Building setback with tree planting creates comfortable environments.
Integrated and balanced design approaches are suggested in urban renewal.
"It is clear that the current incentive schemes allowing a fivefold increase in gross floor area will encourage the setback of buildings from the street. However, this incentive will inevitably also increase both building volumes and development density. Relatedly, the findings of this study highlight the value of employing a balanced approach to enable sustainable urban development. Indeed, an appropriate and moderate level of building setback alongside other planning and design strategies, such as urban tree planting, is beneficial for creating a comfortable, green environment for pedestrians, as well as maintaining urban density control. These factors all represent essential aspects that must be considered during the urban renewal process in Hong Kong and similar cities worldwide."
Tan, Z., Wang, A., Morakinyo, T. E., Yung, E. H., & Chan, E. H. (2022). Assessing the mitigation performance of building setback from street and the combination with roadside tree planting. Building and Environment, 212, 108814.
Similar or different for the thermal-radiant performance of three typical green infrastructure typologies? Measuring green roof, green wall, and ground tree within one site.
Stationary and mobile measurements were taken for six points (three greenery and three bare points); two typical measuring methods, i.e., the globe thermometer and the six-directional methods, were employed to collect irradiant variables.
"The important findings can be drawn as below in three aspects:
For GI typologies, the ground tree showed the highest reduction and the lowest fluctuations in thermal-irradiant features, followed by the green roof and the green wall. Shading effects from the surrounding building had impacts on the thermal-irradiant performance of GI typologies.
For temporal periods, the morning session (9:30–12:00) had the highest AT reduction, RH increment, and MRT variations among the three GI typologies, followed by the noon (12:01–15:00) and afternoon periods (15:01–17:25).
For two MRT measuring methods, the six-directional method performed higher sensitivity towards the radiation variations, and therefore had higher ranges and different responses on three GI typologies; the globe thermometer tended to overestimate MRT with low global solar radiation (GSR) and underestimate MRT with high GSR."
Ouyang, W., Morakinyo, T. E., Ren, C., Liu, S., & Ng, E. (2021). Thermal-irradiant performance of green infrastructure typologies: Field measurement study in a subtropical climate city. Science of The Total Environment, 764, 144635.
A thermo-cardiovascular regulation model developed to predict the thermal physiological response.
Thermal physiological response considering the body temperature, blood pressure, and blood flow in the bathing thermal environments.
Liu, H., Ogura, D., Hokoi, S., & Iba, C. (2023). Study on physiological response considering blood flow volume in transient and non-uniform bathing thermal environment using thermo-cardiovascular regulation model. Building and Environment, 228, 109820.
The thermal effects of urban trees are related to sky view factor.
Site-level planning should consider urban morphology, subtropical sun angle and areal wind direction. Impact of urban morphology on tree effects is more evident in cloudy conditions.
"Tree planting is important, and it has long been believed that “the more we plant, the better”. This study showed that in subtropical high-density cities, where high green coverage ratios are difficult to achieve, “where to plant” is crucial in terms of providing comfort to pedestrians. The current study has demonstrated that planting trees in urban areas with low SVFs will create a more diverse thermal environment with several comfortable green spots here and there, allowing people to restore the thermal comfort level in such intermittent green spaces. Given a background of increasing urban density and climate change, the findings of the study offer a solution to enhance pedestrian comfort with tree planting in highly developed districts in the subtropics. Planners are also advised to combine the morphology-based planning method with site-specific design strategies to offer more comfortable green spaces in the city."
Tan, Z., Lau, K. K. L., & Ng, E. (2017). Planning strategies for roadside tree planting and outdoor comfort enhancement in subtropical high-density urban areas. Building and Environment, 120, 93-109.
How to derive a more accurate mean radiant temperature estimation based on global temperature? Any differences between open space, street canyon, or under tree shading?
ISO formula shows inefficacies in estimating the mean radiant temperature in subtropical climate background.
The globe diameter unit should be m instead of mm!!!
"The findings of this study can be summarized in the following aspects:
• For the performance of different methods, the ISO standard method and recalibrated methods in different cities should be carefully used in the open space in a subtropical climate. Especially for the accuracy requirement at ±2 °C, it is suggested to take a local recalibration for the convection coefficient instead of directly using the formula in the literature.
• For different globe thermometers, Kestrel and Grey globe are more recommended in the outdoor environment in the subtropical climate background, as they presented higher fits, lower errors and biases in contrast to the benchmark. The black globe can be applied when the accuracy requirement is ±5 °C.
• For analysis time intervals, 1min interval is not recommended when estimating MRT based on Tg in the outdoor environment, as the varying wind and solar conditions may bring many uncertainties to the results. This study suggests applying 10min as the analysis interval, 5min is also appropriate when the wind and solar conditions are stable.
• The recalibrated coefficients for the subtropical climate was hc = 0.678 ∗ 10^8 ∗ Va^0.019 / D^0.4"
Ouyang, W., Liu, Z., Lau, K., Shi, Y., & Ng, E. (2022). Comparing different recalibrated methods for estimating mean radiant temperature in outdoor environment. Building and Environment, 109004.
Combining trees with wind path doubles cooling in temperature and
Urban trees design as daytime urban heat island mitigation approach was tested in regular block array and irregular distribution geometries.
"In this study, optimized tree planning strategies for daytime UHI mitigation in high-density subtropical cities are investigated. Morphology-based planning approaches for tree planting are evaluated in two climate-sensitive waterfront areas of Hong Kong. The study demonstrates that small parks in the high SVF areas with highly localized tree planting significantly cools down the air temperature and mitigates daytime UHI effect in the urban centre. On the other hand, mid-size green space and small green fraction under medium and low SVF reduce the radiation load in the urban environment of low-latitude cities during the early-afternoon period. And a relatively comfortable mean radiant temperature is provided in the heavily built areas. For high-density residential blocks with low green space provision, it is recommended to plan the trees in the areal wind paths to enhance the cooling benefits in neighbourhoods."
Tan, Z., Lau, K. K. L., & Ng, E. (2016). Urban tree design approaches for mitigating daytime urban heat island effects in a high-density urban environment. Energy and Buildings, 114, 265-274.
ENVI-met, a widely used microclimate simulation tool, was systematically evaluated based on its recent updates.
What is its performance regarding different thermal-radiative indicators?
This study emphasized proper validation for ENVI-met before applications, when full forcing and localized settings are essential. The strengths and limitations of ENVI-met were discussed and implications were provided for model developers and users.
“Sensitivity analyses were conducted for the inputs and settings of ENVI-met, including new radiation calculation IVS scheme (on and off), meteorological conditions (simple, cloud and radiation forcing), construction surface properties and plant parameters (default and localized settings), and output intervals (10min, 30min, 1 h). Model evaluation was conducted for three GI typologies, nine thermal-radiative parameters, and three output intervals.
The results showed that 1) recent updates of ENVI-met can improve the estimation accuracy, especially with IVS on, radiation forcing, and localized materials settings; 2) ENVI-met was capable of simulating the thermal-radiative performance of three GI typologies simultaneously; 3) mobile measurement can be used for ENVI-met validation, and 4) model evaluation results were sensitive to the metrics.
Ouyang, W., Sinsel, T., Simon, H., Morakinyo, T. E., Liu, H., & Ng, E. (2022). Evaluating the thermal-radiative performance of ENVI-met model for green infrastructure typologies: Experience from a subtropical climate. Building and Environment, 108427.
Microclimate and thermal comfort showed different correlations in
different urban settings.
Subjective perceptions are moderated by seemingly unrelated parameters, such as the acoustic environment, air quality and appreciation of urban landscape.
"Perceived environmental quality including air quality, acoustic environment, site accessibility and aesthetic quality showed significant, positive moderating effects on the associations
between microclimate variables and subjective assessment thermal comfort. Positive perception of environmental quality of the mentioned aspects strengthens the contribution of
microclimate variables to outdoor thermal comfort or subjective assessment on thermal environment, i.e. high sensitivity to microclimate determinant was shown.
Such finding provides further evidence on the complementary interaction between physical environmental and psychological effects in regards of outdoor thermal comfort"
Tan, Z., Chung, S. C., Roberts, A. C., & Lau, K. K. L. (2019). Design for climate resilience: influence of environmental conditions on thermal sensation in subtropical high-density cities. Architectural Science Review, 62(1), 3-13.
The more greening, the cooler for our built environment? Not always true! The cooling effects of greenery is non-linearly related to tree coverage ratio.
When TCR reached 20–30%, the optimal cooling efficiency of trees was achieved, irrespective of building densities and temporal periods.
"This study investigated the cooling efficiency and compared two regression patterns (both linear and non-linear) at a neighborhood scale. Parametric studies were conducted in a validated ENVI-met model for various greenery coverage ratios, three different urban densities, and two temporal periods in typical summer day. Four heat indicators and thermal comfort indices were taken to measure the cooling benefits of greenery. Regardless of heat indicators, urban densities, and temporal periods, greenery coverage ratio and their cooling benefits were found to be related non-linearly in a logarithm pattern. This curve relationship indicated that greenery cooling efficiency increases at the lower end of greenery coverage ratio, then decrease as greenery abundance increase, and reached the highest at 20–30%. Above the 30% coverage ratio, the marginal cooling benefits nearly levelled off.
This understanding could be directly applied to other sub-tropical cities with LCZ 1 and 2 urban morphological characteristics. Furthermore, the findings could be used to efficiently improve the green master plan and design control for creating better microclimatic conditions and achieving thermal comfort at the pedestrian level in urban areas, especially for cities with limited land resources and high-density development needs.
Ouyang, W., Morakinyo, T. E., Ren, C., & Ng, E. (2020). The cooling efficiency of variable greenery coverage ratios in different urban densities: A study in a subtropical climate. Building and Environment, 174, 106772.