Southeast Asia is characterized by high temperatures and humidity, and as urbanization accelerates and climate change intensifies, the region faces an increasing risk of heatwaves. This study investigates the characteristics and trends of heatwaves across Southeast Asia from 1960 to 2024. Using the ERA5-Land dataset, we classify heatwaves into three types—daytime, nighttime, and compound—based on four extreme temperature indices and consecutive days. The trends of these heatwave types are analyzed using the Mann-Kendall (MK) test and Sen's slope estimator. Additionally, we assess heat exposure intensity using the Heatwave Cumulative Intensity Index (HWCI) across five major Southeast Asian regions (Borneo, Java, the Malay Peninsula, Sumatra, and the Mekong Delta), with a focus on the potential health risks associated with prolonged temperatures exceeding threshold values.

The results reveal that nighttime and compound heatwaves are experiencing a significant upward trend, whereas daytime heatwaves do not show such a trend. Coastal areas exhibit longer durations and a marked increase in nighttime heatwaves, with approximately one additional heatwave event every decade, each lasting 1-2 days longer. In the analysis of HWCI, we find that the intensity of different heatwave types remains consistent across regions. Urbanization and land use/land cover changes do not significantly influence the overall heatwave trends, suggesting that heatwaves in Southeast Asia are predominantly driven by global warming and large-scale atmospheric-oceanic phenomena. This study underscores the urgent need to enhance heatwave monitoring systems and adaptation strategies for tropical low-latitude cities.

How to cite: Liu, P., Chew, L. W., Yu, J., Xin, R., Sahany, S., and Moise, A. F.: Characteristics and Trends of different types of Heatwaves over Southeast Asia based on ERA5-Land, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-185, https://doi.org/10.5194/icuc12-185, 2025.

City-scale thermal comfort mapping is essential for identifying urban heat hotspots and assessing residents' thermal responses, especially when integrated with socio-economic data such as population density. However, many existing thermal comfort studies are confined to microscale analyses, short-term durations, or limited scenarios. To address these gaps, we developed a city-scale mapping tool (Fig.1) by integrating a new GIS-based processing scheme of urban morphologies, a high-resolution Digital Surface Model, an urban meteorological observation network, and a mechanistic urban canopy model Urban Tethys-Chloris (UT&C). This tool can rapidly calculate long-term Universal Thermal Climate Index (UTCI) maps for Singapore within a few hours. Model validation against field measurements from three urban precincts and comparisons with mesoscale Weather Research and Forecasting (WRF) simulations demonstrated robust performance. Parametric studies were conducted to investigate the integrated effects of urban features on thermal comfort, including monsoon seasonal variability, greenery coverage changes, and urban gross plot ratio (GPR) adjustments. The key findings are as follows: (1) The calculated results exhibited good performance in both simple street and complex non-street configurations; (2) Incident radiation and air temperature were the dominant background meteorological factors influencing mapping distributions, particularly in Singapore's cloud-prone environment; (3) Analysis at seven representative precincts indicated that high-rise buildings and extensive greenery effectively improved thermal comfort, with a maximum UTCI reduction of 4.0°C; (4) Increasing greenery coverage by 60% resulted in an average UTCI reduction of 0.7°C for the whole Singapore, mitigating extreme heat risk for approximately 10% of built-up areas. The cooling (up to 2.5°C) was most pronounced in low-rise western and eastern regions; (5) Increasing GPR improved thermal comfort citywide but diminished greenery's cooling efficacy and reduced urban ventilation. Our study provides valuable insights into evidence-based greenery planting strategies for urban planning and design, contributing to sustainable urban environments.

Fig.1 Research workflow.

How to cite: Chen, T. and Yuan, C.: A city-scale mapping tool for assessing the effects of urban greenery and morphologies on thermal comfort: A case study in Singapore, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-739, https://doi.org/10.5194/icuc12-739, 2025.

Ensuring comfortable walking environments is essential for promoting healthy lifestyles and supporting active ageing, especially in densely redeveloped urban cores. However, thermal comfort during walking remains underexplored, a concern heightened by increasingly frequent extreme heat events. This study addresses this gap by presenting findings from a series of field experiments examining how microclimate conditions in redeveloped urban settings influence thermal comfort among older and younger adults. Participants walked through various built environments under very hot conditions, which enabled simultaneous collection of microclimate data and physiological responses, alongside self-reported thermal sensations.

Physiologically equivalent temperatures (PET) were then calculated across these different settings for both age groups, providing a quantitative basis for comparing thermal experiences. Results reveal that urban environments differ in their effectiveness at mitigating heat stress. While PET variations account for most changes in younger adults’ thermal sensations, older adults exhibit greater sensitivity to the characteristics of the built areas and green spaces. The study also explores current redevelopment policy in Hong Kong and discusses how it may reshape thermal environments in historical neighbourhoods, ultimately influencing pedestrian comfort. Taken together, these findings underscore the importance of systematically assessing microclimate conditions in urban redevelopment projects—particularly for areas with large senior populations—and highlight the need for design interventions, such as improved streetscape configurations and baseline tree canopy coverage, to ensure more comfortable and health-promoting walking experiences.

How to cite: Tan, T. Z.: Walking Under Extreme Heat: Considering Pedestrian Thermal Comfort in the Redevelopment of Historical Urban Areas with Hot-Humid Climate, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-1075, https://doi.org/10.5194/icuc12-1075, 2025.

Singapore recognises the importance of addressing heat-related risks, particularly with temperatures set to increase against the backdrop of its humid tropical climate. Drawing from the IPCC’s definition of risk as a function of hazard, exposure, vulnerability, and response, this study presents the development of a baseline Heat Risk Index (HRI) for Singapore. Publicly available demographic, climatic, and geospatial data from 2018—including population census, vegetation, park spaces, infrastructure, and resources—are leveraged to build a comprehensive snapshot of the hazard, exposure and vulnerability parameters contributing to Singapore’s heat risk levels at that time. Responses – government policies and interventions – are documented, as the measurable effects of ongoing responses in and before 2018 would have been in part reflected in the hazard, exposure and vulnerability variables. This baseline would thus provide insights into geographic areas and populations that require additional support to address heat risks.  

However, the true value of the HRI lies in its capacity for regular updates. These updates would examine how heat-related risk evolves with future climatic and demographic conditions. By analysing active and measurable responses that would reduce future exposure, hazards, and vulnerability, the HRI would support evidence-based policies and offer insights into the effective responses in mitigating hazards, reducing exposure, and increasing adaptive capacities. By establishing this HRI baseline, the study lays the groundwork for a dynamic, living index that evolves with Singapore’s climatic and demographic landscapes, and support strategies in making Singapore adaptive and heat- resilient.  

How to cite: Heng, S. L., Yik, S. K., Ho, B., Mandelmilch, M., Pak, H. Y., and Chow, W.: Developing a baseline Heat Risk Index for the Humid Tropical city of Singapore, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-535, https://doi.org/10.5194/icuc12-535, 2025.

The urban microclimate model, urbanMicroclimateFoam developed by the authors, is used to simulate thermal comfort in an urban park and adjacent neighborhood in tropical Singapore during a hot humid period. The thermal comfort is analyzed using the Universal Thermal Climate Index (UTCI) which depends on air temperature, relative humidity, wind speed and mean radiant temperature.

To better understand the spatial variability of urban microclimate, we apply clustering approach to classify different local climate zones using scaled values of comfort variables as input. In the case study, six clusters are identified, each representing areas with similar local climate characteristics. One key cluster corresponds to the zone shadowed by trees in the park, where UTCI is significantly lower due to tree coverage. However, unshaded zone in between the park trees is also represented in a distinct cluster, where UTCI are higher since they experience lower wind speed and higher relative humidity, due to the wind blocking and transpiration by trees. This effect, in contrast to the local shading by trees, is referred to as nonlocal heating effect by trees. Interestingly, the proposed approach identifies different clusters with different thermal conditions in between the building blocks, with wind speed emerging as the primary differentiating factor. This highlights the importance of wind on thermal comfort in hot-humid context.

Using a clustering approach with six variables as input, we were able to detect different wind corridors with a wind speed higher than average. Some of these clusters indicate hot and dry air, or cool and wet air ventilation corridors, which are not easily distinguishable using conventional methods and also unfavorable for thermal comfort improvement. One cluster indicates cool and dry air ventilation, which favors thermal comfort improvement.

In conclusion, clustering approach allows to map different urban microclimate patterns and analyze the underlying reasons for the observed thermal comfort.

How to cite: Carmeliet, J., Nevers, C., Kubilay, A., and Derome, D.: Mapping local urban climate and ventilation corridors using clustering approach , 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-512, https://doi.org/10.5194/icuc12-512, 2025.

Urban overheating is a challenge for all cities, leading to increased cooling demand and negative health outcomes. Urban microclimate modeling is a method for exploring the impacts and solutions to urban overheating and have been evaluated and tested in a variety of urban contexts and purposes.  Outdoor thermal comfort of parks is a common area of study with models. However, previous studies have primarily focused on mid- and high-latitude cities with temperate or cold climates. In the tropics, the intensity of incoming solar radiation and high water vapor content of the atmosphere create unique conditions that are under-explored in these microclimate models. As such, evaluation of the models in tropical climates is crucial to construct a more complete understanding of how urban parks impact outdoor thermal comfort. Further, a commonly proposed intervention to urban overheating is to increase vegetation. This promotes more shade and evapotranspiration in the city; however, under hot and humid conditions, this may result in decreased outdoor thermal comfort.

In this study, we use Bishan Ang Mo Kio Park in Singapore as the case study and the testbed for the theoretical impact of increased vegetation in tropical climates. We run an ENVI-met domain and simulations of the park using previously validated idealized weather typologies for Singapore to assess performance of the model. A second set of simulations are then run with 20% more vegetation in the park to explore the impact of vegetation on outdoor thermal comfort. Results indicate that under the typically hottest conditions of the Singaporean Intermonsoon period, the model performs sufficiently well to assess the impact of increased vegetation. Under increased vegetation, the park experiences up to 3° C of air temperature cooling in small pockets of the park, though with low wind speeds, the advection of cooling is limited.

How to cite: Crank, P., Ching, G., Ho, X. T., Acero, J., and Chow, W.: Microclimate modeling in the tropics: case study of the outdoor thermal comfort impacts of increased vegetation in an urban park in Singapore, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-950, https://doi.org/10.5194/icuc12-950, 2025.

As climate change causes more frequent and intense humid heat extremes across the globe, air conditioning (AC) is quickly becoming a necessity that we rely on for cooling as well as dehumidification. However, our understanding of the climate change impact on humidity-driven demand changes is limited over large spatiotemporal scales. This puts climate-sensitive energy planning and future-conscious building design at risk. Here, we dynamically model urban buildings’ latent heat load and the associated dehumidification energy in the urban building energy model of the Community Earth System Model to quantify the AC energy demand for maintaining indoor comfort and health under climate change. We show latent heat load increases by 47% globally under climate change, but its relative contribution to total AC energy demand shows spatially diverging trends. AC energy sensitivity to temperature increases exponentially with humidity regardless of climate zones. Mildly hot and highly humid days may see unexpected demand spikes, and this effect is further amplified by climate change. The interplay of climate-driven temperature and humidity changes leads to divergent humidity-driven shifts in the building energy design space. These results have critical implications for energy planning in preparation for rapid urbanization in the Global South. Our study underscores the importance of dynamically and explicitly modeling urban climate-energy interactions for comprehensive climate risk assessment and climate-aware infrastructure planning.

How to cite: Li, X. "., Zhao, L., Luo, Z., Oleson, K., Xie, X., Sekiya, M., Varquez, A., and Cheng, Y.: Humidity-driven air-conditioning energy demand changes in global cities under climate change, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-282, https://doi.org/10.5194/icuc12-282, 2025.

Urbanization and land cover transformations significantly influence city microclimates, leading to phenomena such as the urban heat island (UHI) effect, where urban areas experience higher temperatures than their rural counterparts. This effect is particularly pronounced in tropical cities like Singapore, which are highly susceptible to climate change impacts.

This study investigates UHI's intensity and temporal dynamics at both surface (UHI_surface) and canopy (UHI_ucl) levels during Singapore's Northeast Monsoon and First Inter-Monsoon seasons. Employing a multi-method approach, we integrated Local Climate Zone (LCZ) classification, meteorological data, and remote sensing techniques. A detailed LCZ map of Singapore was developed using Geographic Information System (GIS) tools, and thermal satellite imagery was analyzed to assess spatial patterns of the UHI_surface. Additionally, data from fixed meteorological stations were utilized to quantify UHI_ucl intensity.

Our findings indicate that UHI intensity at both levels is higher during the First Inter-Monsoon compared to the Northeast Monsoon. Compact LCZs (1–3) and industrial zones (LCZs 8 and 10) consistently recorded the highest temperatures for both UHI_surface and UHI_ucl across seasons. Thermal satellite imagery revealed that during the First Inter-Monsoon, compact LCZs (1–3) were up to 3°C warmer than open LCZs (4–6) during the day and 2°C warmer at night. Moreover, green areas (LCZ A) were found to be approximately 8°C cooler than built-up LCZs during the day and 2–4°C cooler at night.

These findings confirm that tropical cities are consistently hotter than their rural surroundings and exhibit higher UHI intensity, particularly in compact and industrial zones. The study underscores the significant role of climatologists in equipping urban planners, architects, and policymakers with essential data and insights. Such collaboration is crucial for designing sustainable urban environments and implementing effective strategies to mitigate urban overheating in tropical climates, thereby enhancing resilience to climate change.

How to cite: Mandelmilch, M. E., Yik, S. K., Ho, B., Heng, S. L., and Chow, W.: Seasonal Variations of Urban Heat Island Intensity in a Tropical City Using Local Climate Zone Mapping: A Case Study of Singapore, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-562, https://doi.org/10.5194/icuc12-562, 2025.