X. Chen
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5 records found
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Assessing the role of urban blue space in summer outdoor thermal regulation in northwestern Europe
A hectometric Weather Research and Forecasting modelling on idealized urban landscape
Journal article
(2026)
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Xuan Chen, Srinidhi Gadde, Arjan Droste, Gert-Jan Steeneveld, Miriam Coenders-Gerrits, Remko Uijlenhoet
Cities in northwestern Europe face increasingly extreme summer heat under climate change, intensifying the need for effective neighbourhood-scale heat mitigation strategies. Using hectometric (100 m) idealized Weather Research and Forecasting (WRF) simulations during three extreme heat events, this study examines how urban blue space configuration, atmospheric forcing, and physical mechanisms regulate air temperature and thermal comfort (wet-bulb globe temperature index) across coastal and inland cities. We assess how surface energy fluxes interact with horizontal advection to propagate cooling beyond waterbodies, while evaluating whether WRF-Lake produces physically realistic outputs for small, shallow urban blue spaces. Our simulations show near-surface horizontal advection as the dominant cooling mechanism, mixing cooler air from blue spaces with warmer urban air. Around midday, this provides approximately 50 W⋅ m−2 cooling potential, amplified by evaporative cooling enhanced by urban-generated turbulence. Daily mean temperature reductions ranged from −0.1◦C to −0.4◦C, with peak morning effectiveness reaching −1.0◦C in coastal areas. Wind speed emerged as the primary control: moderate winds (4.7–5.8 m⋅ s−1) propagated cooling citywide, extending up to three times the city diameter downwind, whereas light winds (1.2 m⋅ s−1) limited cooling locally. Randomly distributed waterbodies created more homogeneous cooling than canal configurations. Thermal comfort analysis revealed a critical temperature–humidity trade-off. Factor analysis (R2 = 0.93) showed air temperature cooling (50.3%) is counteracted by increased relative humidity (42.3%).We identified limitations of WRF-Lake for shallow urban blue spaces. Default roughness lengths underestimate turbulence and fluxes, likely underestimating cooling and causing unrealistic water temperature increases. This underscores the need for improved parametrizations and targeted observations to advance urban hydrometeorological modelling.
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Cities in northwestern Europe face increasingly extreme summer heat under climate change, intensifying the need for effective neighbourhood-scale heat mitigation strategies. Using hectometric (100 m) idealized Weather Research and Forecasting (WRF) simulations during three extreme heat events, this study examines how urban blue space configuration, atmospheric forcing, and physical mechanisms regulate air temperature and thermal comfort (wet-bulb globe temperature index) across coastal and inland cities. We assess how surface energy fluxes interact with horizontal advection to propagate cooling beyond waterbodies, while evaluating whether WRF-Lake produces physically realistic outputs for small, shallow urban blue spaces. Our simulations show near-surface horizontal advection as the dominant cooling mechanism, mixing cooler air from blue spaces with warmer urban air. Around midday, this provides approximately 50 W⋅ m−2 cooling potential, amplified by evaporative cooling enhanced by urban-generated turbulence. Daily mean temperature reductions ranged from −0.1◦C to −0.4◦C, with peak morning effectiveness reaching −1.0◦C in coastal areas. Wind speed emerged as the primary control: moderate winds (4.7–5.8 m⋅ s−1) propagated cooling citywide, extending up to three times the city diameter downwind, whereas light winds (1.2 m⋅ s−1) limited cooling locally. Randomly distributed waterbodies created more homogeneous cooling than canal configurations. Thermal comfort analysis revealed a critical temperature–humidity trade-off. Factor analysis (R2 = 0.93) showed air temperature cooling (50.3%) is counteracted by increased relative humidity (42.3%).We identified limitations of WRF-Lake for shallow urban blue spaces. Default roughness lengths underestimate turbulence and fluxes, likely underestimating cooling and causing unrealistic water temperature increases. This underscores the need for improved parametrizations and targeted observations to advance urban hydrometeorological modelling.
Comparative analysis of microclimate simulations
Assessing the single-layer urban canopy model and ENVI-met in Hong Kong
As global warming and urban overheating continue to intensify, accurate urban microclimate modeling has become critical for sustainable urban planning. While the Single-layer urban canopy model (SLUCM, a reduced-order surface energy balance model) and ENVI-met (a computational fluid dynamic model) are the two most widely used models, a direct comparison of their performance is missing. This study aims to examine potential biases between SLUCM and ENVI-met using Hong Kong as a case study and provide guidance on model selection for different purposes. Evaluated against pedestrian-level observational data, the results show that both SLUCM and ENVI-met simulate air temperatures reasonably well, with mean absolute errors less than 1.5 °C. However, SLUCM outperforms ENVI-met in simulating relative humidity, which is partially caused by the insufficient representation of sea breeze by both models. To extrapolate SLUCM output to different heights, Monin-Obukhov similarity theory is applied. This leads to large gradient of temperature and humidity in the vertical direction, while ENVI-met simulations yield homogeneous profiles due to explicit modeling of the turbulent mixing. Findings suggest that ENVI-met suits heterogeneous neighborhoods where turbulent mixing is largely regulated by urban morphology, but its accuracy on humidity simulation needs special attention. SLUCM performs reasonably well in simulating air temperature, but it tends to yield large bias in the vertical direction. Based on the findings, we recommend development of enhanced turbulence parameterization for SLUCM, and coupling both models with mesoscale models to better account for the effect of land/sea breeze on urban microclimate in coastal cities.
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As global warming and urban overheating continue to intensify, accurate urban microclimate modeling has become critical for sustainable urban planning. While the Single-layer urban canopy model (SLUCM, a reduced-order surface energy balance model) and ENVI-met (a computational fluid dynamic model) are the two most widely used models, a direct comparison of their performance is missing. This study aims to examine potential biases between SLUCM and ENVI-met using Hong Kong as a case study and provide guidance on model selection for different purposes. Evaluated against pedestrian-level observational data, the results show that both SLUCM and ENVI-met simulate air temperatures reasonably well, with mean absolute errors less than 1.5 °C. However, SLUCM outperforms ENVI-met in simulating relative humidity, which is partially caused by the insufficient representation of sea breeze by both models. To extrapolate SLUCM output to different heights, Monin-Obukhov similarity theory is applied. This leads to large gradient of temperature and humidity in the vertical direction, while ENVI-met simulations yield homogeneous profiles due to explicit modeling of the turbulent mixing. Findings suggest that ENVI-met suits heterogeneous neighborhoods where turbulent mixing is largely regulated by urban morphology, but its accuracy on humidity simulation needs special attention. SLUCM performs reasonably well in simulating air temperature, but it tends to yield large bias in the vertical direction. Based on the findings, we recommend development of enhanced turbulence parameterization for SLUCM, and coupling both models with mesoscale models to better account for the effect of land/sea breeze on urban microclimate in coastal cities.
Temporally compound heatwaves (CHWs), two consecutive heatwaves (HWs) with an intermittent cool break between them, are projected to occur more frequently under a warming globe. However, their spatiotemporal characteristics and interaction with urban heat island (UHI) are unexplored at the continental scale. Using observational data from over 2000 ground-based stations over China, we find that CHWs constitute an increasing portion of HW hazard from 1961 to 2021. The increasing trend is especially evident when using the daily minimum temperature to define hot days, suggesting an aggravated thermal environment at night. Urban-rural contrast of CHW trends illustrates that urbanization contributes substantially to the increased frequency of CHWs in cities, especially in southern China. Results show that mean UHI intensity (UHII) tends to weaken under HW and CHW conditions, which correlates with increased pressure and reduced precipitation. During CHW events, UHII reduces during cool break due to enhanced evaporative cooling in urban areas under precipitation. The interaction between UHI and HW is subject to change with background climate, which is positive for dry regions and negative for wet regions. This study provides insights into CHW evolution over mainland China and demonstrates the need for heat mitigation strategies under climate change.
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Temporally compound heatwaves (CHWs), two consecutive heatwaves (HWs) with an intermittent cool break between them, are projected to occur more frequently under a warming globe. However, their spatiotemporal characteristics and interaction with urban heat island (UHI) are unexplored at the continental scale. Using observational data from over 2000 ground-based stations over China, we find that CHWs constitute an increasing portion of HW hazard from 1961 to 2021. The increasing trend is especially evident when using the daily minimum temperature to define hot days, suggesting an aggravated thermal environment at night. Urban-rural contrast of CHW trends illustrates that urbanization contributes substantially to the increased frequency of CHWs in cities, especially in southern China. Results show that mean UHI intensity (UHII) tends to weaken under HW and CHW conditions, which correlates with increased pressure and reduced precipitation. During CHW events, UHII reduces during cool break due to enhanced evaporative cooling in urban areas under precipitation. The interaction between UHI and HW is subject to change with background climate, which is positive for dry regions and negative for wet regions. This study provides insights into CHW evolution over mainland China and demonstrates the need for heat mitigation strategies under climate change.
Preprint
(2025)
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Xuan Chen, Job Augustijn van der Werf, Arjan Droste, Miriam Coenders-Gerrits, Remko Uijlenhoet
Urban areas, characterized by dense populations and many socio-economic activities, increasingly suffer from floods, droughts, and heat stress due to land use and climate change. Traditionally, the urban thermal environment and water resources management have been studied separately, using urban land surface models (ULSMs) and urban hydrological models (UHMs). However, as our understanding deepens and the urgency to address future climate disasters grows, it becomes clear that hydrological disasters—such as floods, droughts, severe urban thermal environments, and more frequent heat waves—are actually not isolated events but compound events. This underscores the close interaction between the water cycle and the energy balance. Consequently, the existing separation between ULSMs and UHMs creates significant obstacles to better understanding urban hydrological and meteorological processes, which is crucial for addressing the high risks posed by climate change. Defining the future direction of process-based models for hydro-meteorological predictions and assessments is essential for better managing climate disasters and evaluating response measures in densely populated urban areas. Our review focuses on three critical aspects of urban hydro-meteorological simulation: similarities, differences, and gaps among different models; existing gaps in physical process implementations; and efforts, challenges, and potential for model coupling and integration. We find that ULSMs inadequately represent water surfaces and hydraulic systems, while UHMs lack explicit surface energy balance solutions and detailed building representations. Coupled models show potential for simulating urban hydro-meteorological environments, but face challenges at regional and neighborhood scales. Our review highlights the need for interdisciplinary communication between the urban climatology and urban water management communities to enhance urban hydro-meteorological simulation models.
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Urban areas, characterized by dense populations and many socio-economic activities, increasingly suffer from floods, droughts, and heat stress due to land use and climate change. Traditionally, the urban thermal environment and water resources management have been studied separately, using urban land surface models (ULSMs) and urban hydrological models (UHMs). However, as our understanding deepens and the urgency to address future climate disasters grows, it becomes clear that hydrological disasters—such as floods, droughts, severe urban thermal environments, and more frequent heat waves—are actually not isolated events but compound events. This underscores the close interaction between the water cycle and the energy balance. Consequently, the existing separation between ULSMs and UHMs creates significant obstacles to better understanding urban hydrological and meteorological processes, which is crucial for addressing the high risks posed by climate change. Defining the future direction of process-based models for hydro-meteorological predictions and assessments is essential for better managing climate disasters and evaluating response measures in densely populated urban areas. Our review focuses on three critical aspects of urban hydro-meteorological simulation: similarities, differences, and gaps among different models; existing gaps in physical process implementations; and efforts, challenges, and potential for model coupling and integration. We find that ULSMs inadequately represent water surfaces and hydraulic systems, while UHMs lack explicit surface energy balance solutions and detailed building representations. Coupled models show potential for simulating urban hydro-meteorological environments, but face challenges at regional and neighborhood scales. Our review highlights the need for interdisciplinary communication between the urban climatology and urban water management communities to enhance urban hydro-meteorological simulation models.
Urban areas, characterized by dense populations and many socioeconomic activities, increasingly suffer from floods, droughts and heat stress due to land use and climate change. Traditionally, the urban thermal environment and water resource management have been studied separately, using urban land-surface models (ULSMs) and urban hydrological models (UHMs). However, as our understanding deepens and the urgency to address future climate disasters grows, it becomes clear that hydroclimatological extremes – such as floods, droughts, severe urban thermal environments and more frequent heat waves – are actually not always isolated events but can be compound events. This underscores the close interaction between the water cycle and the energy balance. Consequently, the existing separation between ULSMs and UHMs creates significant obstacles in better understanding urban hydrological and meteorological processes, which is crucial for addressing the high risks posed by climate change. Defining the future direction of process-based models for hydrometeorological predictions and assessments is essential for better managing extreme events and evaluating response measures in densely populated urban areas. Our review focuses on three critical aspects of urban hydrometeorological simulation: similarities, differences and gaps among different models; existing gaps in physical process implementations; and efforts, challenges and potential for model coupling and integration. We find that ULSMs inadequately represent water surfaces and hydraulic systems, while UHMs lack explicit surface energy balance solutions and detailed building representations. Coupled models show the potential for simulating urban hydrometeorological environments but face challenges at regional and neighbourhood scales. Our review highlights the need for interdisciplinary communication between the urban climatology and the urban water management communities to enhance urban hydrometeorological simulation models.
...
Urban areas, characterized by dense populations and many socioeconomic activities, increasingly suffer from floods, droughts and heat stress due to land use and climate change. Traditionally, the urban thermal environment and water resource management have been studied separately, using urban land-surface models (ULSMs) and urban hydrological models (UHMs). However, as our understanding deepens and the urgency to address future climate disasters grows, it becomes clear that hydroclimatological extremes – such as floods, droughts, severe urban thermal environments and more frequent heat waves – are actually not always isolated events but can be compound events. This underscores the close interaction between the water cycle and the energy balance. Consequently, the existing separation between ULSMs and UHMs creates significant obstacles in better understanding urban hydrological and meteorological processes, which is crucial for addressing the high risks posed by climate change. Defining the future direction of process-based models for hydrometeorological predictions and assessments is essential for better managing extreme events and evaluating response measures in densely populated urban areas. Our review focuses on three critical aspects of urban hydrometeorological simulation: similarities, differences and gaps among different models; existing gaps in physical process implementations; and efforts, challenges and potential for model coupling and integration. We find that ULSMs inadequately represent water surfaces and hydraulic systems, while UHMs lack explicit surface energy balance solutions and detailed building representations. Coupled models show the potential for simulating urban hydrometeorological environments but face challenges at regional and neighbourhood scales. Our review highlights the need for interdisciplinary communication between the urban climatology and the urban water management communities to enhance urban hydrometeorological simulation models.