Modelling of Urban Heat Island and Quantification of Amelioration Effects of Plant Species on Microclimate and Human Thermal Comfort in Nairobi City, Kenya

Abstract

Rapid urbanisation and global warming have led to the Urban Heat Island (UHI) phenomenon in Nairobi City. The UHI phenomenon is considered the most interpretive indication of an urban climate change in the current time of increasing urbanisation. Proper climate change adaptation planning actions in Nairobi city require recognizing the possible range of UHI intensity and spatial dispersion. However, climate research and its applications in urban planning and improvement in Nairobi city is constrained by inadequate evaluation and irregular descriptions of the local city's characteristics. Therefore, there is a need to study the correlation between urbanisation and UHI and quantify the benefits of plant species on microclimate and human thermal comfort within Nairobi city. The study models and simulates the UHI of Nairobi and further quantifies environmental amelioration effect of different tree species on microclimate and human thermal comfort in the city. The city was first classified into Local Climate Zones (LCZs), then estimated the land surface temperature (LSTs) distribution using a split-window algorithm, and simulated 2m air temperature by MUKLIMO_3 model to show UHI phenomenon for Nairobi city. After that, the attenuation effects of selected plant species on microclimate and human thermal comfort were quantified. The study generated the current LCZ classification for Nairobi city. Both built-up (LCZ 1-10) and natural (LCZ A-G) areas are present. The LCZ classification captured the forms and functions of each zone and indicated the potential UHI distribution pattern of Nairobi city. The study confirmed the presence of urban heat loads in built-up areas with a high percentage of water-resistant, non-reflective surfaces and low vegetation compared to the surrounding rural areas. Statistical analysis of LSTs showed significant differences among typical LCZs. Additionally, the amelioration effects of selected individual plant species demonstrated differences in performance. Ficus benjamina (vast dense canopy) presented the highest ability to attenuate environmental parameters (surface temperature, ambient temperature globe temperature, and relative humidity) and reduce thermal discomfort index, followed by Cassia spectabilis, Warburgia ugandensis, Ficus religiosa, Callistemon citrinus followed by Dypsis decaryi, Bambusa vulgaris, Terminalia mantaly and Schinus molle (small open canopy). In conclusion, the study presented the LCZ Nairobi as a basic spatial unit for synoptic characterization and comprehensive climate-based classification of the city sites for urban planning and management. The spatial pattern of urban heat loads in Nairobi is influenced by local climate formation as differentiated by urban form (urban morphological parameters) and functions. In addition, the attenuation effects of plant species in urban spaces vary based on their allometric properties and planting designs. Therefore, proposals for urban planning and design strategies to manage urban heat in Nairobi City can optimise the form and layout of urban LCZs to enhance ventilation and promote appropriate building materials increasing the use of green and blue urban infrastructure. The study recommended further research on spatial configurations of planting designs with more cooling and fewer energy demands and their application to the whole city.