Urban Climatic Analysis for Heat Stress Mitigation in French Cities: A Case Study for Toulouse
Refereed conference paper presented and published in conference proceedings

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AbstractThe adverse impacts of urban heat islands (UHIs) created by the compact built environment in French cities have been particularly evident during the 2003 heatwave (Cadot and Spria, 2006). There was a call for heatwave risk prevention to be put into action through climate-informed town planning and local mitigation strategies (Poumadère et al., 2005). Therefore, the French national MApUCE project aims to fill this gap, using urban climatic map (UCMap) as an information and evaluation tool to give a spatial representation of local climate characteristics and their corresponding planning recommendations. Being the fourth largest city agglomeration in France, Toulouse Metropolitan Region has been chosen for case study due to its strong urban sprawl (Pumain, 2004) and higher exposure to warming due to its southern and continental location (Mithieux, 2015).
This study aims to develop an UCMap for Toulouse Metropolitan Region based on local climate zone (LCZ) classifications (Stewart and Oke, 2012) and formulate corresponding climatic-sensitive design strategies for heat stress mitigation. First, information on the geography and building morphology, as well as the Toulouse LCZ map (Hidalgo et al., submitted manuscript) are extracted from the MApUCE urban database (Bocher et al., 2018). Secondly, simulated hourly meteorological data, including air and radiant temperatures, at 2m above ground and a horizontal resolution of 250m from the MesoNH-SURFEX model are used for the climatic analysis. To provide a multi-scenario understanding of the thermal conditions, the typical and extreme summer conditions are then selected as 10 warm and dry days with light northwesterly winds (Hidalgo et al., 2014) in July 2004 and 12 days during the heatwave in August 2003, respectively. These data are further averaged over three selected time periods to capture features of the dynamic thermal environment, i.e. early afternoon (13-16h) when radiant temperatures are the highest, late afternoon (17-20h) when air temperatures are the highest, and nighttime (03-06h). Heat stress intensities are reflected by the calculated universal thermal climate index (UTCI).
Under typical summer conditions, results show strong heat stress (UTCI>32°C) in the city centre and the western side of River Garonne in the early afternoon due to high exposure to intense solar heat in areas dominated by compact low-rise buildings (LCZ 3). At night, a pronounced UHI effect of up to 3°C between the city centre and its outskirts is observed. During heat wave conditions, very strong heat stress (UTCI>38°C) is found over the whole Metropolitan Region. Strong heat stress remains until late afternoon, except for areas with trees (LCZ A,B) and water (LCZ G), due to prolonged radiation in the day and slow heat release from building materials. Persistently high air temperatures also led to heat accumulation in the densely built urban areas (LCZ 1-4, 8), resulting in temperatures up to 28˚C at night. Finally, areas are reclassified from 14 LCZs into seven climatopes, namely “Compact Settings” (LCZ 1-3), “Open Settings” (LCZ 4-6), “Large Low-rise” (LCZ 8,10), “Sparsely Built” (LCZ 9), “Trees” (LCZ A,B), “Grass/Shrubs” (LCZ D,E) and “Water Bodies” (LCZ G), based on their dynamic thermal characteristics (see attached figure).
This study presents a pioneering approach for developing the UCMap for Toulouse, which could serve as an exemplary reference for city planners and policy makers of other French cities to undertake climatic-sensitive planning and formulate heat stress mitigation strategies. The multi-scenario and dynamic thermal environment analysis would also be helpful for increasing the preparedness and resilience of Toulouse in face of future warming trends and more frequent heat wave occurrences.
Acceptance Date23/02/2018
All Author(s) ListKevin Ka-Lun LAU, Julia HIDALGO, Chao REN, Robert SCHOETTER, Yu Ting KWOK, Valéry MASSON, Xuyi ZHANG, Xinwei LI
Name of ConferenceThe 10th International Conference on Urban Climate
Start Date of Conference06/08/2018
End Date of Conference10/08/2018
Place of ConferenceNew York
Country/Region of ConferenceUnited States of America
LanguagesEnglish-United Kingdom

Last updated on 2020-12-05 at 16:18