Effect of urban design on microclimate and thermal comfort outdoors in warm-humid Dar es Salaam, Tanzania

Erik Johansson,Moohammed Wasim Yahia,Sofia Thorsson,Fredrik Lindberg,Maria Isabel Rasmussen

doi:10.1007/s00484-017-1380-7

Erik Johansson, Moohammed Wasim Yahia + Show 3 more

Open Access

https://doi.org/10.1007/s00484-017-1380-7

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Abstract

Due to the complexity of built environment, urban design patterns considerably affect the microclimate and outdoor thermal comfort in a given urban morphology. Variables such as building heights and orientations, spaces between buildings, plot coverage alter solar access, wind speed and direction at street level. To improve microclimate and comfort conditions urban design elements including vegetation and shading devices can be used. In warm-humid Dar es Salaam, the climate consideration in urban design has received little attention although the urban planning authorities try to develop the quality of planning and design. The main aim of this study is to investigate the relationship between urban design, urban microclimate, and outdoor comfort in four built-up areas with different morphologies including low-, medium-, and high-rise buildings. The study mainly concentrates on the warm season but a comparison with the thermal comfort conditions in the cool season is made for one of the areas. Air temperature, wind speed, mean radiant temperature (MRT), and the physiologically equivalent temperature (PET) are simulated using ENVI-met to highlight the strengths and weaknesses of the existing urban design. An analysis of the distribution of MRT in the areas showed that the area with low-rise buildings had the highest frequency of high MRTs and the lowest frequency of low MRTs. The study illustrates that areas with low-rise buildings lead to more stressful urban spaces than areas with high-rise buildings. It is also shown that the use of dense trees helps to enhance the thermal comfort conditions, i.e., reduce heat stress. However, vegetation might negatively affect the wind ventilation. Nevertheless, a sensitivity analysis shows that the provision of shade is a more efficient way to reduce PET than increases in wind speed, given the prevailing sun and wind conditions in Dar es Salaam. To mitigate heat stress in Dar es Salaam, a set of recommendations and guidelines on how to develop the existing situation from microclimate and thermal comfort perspectives is outlined. Such recommendations will help architects and urban designers to increase the quality of the outdoor environment and demonstrate the need to create better urban spaces in harmony with microclimate and thermal comfort.

Highlights

Due to the complexity of built environment, urban design patterns considerably affect the microclimate and outdoor thermal comfort in a given urban morphology
In Manzese, the variation is about 5. This can be explained by the fact that due to the low building heights Manzese receives much more short wave radiation which warms up the air at spaces between buildings more than in other areas
The maximum sky view factor (SVF) is 0.8 at Upanga and it corresponds 43.5 °C physiologically equivalent temperature (PET) (Fig. 9a). This means that an increase of 3 °C PET occurs when SVF increases from 0.4 to 0.8. This can be explained by the fact that the compact urban morphologies reduce the time of solar exposure and reduce the amount of direct solar radiation which reaches the ground surface

Summary

Introduction

Due to the complexity of built environment, urban design patterns considerably affect the microclimate and outdoor thermal comfort in a given urban morphology. There have been several studies on thermal conditions of cities in warm-humid climates in later years (e.g., Johansson and Emmanuel 2006; Lin 2009; Johansson and Yahia 2011; Ng and Cheng 2012; Yang et al 2013; Ndetto and Matzarakis 2013, 2015, 2016). These studies have shown that thermal conditions are stressful for the urban dwellers and that increased air temperatures will lead to increased heat stress and to increased energy use for air-conditioning and increased emissions of greenhouse gases. In warm and humid regions, it has been found that on average about 80% of the total cooling effect of the sites is contributed by tree shading (Yoshida et al 2000; Shashua-Bar and Hoffman 2000)

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