Mediterranean (EURO–MED) region for the 1979–2016 period. As a holistic approach, mCEI provides spatiotemporal information, and the high-resolution grid-based data allow us to accomplish detailed country-based and city-based analyses. The analyses indicate that warm temperature extremes rise significantly over the EURO–MED region at a rate of 1.9% decade⁻¹, whereas the cold temperature extremes decrease. Extreme drought has a significant increasing trend of 3.8% decade⁻¹. Although there are regional differences, extreme precipitation indexes have a significant increasing tendency. According to the mCEI, the major hotspots for the combined extremes are the Mediterranean coasts, the Balkan countries, Eastern Europe, Iceland, western Russia, western Turkey, and western Iraq. The decadal changes of mCEI for these regions are in the range of 3–5% decade⁻¹. The city-scale analysis based on urbanized locations reveals that Fes (Morocco), Izmir (Turkey), Marseille and Aix-en-Provence (France), and Tel Aviv (Israel) have the highest increasing trend of mCEI, which is greater than 3.5% decade⁻¹.

The frequency and severity of extreme weather events, including temperature and precipitation extremes, have been increasing globally due to human-induced climate change. The Black Sea Basin (BSB), with its complex topography and strong air-sea interactions, is particularly susceptible to climate change and serves as a hot-spot for studying regional climate extremes. To obtain reliable information in BSB, high-resolution convection-permitting simulations are necessary. In this research, we performed convection-permitting climate simulations for historical (2005–2014) and future (2061–2070) periods to investigate the changes in temperature and precipitation extremes and underlying mechanisms based on the SSP3-7.0 climate change scenario over the BSB. To achieve this, we downscaled the CMIP6-based MPI-ESM1.2-HR outputs to 3 km horizontal resolution using the WRF model. The future simulation demonstrates an increased exposure to warm extremes as indicated by the positive change of the TX90P index by about 18% and an increase of the heat wave duration index (HWDI) reaching 55 days per year over the BSB. These changes primarily occur over the highlands of Eastern Anatolia due to enhanced land-atmosphere interactions. In March, a change in low-level circulation leads to a sudden warming of approximately 6°C and an early onset of the melting season, resulting in a 20% reduction in snow cover over Eastern Anatolia. This shift increases extreme temperatures due to a substantial snow albedo feedback caused by a 10% reduction in surface albedo in this area. Furthermore, our analyzes highlight the intensification of daily and sub-daily precipitation along the coastal regions of the Black Sea. Particularly in winter and autumn, the ratio of daily extreme precipitation amounts to the seasonal total precipitation (R90PTOT index) reaches 45% in the future over the eastern Black Sea. Additionally, daily precipitation probabilities shift towards higher values for extreme precipitation amounts in the same area with maximum precipitations exceeding 280 mm/day. At the sub-daily scale, this region experiences an intensification in hourly precipitation throughout the day due to a 22% increase in low-level moisture flux resulting from 1°C warmer sea surface temperatures in winter. The increased extreme precipitation in the autumn is associated with the intensification of afternoon precipitation along the Black Sea coasts of Türkiye. This study emphasizes the importance of convection-permitting climate simulations in improving our understanding of climate extremes in the topographically complex BSB. It provides valuable insights for mitigation and adaptation efforts in this climate change hot-spot.Acknowledgment: The numerical calculations reported in this paper were fully performed at TUBITAK ULAKBIM, High Performance and Grid Computing Center (TRUBA resources).

Detailed information about extreme precipitation is crucial due to the impacts on the human environment. Recently, high-resolution regional climate models (RCMs) are run at convection-permitting scales to investigate the regional precipitation extremes. The Black Sea region is one of the intriguing regions for modelling studies because of its distinctive topographical features where orographic forcing and strong air-sea interactions intensify destructive heavy precipitation. Recently, RCMs have been tested in order to find the most suitable configuration to represent precipitation over this region. Although the historical simulations are beneficial to test the model performance, model configurations may exhibit different spatiotemporal characteristics in simulating extreme precipitation due to the shift of the seasons in a possible warmer future. Recent studies focusing on the intensification of extreme precipitation events highlighted the model sensitivity to increasing sea surface temperature (SST) over the Black Sea. Therefore, future simulations focusing on different model configurations may provide valuable information to understand the response of RCMs in a changing climate. In this study, we downscaled the last generation CMIP6 MPI-ESM1.2-HR outputs by using the WRF model at 3 km horizontal resolution to test the model’s sensitivity for different microphysical and planetary boundary layer (PBL) parameterization options under the SSP5-8.5 future socioeconomic global change scenario. We selected cold and warm extreme precipitation cases and performed 3-days convection-permitting simulations over the complex topography of the Black Sea region. For the cold case, simple single-moment schemes produced less precipitation compared to more complex schemes, especially over the mountains, because of the insufficient representation of snowfall. For the warm case, the difference between the simulations is similar to the cold case but, the magnitude is lower. The change of the PBL scheme affects the vertical and horizontal distribution of microphysical properties and precipitation distribution near the coasts and the mountains.

The Black Sea Basin (BSB) is one of the climate change hot-spots where intense atmosphere–sea interactions and complex topography shape temperature distribution, precipitation regimes, and extreme weather events. Accurate simulation of these regional climate patterns in high-resolution atmospheric models requires microphysical parameterization schemes that realistically represent hydrometeor evolution and temperature tendencies. In this study, we evaluate a suite of convection-permitting WRF simulations at 3 km resolution to identify the microphysics scheme that most reliably reproduces temperature, precipitation, hail occurrence, and snow cover across three meteorologically contrasting years over the BSB. Annual sensitivity simulations for 2008 (a dry year), 2010 (a warm SST year), and 2017 (a wet year) using the Goddard, Milbrandt, NSSL, WSM7, WDM7, and Thompson schemes show that the model reproduces the spatial and seasonal patterns of Tmax and Tmin across the basin. Seasonal temperature patterns are robustly captured by all schemes, with biases generally limited to 2–3°C and partly shaped by mountainous terrain. Similarly, precipitation evaluations indicate that the model represents basin-wide spatial distributions and seasonal cycles with high fidelity, with remaining biases of 6–10 mm/day over the Caucasus and eastern Anatolian highlands, reflecting the observational limitations in complex topography. RMSE metrics show that the microphysics schemes vary in performance, with the Milbrandt scheme standing out for its consistently year-to-year uniform behavior. It maintains low Tmin RMSEs of 1.30–1.40°C and precipitation RMSEs of 0.87–1.15 mm/day across all three years, indicating that it reproduces temperature and precipitation fields with stable accuracy under different meteorological conditions. Hail and snow cover evaluations further reinforce this result. The Milbrandt scheme adequately represents the spatial distribution and spring/early-summer evolution of hail-prone areas across Türkiye. Simulated spring snow cover closely matches satellite observations over the Upper Euphrates Basin, located in eastern Türkiye. Additionally, preliminary results from ERA5-driven simulations strengthen these findings by realistically reproducing the long-term characteristics of temperature, precipitation, and snow cover over the BSB. Overall, the Milbrandt scheme serves as the most suitable microphysical parameterization option for the upcoming long-term fully coupled atmosphere–ocean simulations aimed at improving the representation of two-way air–sea feedbacks, which are crucial for understanding climate processes over 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.

Anthropogenic climate change is projected to intensify hydrometeorological hazards in megacities, with Istanbul emerging as a highly vulnerable hotspot due to its complex topography, strong air–sea interactions, and rapidly urbanizing structure with its growing population of 16 million people. Therefore, high-resolution extreme precipitation simulations are crucial in risk assessment and adaptation planning in Istanbul. In this study, we performed high-resolution convection-permitting simulations over the Black Sea Basin using the WRF model at 3 km horizontal resolution to assess the future evolution of extreme precipitation in Istanbul. The simulations cover a reference period (2005–2014) and two future periods (2041–2050 and 2061–2070) under the SSP3-7.0 scenario. In addition, three future extreme precipitation events (November 2043, December 2050, and October 2064) are dynamically downscaled by nesting a 1-km domain within the 3-km Black Sea domain and applying an ensemble of microphysical parameterization schemes (ETA, Goddard, Kessler, Lin, Milbrandt, Morrison, NSSL, WSM6, WDM6, and Thompson) to better quantify short-duration, localized hazards. Model evaluation indicates that WRF successfully reproduces the precipitation patterns of the reference period. Future projections show that while moderate precipitation remains relatively unchanged, extreme daily totals intensify substantially, with maximum 24-hour precipitation increasing from 210 mm in the reference period to 290 mm and 435 mm in the 2041–2050 and 2061–2070 periods, respectively. Percentile-based indices similarly indicate more frequent and more intense extreme precipitation events, particularly across northern Istanbul. Microphysics-based 1-km ensemble simulations demonstrate that extreme events in future decades may produce both intense 3–6 hour and prolonged 12–24 hour precipitation episodes, elevating the risk of flash urban flooding and longer-duration flood impacts across extensive parts of Istanbul. The spatial extent of areas receiving more than 100 mm of precipitation within 24 hours is substantial, covering 30–36% of the city in the 2043 event (Lin scheme), 25–32% in the 2050 event (WDM6 scheme), and reaching up to 69% in the 2064 event (Kessler scheme). The simulations using the NSSL microphysics scheme in the October 2064 case produce more than 800 mm of 24-hour total precipitation. Furthermore, the Milbrandt scheme yields hail accumulations exceeding 50 mm in the same event, particularly over the vicinity of Istanbul Airport. Overall, these findings indicate that extreme precipitation in Istanbul will become more intense, more widespread, and more frequent under future climate conditions, highlighting the importance of km-scale ensemble simulations for reliably characterizing high-impact precipitation and hail hazards. The high-resolution, city-scale modeling framework developed in this study provides essential scientific input for urban planning and climate adaptation strategies in the Istanbul megacity.Acknowledgments: This study was funded by the Istanbul Metropolitan Municipality, Disaster Coordination Center. The numerical calculations reported in this paper were fully performed at TUBITAK ULAKBIM, High Performance and Grid Computing Center (TRUBA resources).