Researchers led by Associate Professor Hideki Kikumoto at the Institute of Industrial Science, The University of Tokyo have developed a new method to fuse climate analysis datasets with weather observations. This allows researchers to create fine-scale maps of the location and magnitude of changes in both day and night temperatures and on extremely hot days. Their method can be used to help urban planners to improve thermal comfort in Tokyo.

 Global climate change is making temperatures hotter, particularly in densely populated cities, which can adversely affect the health of residents. While mitigation efforts are urgent, it is hard for urban planners to identify exactly where to target as accurate, long-term climate records created over fine spatial scales have been unavailable.

Now, a recent study published in Sustainable Cities and Society reveals an innovative approach to reconstruct hourly changes in urban air temperatures, created by researchers and collaborators at UTokyo-IIS. By fusing climate analysis datasets and weather observations together, climate scientists and urban planners can use this powerful tool to assess climate change in urban regions more accurately.

“Existing climate data can’t capture how people actually experience the temperature in the city hour-by-hour, because variables like trees, buildings, and parks create a complex urban microclimate,” says lead author, Xiang Wang. “Our fusion approach allows us to develop a very detailed picture of how heat and humidity have changed in Tokyo city and suburbs.”

The novel method combined mathematical concepts and a probabilistic estimation method to fuse a regional-scale numerical model dataset with 30 years of observational data from weather stations, taken from surface air temperature and vapor pressure, around the Tokyo area. As a result, the new method can overcome limitations seen in previous approaches, as the high-resolution spatial data can improve the accuracy and length of analysis period in mapping temperature variation within a region.

Ultimately, the reconstruction found a maximum increase in average daytime temperatures of over 1°C and in central Tokyo, the summer temperatures increased by 2°C at night.

Even more worryingly, the number of extremely hot days , defined by a heat index which considers both temperature and humidity, more than doubled in Tokyo after 2021, compared against the 1990s, posing a substantially greater risk to human health. The trend is linked to expanding urban development and associated impervious surfaces. Certain urban areas like Kumagaya, to the northwest of central Tokyo, were more affected by extremely hot days than the city. Most places in the west of the study area showed a significant increase in hot days after 2011.

Trend of the spatial distribution of average nighttime surface air temperature during summer between 1991–2023 in the Tokyo area (the green dashed line is the contour line of 25 ℃ used as a reference of change; the average nighttime surface air temperature from 9 pm to 3 am of the next day from June to August)

Trend of the spatial distribution of the average number of extremely hot days that daily maximum heat index exceeds 40.6 ℃ for several annual periods ranging between 1991–2023 in the Tokyo area.

“Our study clearly identified where, when and by how much temperatures and humidity have risen around the Tokyo area over the past 30 years,” explains senior author, Hideki Kikumoto. “The higher temperatures at night are concerning, as this can affect human health in many ways.”

The fusing of numerical climate analysis and observed climate data provides a new perspective on mapping air temperatures and humidity in urban areas at fine spatial and temporal scales. However, the researchers acknowledged that the method requires refinement, as the accuracy varied depending on the observation sites and, despite high-resolution, the method could not account for all variables (like parks or buildings) in a particular area.

It is hoped that this new modeling approach creates a scientific foundation for urban planners to design mitigation measures, alongside informing heat adaptation policies, which could potentially improve livability in Tokyo for current and future residents.

“Decadal assessment of local climate utilizing meteorological analysis and observation data: Thermal environment changes in the Tokyo area “, is published in Sustainable Cities and Society atDOI: 10.1016/j.scs.2025.106138

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Associate Professor Hideki Kikumoto

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