Microphysics-based 1-km city-scale ensemble simulations of future extreme precipitation events over Istanbul

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).