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Urban Form and Microclimate: Impacts on outdoor thermal comfort and building energy performance in a high-density, tropical context- In Focus: the research of PhD student Tania Sharmin

The unbridled urbanisation in many tropical cities has eradicated the green cover and intensified the vulnerability to climate change. Furthermore, declining air quality caused by the exhausts from traffic and industry and the generation of urban heat islands (UHIs) resulting from the unplanned growth of the built environment have worsened the microclimatic conditions in tropical cities. The fact that almost half of the world’s population lives in the tropics indicates that improving the microclimatic conditions will have a significant impact on the life of the people living there. Ironically, most of the countries in this climate are developing countries and consequently, it is not easy to apply contemporary, high-tech urban design and building design techniques in this context. Therefore, the implications of thermal stress on health and productivity needs to be tackled largely by proper urban and building design details that are affordable. This allows urban planners and designers to play an important role in devising strategies that are balanced with the available resources and are relevant to the local context.

In this situation, passive means for tackling the microclimatic degradation through the modification of urban form is gaining attention among researchers. The current research on urban form and microclimate mainly focuses on temperate climates in western countries. A limited number of studies have focused on tropical climates and even fewer examples exist in developing countries in the tropics. The urban context and settlement patterns in the

developing country megacities are substantially different from those found in the Western (usually colder) countries and therefore the strategies applied to improve their urban microclimate are not necessarily appropriate for use in these cities. To address this gap in knowledge, this study primarily focuses on investigating the impact of urban form and microclimate on outdoor thermal comfort conditions and building energy performances in the tropical megacity Dhaka.

The research unfolds an investigation of the built environment from outdoor to indoor conditions as a narrative of inter-related episodes. The understanding from one experiment is carried forward to the next one. It investigates the challenges related to the microclimatic analysis of urban geometry characteristics, CFD (Computational Fluid Dynamics) simulations of theoretical and real-world case studies, statistical analysis of thermal comfort surveys and building energy performance evaluation of parametric and actual examples. The multidisciplinary approach in this research attempts to link urban design and planning with urban climatology and many other relevant fields of expertise that are essential for making liveable and sustainable cities.

The research identifies urban geometry as one of the most important factors governing outdoor microclimatic conditions in a high-density, tropical climate. Due to rapid urbanisation and limited resources for managing the urban planning, many parts of the case-study city Dhaka have grown spontaneously. These traditional areas, mainly residential, with irregular plot sizes and building heights, were found to have relatively better microclimatic conditions than the contemporary, formal built areas with more uniform building heights and sizes. Significant variation is identified in terms of air temperature, mean radiant temperature and wind speed between the two areas. The research identifies the need to appreciate and analyse the diversity as evident in these less orderly environments. These findings can have important implications for future urban planning in a high-density, tropical context, especially because existing urban planning strategies for such a climate call for a dispersed arrangement that contradicts with the increasing population densities in the tropical cities.

At the next stage, the study investigates how urban form and its microclimatic conditions shape pedestrian thermal comfort responses. Pedestrians in the more diverse traditional areas are found to be more thermally comfortable than those in the formal areas. This suggests that the urban form that governs microclimatic conditions in a city subsequently affects the people who use the urban spaces. Finally, the study focuses on the indoor conditions of the surrounding buildings. It was found that alongside fostering the public realm, the microclimate inside urban canyons can play an important role in lessening overheating and energy demand of the flanking buildings.

In summary, one of the more significant findings to emerge from this study is that the arrangement of urban form and, in particular, its diversity, can be advantageous for improving urban microclimate in a tropical, high-density climate. The same component is found essential for enhancing outdoor thermal comfort conditions in urban spaces as well as reducing overheating and cooling energy demand in the flanking buildings. Apart from answering the core research questions, the study has explored various innovative and original analysis techniques and methods from which several additional findings have emerged. These include understanding the process of applying CFD microclimatic simulation and building energy simulation in a tropical context, devising PET (Physiological Equivalent Temperature) ranges and formulating predictive thermal comfort models for a tropical climate. In order to disseminate the new urban microclimate knowledge learnt from this study, some guidelines have been proposed. The findings from this study may provide useful insights for planners, designers, architects, and engineers in developing a climate-responsive city and shed light on outdoor thermal comfort and energy issues in high-density, tropical cities.

Figure. Traditional urban pattern in Dhaka

Figure. Typical example of a traditional area