Convection permitting simulations driven by pseudo-global warming for a heavy precipitation event in the black sea region

Abstract

Anthropogenic climate change has a strong influence on the climate of the Mediterranean and Black Sea basins. The rapid increase in air temperature and sea surface temperature (SST) creates a more conducive environment for extreme weather events, such as short-duration torrential precipitation. Previous studies using General Circulation Models (GCMs) and in-situ observations have identified the Mediterranean Basin as a climate-change hotspot. Modeling studies for future periods suggest that near-surface air temperature increases over the Mediterranean are projected to range from 1.83°C to 8.49°C in CMIP6 models and from 1.22°C to 6.63°C in CMIP5 models. A recent modeling study focusing on the Black Sea basin indicates that an air temperature increase of approximately 3°C is expected for the period 2061–2070 relative to the 2005–2014 reference period. Combined with increasing SSTs, these changes make the Black Sea basin highly sensitive and vulnerable to extreme weather events, as demonstrated by case studies showing that SST warming can lead to extreme precipitation and even the formation of tropical-like cyclones, commonly referred to as Blackcanes. In this study, the Pseudo-Global Warming (PGW) method is applied to the Bozkurt flash flood event that occurred in August 2021, which resulted in approximately 350–450 mm of precipitation at several weather stations in northern Türkiye. The PGW method provides an efficient framework to investigate how such an event may evolve under future climate conditions by perturbing reanalysis data, rather than dynamically downscaling Earth System Models (ESMs). Three emission scenarios are considered: Shared Socioeconomic Pathways SSP2–4.5, SSP3–7.0, and SSP5–8.5, using 20 models for SSP2–4.5 and SSP3–7.0 and 25 models for SSP5–8.5. Climate-change deltas (Δ) are computed from these models and added to the reanalysis data. The reference period is defined as 1990–2014, while three future periods, 2025–2049, 2050–2074, and 2075–2099, are selected and referred to as NearFut, MiddleFut, and FarFut, respectively. Climate-change deltas are computed for air temperature, SST, relative humidity, horizontal wind components, and surface pressure. The simulations are divided into two parts: first, perturbing all variables, and second, conducting selective-forcing sensitivity experiments, including cases with no dynamical changes (horizontal wind components unperturbed), no dynamical and no surface pressure changes, and no dynamical and no relative humidity changes. For the selective-forcing experiments, additional simulations are performed using a slab ocean model to account for atmosphere–ocean coupling effects. All simulations are conducted using the Weather Research and Forecasting (WRF) model at 3 km horizontal resolution, with a domain centered over the Black Sea and covering its surrounding area, driven by ERA5 reanalysis data. The results indicate a clear intensification of the Bozkurt heavy precipitation event under future climate conditions. The control simulation reproduces the event with a maximum accumulated precipitation of 632 mm, a maximum near-surface wind speed of 89 km h⁻¹, and a minimum sea-level pressure of 1003 hPa. In the full-forcing PGW experiments, maximum accumulated precipitation increases to 803–1220 mm for SSP2–4.5 (27% to 93%), 911–1138 mm for SSP3–7.0 (44% to 80%), and 923–1208 mm for SSP5–8.5 (46% to 91%). The spatial extent of extreme precipitation also expands substantially, with the number of grid cells exceeding 100 mm increasing from approximately 15000 in the control simulation to about 32000–35000 in the SSP5–8.5 FarFut simulations. Areal-total precipitation increases systematically with warming, reaching nearly a twofold increase in the SSP5–8.5 FarFut five-day spin-up simulation relative to the control. Selective-forcing PGW simulations produce maximum accumulated precipitation ranging from approximately 720 to 1450 mm (14% to 129%), and the strongest FarFut simulations reach nearly three times the control precipitation in the configuration where relative humidity is unchanged. These increases are attributed to enhanced low-level convergence and stronger vertical velocities. In addition, increases in SST and air temperature increase the moisture-holding capacity of the atmosphere, leading to an intensification of precipitation. In many simulations, a tropical-like cyclone structure develops, and its intensity is highly dependent on the applied perturbation configuration. In the selective-forcing experiments, maximum wind speeds range from approximately 132 to 181 km h⁻¹, while minimum sea-level pressure values range from about 995 hPa to below 975 hPa, reaching approximately 969–970 hPa in the most intense configuration. Slab ocean model experiments further demonstrate that atmosphere–ocean coupling weakens air–sea fluxes relative to prescribed SST simulations by allowing SSTs to evolve in response to atmospheric forcing, primarily due to cyclone-induced SST cooling. As a result, cyclone intensification is reduced compared to prescribed-SST PGW simulations, and the deep warm-core structure is generally weaker. Overall, these results indicate that future warming can substantially amplify extreme precipitation and Blackcane formation over the Black Sea, while interactive atmosphere–ocean coupling is crucial for avoiding overestimation of cyclone intensification in prescribed-SST PGW experiments. The PGW method employed in this study provides a useful framework for examining how a past event may evolve under future climate conditions. However, this approach does not explicitly represent potential shifts in large scale atmospheric circulation. Therefore, the results should be interpreted under the assumption that similar synoptic conditions occur.