Intra Urban heat Island assessment and heat stress implications in a tropical humid city, Lagos Nigeria: A modelling framework in the local context

The impacts of global climate change and global warming are most prominent in tropical African cities due to high levels of vulnerability. Concurrently, sub-Saharan Africa is undergoing unprecedented urbanization, posing imminent socio-economic and environmental challenges. Notably, since the 1970s oil boom, Lagos city has experienced rapid population growth, significantly transforming its landscape. This transformation has led to the emergence of extreme urban heat as a critical issue linked to urban expansion and landscape changes, posing a substantial threat to human life. Addressing this challenge is complicated by the diverse urban landscape and varying vulnerability levels in the African urban context, necessitating a more comprehensive and high-resolution analysis of heat risk patterns within cities. This study aims to map the intra-urban pattern of heat stress and heat risk in Lagos, Nigeria, serving as a case study for other sub-Saharan African cities.
The project proposes and implements a framework utilizing advanced high-resolution numerical modeling and open-source geospatial data to assess and map the patterns of heat risk. The urbanized Weather Research and Forecasting (uWRF) model is employed to simulate the urban heat island (UHI) and Humidex-based heat stress during a specific heatwave event in March 2020. Open-source high-resolution geospatial datasets were used to assess heat exposure and vulnerability. The urban areas were classified based on the Local Climate Zone (LCZ) scheme, and the LCZ urban canopy parametrization scheme was used as model input upon extensive validation with fieldwork-derived urban canopy data. Model evaluations were conducted by comparing simulated data with observations from standard meteorological data across distinct urban landscapes and satellite data (MODIS). Spatial analysis techniques, including Moran’s I test and Optimized Hot Spot Analysis (OHSA), were used to identify spatial clustering patterns and hot spots of heat risk areas.
The results revealed that the LCZ-based uWRF effectively replicates the spatial and diurnal patterns of basic meteorological variables essential for analyzing the UHI and heat stress at a high resolution. The intra-urban analysis of Humidex-based heat stress showed a generally high incidence of intense discomfort in highly urbanized areas and noted the significant influence of urban morphology on the pattern of heat stress, particularly at night due to the combined effect of urban warming and higher relative humidity. The most socioeconomically disadvantaged urban areas, LCZ 7, were most affected, with ‘hot’ heat stress conditions observed over 90% of the time. Moreover, combining this with socioeconomic and exposure data in heat risk mapping using Gi* statistics in OHSA and mapping the hot spot areas at the highest confidence level of 99% identified Critical Heat Risk Zones (CHRZ), covering an area of approximately 423 km2.
Generally, high-density and informal urban areas exhibited the highest UHI and heat stress, while high heat risk areas were predominantly associated with informal urban areas in low compact urban areas (LCZ 3), open low rise (LCZ 6), and high dense (LCZ 7). The proposed heat risk assessment framework is designed to be transferable to other global South cities, especially in Africa, where key data sparsity often impedes such assessments.