As climate-related disasters intensify globally, cities are increasingly vulnerable to extreme rainfall and urban flooding. Hong Kong has also experienced increasingly intense and frequent heavy rainfall events in the recent years. One striking example was the unprecedented black rainstorm in September 2023 in Hong Kong, which exposed critical vulnerabilities in urban infrastructure and community preparedness. This public forum, part of the 9th CPCM Workshop, brought together experts from science, policymakers, and industry to explore collaborative strategies for managing these escalating risks.

Technical Summary:

The 9th CPCM Workshop took place from 5–7 August 2025 at the Hong Kong University of Science and Technology (HKUST), marking the first time the event was held physically in Asia. It was co-organized by the WMO World Weather Research Programme (WWRP), the WMO World Climate Research Programme (WCRP), GEWEX, CORDEX, and the Hong Kong Observatory, reflecting strong institutional support from the WMO and its affiliated programs.

Leading scientists from mainland China, Hong Kong, and 13 other countries – Australia, Canada, Denmark, Finland, Italy, Japan, the Philippines, South Korea, Sweden, Switzerland, Thailand, the United Kingdom, and the United States. Many participants were from national meteorological services and leading national research institutes, representing the urban modelling teams behind all major numerical weather prediction (NWP) models used by weather services worldwide. This made the workshop a rare global gathering of those who set the technical direction for extreme precipitation research and urban climate prediction.

The meeting opened under extraordinary circumstances, with extreme precipitation—our “star” but “uninvited guest”—joining the workshop. On 5 August 2025, the Hong Kong Observatory issued at 5:50 a.m. the first-ever fourth Black Rainstorm Warning within a single year. The warning remained in force throughout the day, forcing the workshop to move online from 2:00 to 5:00 p.m. Fittingly, it was only cancelled at 5:05 p.m., just after the close of the first day’s program—almost as if our “guest” had insisted on staying for the entire session. In total, the Observatory recorded the highest-ever August daily rainfall (368.9 mm) since records began in 1884. The episode offered a vivid reminder of the challenges under discussion, underscoring both the urgency and the real-world relevance of the workshop’s themes. Revisit the 9th CPCM Workshop

Public Forum Panel 1 – Understanding the Challenges: Scientific Prediction of Urban Climate Extremes

The first panel addressed three major themes: assessing current capabilities and gaps, advancing high-resolution modeling and AI integration, and defining future research priorities.

Predictability and Gaps

• Prof. Andreas Prein (Chair, CPCM Science Committee, ETH Zurich) highlighted the inherent difficulty of forecasting convection in the tropics. “Pinpointing where thunderstorms will strike hours in advance is almost impossible,” he explained, emphasizing that ensemble forecasting is critical to providing probabilities and quantifying uncertainty. He also stressed the need for computationally affordable ensembles and noted the promise of AI in learning from them.
• Prof. Fei Chen (Co-chair, WWRP Urban-PREDICT Project; HKUST) agreed, noting that current models often fail to capture extreme events ahead of time, particularly in tropical regions. He argued for better representation of clouds, cities, terrain, and coastlines in models, as well as denser observation networks: “Even Hong Kong’s world-class system still misses extremes in some places.”

High-Resolution Modeling and AI

• Prof. Prein underscored that physical modeling and AI should complement each other, with the merging of the two holding the greatest promise. At the same time, he cautioned that “AI is only as good as the data we provide” and must be subjected to rigorous, model-like validation.
• Prof. Chen pointed to WMO’s Urban-PREDICT initiative as a pathway, combining ultra-high-resolution modeling with physical sciences, social sciences, and AI. He stressed the importance of new reanalysis datasets: “Unprecedented high-resolution data for Southeast Asia will be critical to train AI and advance both physics-based and AI forecasts.”

Future Research Priorities

Looking ahead, panelists highlighted several directions for advancing prediction in tropical and coastal cities. Prof. Prein emphasized the development of multi-model, high-resolution ensembles to better quantify uncertainty and provide more reliable guidance. Prof. Chen pointed to new regional reanalysis datasets, particularly for Southeast Asia, as a foundation for both physical models and AI-based methods.

Both stressed that the future lies in integrating AI with physics-based modeling—AI for speed and efficiency, physics for interpretability and robustness—so forecasts are both faster and more trustworthy. They also noted the importance of expanding urban and coastal observation networks, including radars, IoT sensors, and satellites, to capture the fine-scale processes driving extremes.

Moderator Prof. Chi-Ming Shun (former HKO Director; HKUST) added that future research must also focus on the “last mile” of forecasting: ensuring that advances in science translate into practical, actionable early warnings for densely populated cities.

Public Forum Panel 2 – Collaborative Resilience: How Coastal Countries and Local Industries Are Preparing for Extreme Rainfall

The second panel explored how advances in science connect to resilience and adaptation, drawing on both international experiences and Hong Kong’s local practices.

International Perspectives

• Prof. Jason Evans (Science Advisor, WCRP CORDEX Program; University of New South Wales) reminded participants that “the climate system by its nature is global.” He stressed the importance of multi-model ensembles and international collaboration to capture uncertainty. He also described how unified datasets in Australia have informed government planning, leading to new engineering guidelines that require 8–15% higher design rainfall standards for each degree of warming.
• Dr. Faye Abigail Cruz (Co-Chair, WCRP CORDEX–SE Asia; Manila Observatory) emphasized the particular challenges for developing and archipelagic countries. She noted that “high-resolution climate information is necessary… [because] global climate models have poor spatial resolutions,” and stressed the need for “co-production of information” so that scientific outputs are tailored to the needs of local communities and decision-makers.

Hong Kong Perspectives

Local institutions provided concrete examples of resilience-building in practice:

• Mr. Ringo Mok, Director of Drainage Services, explained that decades of sustained investment in drainage infrastructure have significantly reduced flood risk, even under record-breaking rainfall. He added that continued adaptation will be essential as extreme events intensify.
• Mr. Woody Chan, Senior Manager – Sustainability at Link REIT, observed that recent flooding events had accelerated adaptation planning in the property sector, from infrastructure improvements to new operational protocols.
• Mr. Clement Hau, Chairman of the Fire Insurance Association at the Hong Kong Federation of Insurers, underscored the importance of better data and risk assessment: “The rising frequency of extreme events underscores the need to better quantify and share climate risks, so that insurance can remain a tool to support resilience rather than just cover losses.”

Moderator Prof. Christine Loh (HKUST) closed the discussion by underlining the connection between science and practice: “The real test of these advances will be whether they can inform how governments, businesses, and communities prepare for the climate risks ahead.”

Final Note

The discussions across both panels underscored the importance of not only advancing scientific prediction, but also sharing best practices in resilience and adaptation. This workshop was unique in that participants witnessed first-hand the impact of extreme precipitation in Hong Kong, as the city recorded its highest-ever August daily rainfall while the meeting was underway.

International participants were all impressed by how effectively Hong Kong managed the situation, pointing to the government’s long-term investments in drainage infrastructure, early warning systems, and emergency response, together with the preparedness of businesses and communities. Many remarked that Hong Kong’s experience offered valuable lessons for other coastal cities on how to strengthen resilience and adaptation in the face of intensifying climate risks.