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# Mapping effects of heatwaves on urban systems: a comparison Greater London -Métropole du Grand Paris

Romain Courault, Marianne Cohen, Laurence Eymard, Serge Muller

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Romain Courault, Marianne Cohen, Laurence Eymard, Serge Muller. Mapping effects of heatwaves on urban systems: a comparison Greater London -Métropole du Grand Paris. 7ème édition du colloque "Climat et Impacts", Nov 2022, Gif-sur-Yvette, France. . hal-03874792

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Submitted on 28 Nov 2022

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Distributed under a Creative Commons Attribution 4.0 International License
# Mapping effects of heatwaves on urban systems: a comparison
## Greater London – Métropole du Grand Paris

¹UMR 7533 LADYSS, Univ Paris I, * [romain.courault@univ-paris1.fr](mailto:romain.courault@univ-paris1.fr), ²EA Médiations, Univ Paris IV, ³UMR 8190 LATMOS, Univ Paris VI, ⁴UMR 7205 ISYEB, MNHN

Romain COURAULT¹*, Marianne COHEN¹,², Laurence EYMARD³, Serge MÜLLER⁴

### I. Context & objectives

Global warming drives **recurrent anomalies of atmospheric circulation** (fig. 1) at mid-latitude and gives **more frequent and intense heatwaves** in European region and major cities.

It has severe consequences on **human wellbeing** and **urban ecosystems**, especially during **extremely hot nights**.

Urban **morphology** and **land composition** drive the behavior of **heat urban islands effects** (figure 3), more generally the capacity of urban systems to discharge excess of accumulated heat during night.

Moreover, a few studies were carried to examine links between heatwaves' land surface temperatures, land cover features, and socio-economic attributes of neighborhoods of European major cities.

We did a geographical comparison between Greater London and Métropole du Grand Paris (figure 3-4), to enquire those questionings.

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### II. Material & Methods

1) **Selection of extremely hot nights** using thresholds given by London and Paris authorities to define heatwave alerts, based on 2002-2022 temperature time series (Heathrow Airport & Paris-Montsouris meteorological station, source: MetOffice & Météo-France)

2) **Mean of selected nights for emissivity/Land Surface Temperature** (LST, MODIS 11A1, 2002-2022, source: LPDAAC-USGS)

3) **Geographical aggregation of LSTs by metropolitan neighborhoods** (geostatistical units: wards/IRIS) for :

- Urban land cover (2018, source: Copernicus-EEA)

- Median household incomes (sources: London Data Store & INSEE) and

4) **Geostatistical tests** : Pearson & Spearman correlations; ANOVA

**Figure 1** (upper-left): Reanalysis of 850m geopotential-height anomaly for JJA 2022 compared to 1991-2020 normal, for northern hemisphere (source: Climate Change Institute); **Figure 2** (upper-right): graphical abstract of heat urban island (real. R. Courault, 2021); **Figure 3**: Urban land cover map in 2018 for Greater London; **Figure 4**: Urban land cover map in 2018 for Métropole du Grand Paris. Legend colors are reported for land cover categories of Table 1 (source: Copernicus & European Environmental Agency; real. R. Courault 2022)

### III. Results

1) & 2) Selection criteria used to define heatwave, as given by British and French health & meteorological institutes, gave too little days/nights to sample MODIS LST images. We then based our selection on T°C thresholds overrun, excluding length of episodes.

3) On a regional basis, night LSTs are cooler for London compared to Paris (total average, respectively: 18,7 and 24,6°C), due to overall fresher climate, presence of the tidal river Thames and different distribution of land cover types (fig. 3-4 and table 1).

Averaged to land covers, **Continuous urban fabric** are the hottest land cover found for both cities (Paris: 24,9°C; London: 19,6°C). For Paris, Port areas and Water are the second and third hottest; while in London it is **Discontinuous dense urban fabric and Railways and associated land** (table 1).

Freshest land cover are found both for **Permanent crops** (vineyard, fruit trees etc), **Isolated structures** and **Arable lands** (annual crops). Those results need to be weighted according to land cover proportion and distribution, which vary significantly between London and Paris (visual comparison on figures 3 & 4).

4) **Pearson and Spearman correlations** on LST 'explained' by median household income did not show significant results for London. For Paris, weak but existing signals are found, with respectively R=0,2 and R=0,19 (p<0,0001).

ANOVA showed heterogenous results for differences inter-land covers and LST°C. For both city models showed significant results, with better performances for London.

<table><caption>**Table 1:** Night MODIS 11A1 Land Surface Temperatures averaged by urban land cover type, at the extent of Greater London and Métropole du Grand Paris. Green to red colors for LST depicts distribution between hottest and freshest land cover for night LST during heatwaves (Real. R. Courault, 2022)</caption><thead><tr><th>Land cover - Urban Atlas (2018)</th><th>Night LST (°C) - Grand Paris</th><th>Night LST (°C) - Greater London</th></tr></thead><tbody><tr><td>Airports</td><td>24,0</td><td>18,3</td></tr><tr><td>Arable land (annual crops)</td><td>22,5</td><td>16,6</td></tr><tr><td>Construction sites</td><td>24,6</td><td>18,8</td></tr><tr><td>Continuous urban fabric (S.L. : > 80%)</td><td>24,9</td><td>19,6</td></tr><tr><td>Discontinuous dense urban fabric (S.L. : 50% - 80%)</td><td>24,2</td><td>19,0</td></tr><tr><td>Discontinuous low density urban fabric (S.L. : 10% - 30%)</td><td>23,3</td><td>17,5</td></tr><tr><td>Discontinuous medium density urban fabric (S.L. : 30% - 50%)</td><td>23,5</td><td>18,1</td></tr><tr><td>Discontinuous very low density urban fabric (S.L. : < 10%)</td><td>23,9</td><td>17,3</td></tr><tr><td>Fast transit roads and associated land</td><td>24,5</td><td>17,6</td></tr><tr><td>Forests</td><td>22,5</td><td>16,3</td></tr><tr><td>Green urban areas</td><td>24,5</td><td>18,7</td></tr><tr><td>Herbaceous vegetation associations (natural grassland, moors...)</td><td>22,3</td><td>16,2</td></tr><tr><td>Industrial, commercial, public, military and private units</td><td>24,6</td><td>18,6</td></tr><tr><td>Isolated structures</td><td>22,2</td><td>16,1</td></tr><tr><td>Land without current use</td><td>24,2</td><td>18,6</td></tr><tr><td>Mineral extraction and dump sites</td><td>23,8</td><td>17,3</td></tr><tr><td>Open spaces with little or no vegetation (beaches, dunes, bare rocks, glaciers)</td><td>NA</td><td>17,5</td></tr><tr><td>Other roads and associated land</td><td>24,6</td><td>18,9</td></tr><tr><td>Pastures</td><td>23,2</td><td>16,7</td></tr><tr><td>Permanent crops (vineyards, fruit trees, olive groves)</td><td>21,2</td><td>16,0</td></tr><tr><td>Port areas</td><td>24,8</td><td>18,4</td></tr><tr><td>Railways and associated land</td><td>24,6</td><td>18,9</td></tr><tr><td>Sports and leisure facilities</td><td>24,4</td><td>18,3</td></tr><tr><td>Water</td><td>24,7</td><td>18,7</td></tr><tr><td>Wetlands</td><td>NA</td><td>17,4</td></tr><tr><td>Total average</td><td>24,6</td><td>18,7</td></tr></tbody></table>