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Study Notes
Urban Heat Island Effect and Interventions
- The Urban Heat Island (UHI) effect increases urban temperatures, leading to higher mortality and morbidity rates, especially during heatwaves.
- To mitigate this, various urban interventions have been proposed, including passive (e.g., green roofs, cool roofs) and active strategies (e.g., air conditioning).
Study Methodology
- The study used the Weather Research Forecast (WRF) model to simulate the impact of 9 urban interventions on air temperature in the Greater London Authority (GLA) area during two hot summer days in 2018.
- The interventions were applied to different scenarios, including:
- 100% deployment of cool roofs, green roofs, solar panels, and air conditioning
- Practicable deployment of green roofs and solar panels in existing buildings
- Deployment of cool roofs in specific areas (e.g., compact mid-rise, open low-rise, and large low-rise areas)
Results
- Cool roofs were found to be the most effective intervention, reducing temperatures by ~1.2°C on average and up to 2.0°C in certain locations.
- Green roofs had a minor impact, reducing temperatures by ~0.3°C on average, while solar panels reduced temperatures by ~0.5°C.
- Air conditioning increased temperatures by ~0.15°C on average, although it could be used to power air conditioning systems.
- The study also found that the impact of interventions varied depending on the location and type of urban area.
Surface Energy Balance
- The study analyzed the impact of interventions on the surface energy balance, including:
- Albedo: Cool roofs increased albedo by ~0.3, reducing incoming net solar shortwave radiation by ~50 W/m².
- Sensible heat fluxes: Cool roofs reduced sensible heat fluxes by ~30 W/m², while air conditioning increased them by ~5 W/m².
- Latent heat fluxes: Green roofs increased latent heat fluxes by ~12 W/m², although this was offset by an increase in sensible heat fluxes.
Discussion and Implications
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The study highlights the complex interactions between local cooling capacity and the total cooling load that can be brought to a city by each intervention.
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The results suggest that cool roofs are the most effective intervention, but practicable deployment of green roofs and solar panels may not be as effective in reducing temperatures.
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The study's findings can inform urban planning and policy decisions aimed at mitigating the Urban Heat Island effect and improving public health.### Urban Climate Interventions
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Cool Roofs: can reduce outdoor temperature by up to 3.2°C, with peak temperature reduction in the late afternoon and evening; effectiveness depends on the area of application, with larger impact when applied to central mid-rises in London.
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Green Roofs: can reduce temperature, but only during the night, and have mixed effects during the day due to increased sensible heat emission; sedum vegetation is the most common type of green roof in London.
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Rooftop Solar PV: can reduce temperature, but only by a small amount (∼0.3°C), and is dependent on the building characteristics and solar radiation; can also generate electricity, with a peak production of 15.15 MW during high solar incidence.
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Changing Vegetation: changing natural spaces to deciduous trees can reduce outdoor temperature, but only at night, and has mixed effects during the day due to increased latent heat emission.
Urban Heat Island Mitigation Strategies
- Widespread Adoption of Cool Roofs: can be an efficient way to reduce urban temperatures and adverse health impacts during hot spells.
- Rooftop Solar PV: can provide a small amount of cooling as an additional benefit to power production.
- Green Roofs: have other environmental benefits, but appear less effective in reducing temperatures.
- Air Conditioning: will lead to increased outdoor temperatures, despite improved thermal comfort indoors.
Limitations and Future Research Directions
- Model Uncertainty: results are dependent on the urban climate model used and may vary depending on the model's dynamics and physics.
- Short Simulation Period: results are based on a short simulation period and may not be generalizable to other cities or seasonal impacts.
- Single-Type Application: results assume a single type of application per intervention and do not explore the benefits of more recent technological advancements.
- Local Climate Effects: results may not capture local climate effects dependent on weather conditions during the short period of study.
Future Research Directions
- Estimate Impacts on Longer Time Periods: estimate the impact of each intervention on longer time periods (e.g., monthly or seasonal).
- Perform Multi-Model Simulations: perform multi-model simulations to better quantify the uncertainty around the impact of each intervention.
- Investigate Complex Interactions: investigate the complex interactions between local and city-scale impacts of tailored interventions.
- Estimate Costs: estimate the costs related to the deployment of each intervention to perform cost-benefit analyses.
- Foster Urban Scale SEB Monitoring Systems: foster the deployment of urban scale SEB monitoring systems to validate model simulations.
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Description
Learn about the Urban Heat Island effect, its impact on urban temperatures and health, and explore various passive and active interventions to mitigate its effects.
