Japan's Growing Challenge with Extreme Rainfall Events

Japan, an archipelago characterized by its mountainous terrain and diverse climate zones, faces persistent threats from heavy rainfall, typhoons, and subsequent flooding. These events have intensified in recent years due to climate change, with the Japan Meteorological Agency noting a rise in extreme precipitation incidents. For instance, in 2025, record-breaking rains followed Japan's hottest day on record, exacerbating flood risks across the southwest. Projections indicate that under +2°C warming, flash flood risks could increase by over 10% in most basins, rising to 20% at +4°C, highlighting the urgency for advanced predictive tools.

Rural areas, often underserved by dense observation networks concentrated in urban centers, suffer from data gaps that hinder accurate forecasting. Traditional hazard maps may underestimate risks, leaving infrastructure vulnerable. This backdrop underscores the critical role of innovative research from institutions like Osaka Metropolitan University (OMU) in bridging these gaps through sophisticated modeling techniques.

Osaka Metropolitan University's Breakthrough Research

A team from OMU's Graduate School of Human Life and Ecology has pioneered a novel approach to extreme rainfall prediction, published on January 6, 2026, in the Journal of Hydrology: Regional Studies. Titled "Assessing the risk of extreme precipitation in Japan through GEV distribution and spatial modeling," the study leverages four decades of hourly data to forecast risks up to 100-year return levels across Japan's four main islands.

The research addresses key limitations in existing methods: ordinary kriging (OK) and kriging with external drift (KED) often underestimate extreme values, while Markov Chain Monte Carlo (MCMC)-based Bayesian hierarchical models demand excessive computation. Instead, the team employs the Integrated Nested Laplace Approximation - Stochastic Partial Differential Equation (INLA-SPDE) approach, offering efficiency and accuracy for Japan's complex topography.

The Research Team Behind the Innovation

Leading the effort is Associate Professor Jihui Yuan, alongside Emeritus Professor Kazuo Emura and Professor Craig Farnham from OMU, with Visiting Researcher Zhichao Jiao from Yantai University's Architecture School. Yuan's team specializes in environmental risk assessment, building on prior work in statistical hydrology tailored to Japan's unique geography.Aspiring researchers in environmental science can draw inspiration from their interdisciplinary collaboration, blending statistics, ecology, and spatial analysis.

"This study contributes to improving disaster prevention plans by identifying limitations of conventional hazard maps," Yuan emphasized, highlighting the practical orientation of OMU's research. Their work, funded by JSPS KAKENHI grants (23KJ1840, JP22K02098), exemplifies how Japanese universities foster cutting-edge climate adaptation studies.

Data and Methodology: A Rigorous Foundation

The study utilized hourly precipitation records from 752 Japan Meteorological Agency (JMA) stations spanning 1981–2020, dividing Japan into four climate regions for targeted analysis. At each station, Generalized Extreme Value (GEV) distributions— a statistical model for maxima in hydrological extremes (full name: Generalized Extreme Value distribution)—were fitted using MCMC to derive return levels for 2-, 5-, 10-, 25-, 50-, and 100-year events.

Spatial extrapolation to ungauged areas incorporated covariates like annual precipitation, distance from the coast, and population density. Three methods were compared:

• Ordinary Kriging (OK): Basic spatial interpolation.
• Kriging with External Drift (KED): Incorporates covariates.
• INLA-SPDE: Bayesian approximation via stochastic partial differential equations, with variants SPDE1 (annual precip covariate) and SPDE2 (coast/population).

Leave-One-Out Cross-Validation (LOOCV) validated performance, confirming INLA-SPDE's superior stability, especially for rarer events.Read the full paper for step-by-step GEV fitting and INLA implementation details.

Key Findings: Mapping Japan's Extreme Rainfall Risks

Precipitation intensity generally decreases from south to north, but extremes intensify in mountainous regions. Spatial variability escalates with return period: high-risk zones expand northward, with INLA-SPDE revealing narrower confidence intervals than kriging for 100-year events.

In one region, kriging's predictive skill faltered at century-scale extremes, underscoring INLA-SPDE's robustness. These maps enable precise identification of vulnerable rural areas, where data scarcity previously obscured threats.

This step-by-step enhancement—from station-level GEV to covariate-driven spatial modeling—provides actionable insights for Japan's disaster-prone landscape.

Advantages Over Traditional Approaches

• Efficiency: INLA-SPDE approximates MCMC integrals rapidly, ideal for large datasets.
• Accuracy for Extremes: Avoids kriging's underestimation, capturing variability in sparse data regions.
• Topography Adaptation: Suited to Japan's rugged terrain via stochastic PDEs modeling spatial dependence.

LOOCV metrics showed SPDE1 outperforming others, with lower root mean square error (RMSE) and mean absolute error (MAE) for long returns. This positions INLA-SPDE as a game-changer for national-scale hydrological risk assessment.Explore research positions in spatial statistics at Japanese universities.

Implications for Disaster Management and Infrastructure

Japan's annual flood damages exceed billions of yen, with 2025 events amplifying calls for refined hazard maps. OMU's model informs resilient infrastructure planning, prioritizing reinforcements in emerging high-risk zones. Rural municipalities, often under-resourced, benefit most from these predictions, enabling targeted evacuations and land-use policies.

Integrated with dynamic factors like typhoon tracks, as planned, it could evolve into real-time forecasting tools. Policymakers can now quantify climate-amplified risks, aligning with Japan's Climate Change in Japan 2025 report, which projects intensified extremes.

Climate Change Context in Japan

Global warming has boosted atmospheric moisture, fueling heavier rains. JMA data shows increased hourly extremes, with southern regions like Kyushu facing 30 mm/h delays in ambulance response during peaks. Projections warn of multiscale flood rises, urging adaptive strategies. OMU's work complements national efforts, providing granular spatial insights absent in coarser climate models.

Cultural context: Japan's sonae (preparedness) ethos aligns with proactive modeling, yet rural-urban disparities demand equitable tools like this.

Future Directions and Expansions

The team aims to integrate typhoon dynamics and transition to full spatio-temporal models for process-level forecasting. High-resolution variants could support urban planning in flood-vulnerable Osaka. Collaborations with JMA could operationalize outputs, enhancing early warning systems.

This positions OMU as a leader in hydrological statistics, inspiring similar applications globally.Discover more Japan higher ed news.

OMU's Role in Japan's Higher Education Landscape

Osaka Metropolitan University, formed by merging Osaka City and Prefecture Universities, excels in applied environmental sciences. Its Graduate School of Human Life and Ecology drives interdisciplinary research addressing societal challenges. Faculty like Yuan exemplify mentorship, with JSPS funding fueling student-led extensions.Rate professors and share experiences from OMU or similar institutions.

In Japan's competitive academic scene, such publications bolster NIRF-like rankings and attract faculty positions in climate modeling.

Actionable Insights for Stakeholders

• Engineers: Update designs for 100-year extremes using INLA-SPDE maps.
• Planners: Prioritize northern expansions in risk zoning.
• Researchers: Adapt INLA-SPDE for other hazards like landslides.
• Students: Pursue stats-hydrology theses; check scholarships for Japan studies.

This research not only predicts but empowers resilience, showcasing higher education's pivotal role.

Photo by Pat Krupa on Unsplash

Conclusion: Paving the Way for Resilient Japan

OMU's novel spatial modeling marks a milestone in extreme rainfall prediction, offering precise, stable forecasts amid escalating climate risks. By overcoming data sparsity and methodological hurdles, it equips Japan for safer futures. Explore opportunities at higher-ed-jobs, rate-my-professor, or higher-ed-career-advice to join this vital field. For Japan-specific roles, visit /jp and university-jobs.