Etude longitudinale des Ilots de Chaleurs Urbains (ICU) dans le contexte de changement climatique : facteurs spatiaux, bilan énergétique de surface et implications pour la santé publique
Abstract
Les îlots de chaleur urbains (ICU) constituent l'effet microclimatique de l'urbanisation le plus étudié, alors que les villes d'Afrique tropicale restent peu représentées dans la littérature scientifique. Cette étude examine la distribution spatiale et les variations diurnes/nocturnes des IUB à Brazzaville (République du Congo) à partir d'une analyse par télédétection multi-indicateurs (température de surface terrestre, indice de végétation par différence normalisée, indice de surface bâtie par différence normalisée, albédo, vitesse du vent, densité de l'air). Les résultats révèlent une distribution spatiale contrastée des IUB : pendant la journée, l’intensité maximale (8,7 °C) se situe au nord, dans des quartiers informels à faible inertie thermique ; pendant la nuit, le centre des IUB se déplace de 7,2 km vers le sud (quartier historique central à forte densité, 2,6 °C), tandis que la partie nord se transforme en îlot de fraîcheur (−2,1 °C). Les prédicteurs classiques (NDVI, NDBI, albédo) expliquent 74 % de la variance pendant la journée, mais moins de 4 % de la variance pendant la nuit. Le processus d’UHI nocturne dépend de la stabilité de la couche limite inférieure. Ces résultats remettent en question l'applicabilité des modèles basés sur un climat tempéré et soulignent l'important enjeu sanitaire lié à l’exposition de la quasi-totalité de la population à un stress thermique maximal pendant les heures de sommeil.
Urban heat islands (UHIs) are the most widely studied microclimatic effect of urbanization, yet cities in tropical Africa remain underrepresented in the scientific literature. This study examines the spatial distribution and diurnal/nocturnal variations of UHIs in Brazzaville (Republic of the Congo) using a multi-indicator remote sensing analysis (land surface temperature, normalized difference vegetation index, normalized difference built-up area index, albedo, wind speed, and air density). The results reveal a contrasting spatial distribution of UHI: during the day, the maximum intensity (8.7 °C) is located in the north, in informal neighborhoods with low thermal inertia; at night, the center of the UHI shifts 7.2 km southward (to the high-density historic downtown area, 2.6 °C), while the northern part becomes a cool island (−2.1 °C). Conventional predictors (NDVI, NDBI, albedo) account for 74% of the variance during the day, but less than 4% of the variance at night. The nocturnal UHI process depends on the stability of the lower boundary layer. These results call into question the applicability of models based on a temperate climate and highlight the significant public health concern associated with the exposure of nearly the entire population to maximum heat stress during sleeping hours.
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