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Urban greenery especially trees has been identified as one of the most effective measures for urban climate moderation through the mechanism of evaporative cooling and shading. For subtropical high-density cities like Hong Kong, intense heat stress arises from the compact urban form, poor ventilation, and the high percentage of impervious urban cover. Thus, concerted actions are being taken to harness the cooling potential of urban greenery for improved microclimate. However, the cooling effects of urban greenery vary based on multiple factors, including background climates, urban morphology, anthropogenic heat emission, vegetation types and quantity, etc. This study aims at quantifying and comparing the cooling efficiency of different urban greenery coverage for a typical residential neighbourhood in Hong Kong. The questions guiding our research are: what is the relationship between tree coverage and ambient environment, including air temperature and thermal comfort during a typical summer day in the selected neighbourhood? To achieve this objective, we employed a previously validated ENVImet model setting of a typical summer day to conducted parametric studies of cooling benefit of variable tree coverage ratio from 0 to 30% at the interval of 2% on a typical summer day. The results verified the greenery had a higher cooling potential during period of maximum temperature than the whole diurnal time, 1 °C and almost 0.8 °C respectively for air temperature. This study also showed that increasing greenery coverage indeed lowers diurnal temperatures, but this relationship is logarithmic instead of linear. The ratio of change in cooling effects to greenery increase and then decreased gradually. Between the coverage ratio of 8%-35%, the cooling potential would be underestimated in a linear relationship. Greenery measures may lead to ineffective results if greening principles have not been comprehensive understood, such as greenery abundance, composition and types. The present study allows urban planners and managers to understand more in greenery amount for urban micro-climate in high-density cities.

The Cooling Efficiency of Urban Greenery Coverage in a High-density City