Our cities will feel the heat in a changing climate

Transcription

1 Greenhouse 2013 Our cities will feel the heat in a changing climate CM Ewenz & JM Bennett School of the Environment Flinders University

2 The Urban Heat Island Effect

3 Surface temperature from airborne measurements 7 March 2011

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5 The Adelaide UHI City Hotspots (Urban Forms - Sky View Factor) SVF Currie St, SVF=0.34 Flinders St, SVF=0.45

6 Change in Bldg Height Building height increase in metre

7 UHI Impact summer winter Estimated increase of temperature in ºC for clear sky at 10pm. UHI intensity from projected building height increase.

8 UHI Intensity Temperature difference CBD - parkland Seasonal and annual average difference. Sea-breeze effect in summer.

9 City & Parkland 1pm, cooling sets in CITY 4 hrs SS SR PARK

10 Street Canyons Canyon - Change with Height Temperature in C Temperature in C Canyon - Changing Width Width in m Height above Ground in m Predicted temperature at ground level in centre of two 25 m high buildings. Predicted temperature with height in centre of two 25 m high buildings. Also changes of temperature in a canyon with humidity - linear increase

11 Electricity & Weather Total electricity consumption over daytime (8am to 6pm) on weekdays for three buildings in the CBD.

12 Electricity & Weather Increase in annual electricity consumption and CO2 production for T increase of 1 C.

13 Summary Urban Forms (SVF) determine UHI intensity Lower SVF ==> Higher UHI intensity Increase of building height Stronger canyon effect ==> Increase in UHI intensity Maximum UHI intensity around sun rise Adelaide (June Nov 2012) maximum of 5.4 C Street canyons Narrower canyons ==> Temperature increase With height ==> Temperature decreases slowly Electricity and Weather Cooling demand increase with 1 C temperature increase

14 Acknowledgements The presented summary is work performed by the UHI research group of Flinders University and Adelaide University (Guan H., Bennett J.M., Vinodkumar, Benger S., Zhu S., Clay R., Sobarto V. and the presenter Ewenz C.M.) Funding and additional data came from a number of organisations in Adelaide, South Australia and Australia Many students have assisted in data collection and analysis Many more people supported this work in a number of ways.

15 Abstract Our cities will feel the heat in a changing climate Cäcilia M Ewenz and John M Bennett School of the Environment, Flinders University, Adelaide Cities are generally warmer than their rural surrounding. The Urban Heat Island phenomenon is responsible for this. The urban heat island (UHI), is a result of the high heat storage capacity of concrete and bitumen as well as the generally low albedo of urban surfaces. Vegetation in contrary stores much less energy and additionally reduces temperature due to evaporative cooling during transpiration. The urban heat island effect contributes towards increasing energy for cooling in cities, however in winter it can reduce energy consumption due to less heating demand. An increase in energy use from fossil fuel sources leads to increased GHG emissions, so any means of reducing the strength of the UHI in times of cooling demand has important energy and climate ramifications. The current state of the temperature distribution in the Adelaide Central Business District (CBD) and its surrounding parkland belt is evaluated by the deployment of 20 temperature sensors since The observations exhibit a maximum temperature difference between CBD and parkland of 6 C with an average of 1.5 C. The maximum occurs around sun rise. The UHI is more intense in winter than in summer. The effect is most pronounced during clear and calm nights and the observed temperature difference vanishes above wind speeds of 8m/s for both clear and cloudy conditions. Daytime differences sharply decrease at temperatures above 17 C,. At night, even in cases with ambient air temperature more than about 27 C, the parks are cooler by about 2 C on average. A micro-scale urban model was employed to study the individual street level variations due to surrounding building height and the effect of green spaces. The study of hot spot locations in the city with current and future temperature distributions and building height variations displays changes in the wind tunnel effect at street level, heat storage over the day and heat release from surface and buildings during night time. The change in temperature depending on the height variation is in excess of 1 C. This in connection to the projected temperature increase due to climate change has major implications to energy use and carbon emissions in the future. Green spaces clearly augment the temperature field in the vicinity. Depending on the synoptic situation UHI mitigation can be achieved by placing green spaces close to hot spot locations.