Many previous studies have investigated the role of urban greening (especially trees) on thermal comfort condition of open spaces while its influence in street canyons has, however, received ...relatively lower research attention. In this study, numerical experiments involving street canyons of varying aspect ratio (ARB) with embedded trees of varying aspect ratio (ART), leaf area index (LAI), leaf area density (LAD) distribution and trunk height under different wind conditions were conducted using a micro-meteorological model, ENVI-met. Employing physiological equivalent temperature (PET), the in-canyon thermal comfort was characterized. Results show variable magnitude of PET reduction (ΔPET) with trees of different vertical LAD distribution even though they are of similar LAI. While larger tree-covered area (TCA) ensures improved thermal comfort, the magnitude varies with tree-planting pattern. Between the tree-planting patterns in TCA = 0.6, ΔPET with double-rows is higher than centre tree-planting. Between eastern and western side tree-planting (TCA = 0.3), the former is more temporally effective than the latter. Furthermore, strong oblique wind condition (“without trees”) was found to be more efficient in improving thermal comfort than trees under calm perpendicular wind, reinstating the importance of desirable ventilation in achieving a thermally comfortable environment. However, vegetation offers improved PET reduction under calmer perpendicular than stronger oblique wind condition. Finally, recommendations on applications of our key findings were presented.
•Numerical experiments involving street canyons with trees of varying configuration were conducted.•Apart from LAI, vertical LAD distribution influences ΔPET while trunk height is less influential.•Variable temporal ΔPET was observed with similar tree-covered area but different planting pattern.
Urban greenery, especially trees, has been proven to be one of the effective measures for urban heat island mitigation. However, no consistent findings have been found for the relationship between ...greenery abundance and cooling magnitude; while some previous studies discovered a linear relationship, others opined it could a non-linear one. In addition, there are rare studies exploring whether or not the strength of the relationship is dependent on urban density. Therefore, in this study, we aim to discuss cooling efficiency by measuring the relationship between greenery coverage ratio and the cooling effects of greenery. Parametric studies were conducted in a validated ENVI-met model, with different combinations of urban densities (Low, Mid, and High) and tree coverage ratios (2–30% at 2% interval, and 56% for full area coverage other than building). Then the pattern of the cooling efficiency was explored under three urban densities scenarios and two selected temporal periods on a typical summer daytime. For a subtropical climate background, results showed a non-linear (logarithmic) pattern for tree coverage ratio (TCR) and cooling effects, irrespective of the urban density, temporal periods, and heat indicators. When TCR reached 20–30%, the optimal cooling efficiency of trees were achieved, irrespective of building densities and temporal periods. The optimal threshold for greenery coverage in this study can provide science-based suggestions to urban planners and designers for better microclimate and thermal comfort environments at the neighborhood scale.
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•Relationship between tree coverage and its cooling efficiency was established.•The cooling efficiency of tree coverage under different urban densities were studied.•Non-linear pattern between tree coverage ratios and cooling effects was revealed.•The optimal cooling efficiency could be achieved by 20–30% tree coverage ratio.•The implication of cooling efficiency pattern for urban planning was discussed.
Urban green and blue infrastructures (GBI) are considered an effective tool for mitigating urban heat stress and improving human thermal comfort. Many studies have investigated the thermal effects of ...main GBI types, including trees, green roofs, vertical greenings, and water bodies. Their physical characteristics, planting designs, and the surrounding urban-fabric traits may impact the resultant thermal effects. ENVI-met, a holistic three-dimensional modeling software which can simulate the outdoor microclimate in high resolution, has become a principal GBI research tool. Using this tool, the GBI studies follow a three-step research workflow, i.e., modeling, validation, and scenario simulation. For providing a systematic and synoptic evaluation of the extant research workflow, a comprehensive review was conducted on GBI-targeted studies enlisting ENVI-met as the primary tool. The findings of 79 peer-reviewed studies were analyzed and synthesised for their modeling, validation, and scenario simulation process. Special attention was paid to scrutinising their data sources, evaluating indicator selection, examining main analytical approaches, and distilling recommendations to improve the research workflow. This review provides researchers with an overview of the ENVI-met methodology and recommendations to refine research on GBI thermal effects.
The re-integration of trees into the urban landscape is a veritable strategy for urban climate mitigation and adaptation. However, dysfunctional trees in terms of urban heat mitigation are dominant ...in many sub-tropical cities' landscapes due to the lack of scientific basis of tree selection. Therefore, this study proposes and evaluates a methodological framework as an approach for “right tree, right place” for urban heat mitigation through parametric ENVI-met simulations that involve the combination of 54 generic tree forms and 10 characteristic urban morphology – Sky-View Factor (SVF). Results show variable temperature regulation by tree forms (species) with varying magnitude in different urban morphology. Daytime and nighttime temperature regulation effects were between 0.3 °C – 1.0 °C and 0.0 °C – 2.0 °C, respectively depending on tree forms and SVF value. Furthermore, the Heat Reduction Potential (HRP) of trees forms were determined in terms of their human thermal comfort improvement. In general, we found a range of +5% and − 20% depending on SVF, negative and positive values imply heat reduction and increment, respectively. With the competing shading effect of buildings, the HRP of trees reduces from high to low SVF area with variable magnitude among tree forms (species). Hence, the proposed morphology-based tree selection approach was evaluated by comparison with two uninformed selection approaches in a realistic urban neighborhood in Hong Kong. Results clearly indicate the proposed approach's capability in improving human thermal comfort by up two times more than either of the other approaches. Finally, evidence-based recommendations were given for the reference of policy-makers when they make urban green development plan.
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•A “Right tree, right place” approach for optimum urban heat mitigation was proposed.•The approach was developed and evaluated through a parametric case study under hot-humid prevailing climate.•Proposed approach was found significantly more efficient compared to two others in a realistic urban neighborhood.•Heat reduction potential of trees was location-dependent and species-specific.
Tree-planting is one of the veritable tools for combating urban heat island and improving thermal comfort in the wake of global warming and urbanization. However, trees of different species and ...morphological properties have variable solar attenuation capacity and consequently, thermal comfort regulation potential. Besides, the shadow-cast effect by buildings helps in reducing pedestrian radiant load and consequently improves thermal comfort especially in high-density cities even though ventilation is reduced. Therefore, a holistic and contextual understanding of tree-planting and shadow-casting can help in designing climate-proof cities. In this study, we employed the ENVI-met model to better understand the interaction between these two forms of shading (trees and buildings) on the pedestrians' thermal comfort in Hong Kong and the influence of one over the other. The impact of different urban densities on the thermal comfort improvement potential by eight (8) common tree-species in Hong Kong was specifically studied. Results show that shallow canyons are susceptible to worse thermal condition when compared to their deeper counterparts with similar aspect ratio value. Of all tree configuration parameters, leaf area index, tree height and trunk height are most influential in improving and aggravating daytime and nighttime thermal comfort, respectively. We also found that trees' effectiveness in improving daytime thermal comfort reduces with increasing urban density and vice-versa for nighttime. For the reference of planners and landscape architects, this study recommends tall trees of low canopy density with high trunk in deeper canyons and vice-versa for shallow canyons and open-areas.
•Worsened thermal condition was observed in shallow than deeper canyons of similar aspect ratio value at mid-day.•The impact of shadow and tree shading on daytime and nighttime thermal conditions is determined by different mechanisms.•The efficacy of trees in improving daytime thermal comfort reduces with increasing urban density.•Tall trees of low leaf density and high trunk are proposed for deep canyons and reverse for shallow canyons and open-areas.•Hong Kong's Greening Master Plan can be updated using tree-species selection based on urban density mapping.
Using a validated ENVI-met model, a parametric study was conducted to investigate the thermal and energy saving benefits in a selected neighbourhood with its current greenery coverage ratio (GCR) of ...7.2%, compared to the recommended 30%. To provide information for efficient tree species selection, nine scenarios were tested for the case of 30% GCR. In eight of them, only one tree species was used in each case. The trees represent the eight most common species in Hong Kong. The remaining one featured a mix of tree species.
In comparison with the reference case (no trees), results revealed a reduction in maximum temperature of 0.4 °C and 0.5–1.0 °C under the current and 30% GCR situations respectively; and a decrease in average Physiological Equivalent Temperature of 1.6 °C and 3.3–5.0 °C. The area coverage of “Very Hot” thermal sensation reduced from ∼60% in the reference case to ∼50% with the current GCR and 17–21% with 30% GCR. Lastly, a decrease in cooling energy of 1500 kWh per typical summer day was observed with the current GCR, which increased to ∼1900–3000 kWh with 30% GCR, equivalent to 200–450 US$ savings within the 500 × 500 m2 domain. The variations in the estimated benefits between the 30% GCR scenarios reflect the importance of species-specific analysis. Statistical analysis revealed leaf area index was the main driver of the observed benefits, followed by trunk height, tree height and crown diameter. Our findings will encourage city planners and citizens to take actions for urban greening.
•The thermal environment of a neigbhourhood was simulated with a validated ENVI-met model.•3D spatial temperature regulation and energy saving of different trees species was estimated.•Variations of estimated benefits is dependent on trees' physical configuration.•Temperature reduction and energy saving cost of trees varies between 0.5 and 1.0 °C and 200–450US$, respectively.
To better understand the relationship between energy consumption, and prevailing climatic condition, the present study uses Hong Kong’s observed air temperature records, end-use electricity ...consumption, and population datasets to: (a) investigate the spatial pattern of cooling energy requirement i.e. cooling degree days on a typical normal and extremely hot summer day using co-kriging geospatial mapping technique; (b) analyze the annual trend of cooling degree days in the city; and (c) quantify the impact of extreme heat events on the summer cooling energy requirements. Results revealed reasonable predictability of city-wide cooling degree days with the co-kriging method which uses two covariates i.e. “elevation of the weather station” and “building volume density within the 1000 m radius neighboring area”. Homogeneity and heterogeneity in cooling degree days’ distribution were found during the summer daytime and nighttime, respectively indicating the method’s ability to delineate the urban heat island effect with increased magnitude during extreme heat events. Quantitatively, the extreme heat events increased cooling degree days by 80–140% depending on the event type, a range consistent in recent years (2011–2015). Lastly, we provided the implications of our findings to building and urban design; and future energy planning.
•Co-kriging method was adopted for city-wide mapping of cooling degree days (CDD).•The impact of extreme heat events on summer CDD was quantified.•CDD increased by 80–140% due to extreme heat events in recent years.•Higher CDD was observed in the built environment especially at nighttime.
•We investigate the effect of tree-shading on building energy demand.•Indoor and outdoor cooling degree days were more for un-shaded building.•More energy (and cost) needed to cool un-shaded ...building.•Tree-shades reduce cooling energy demand and cost in buildings.
This study investigates the effect of tree-shading on energy demand in two similar buildings. Outdoor and indoor air temperature was measured simultaneously for a period of 6 months. Five different base temperatures ranging from 20°C to 25°C were chosen and used to calculate cooling degree-days. Degree-day and cooling/warming rate methods were used to estimate cooling energy requirements for the two buildings. Indoor and outdoor cooling degree days were observed to be more for the un-shaded buildings compared to the tree-shaded one. Indoor warming and cooling rate show that the un-shaded building warms earlier and faster than the tree-shaded. Results indicate that tree-shading can save up to 34,500 NGN (US$218) on energy costs. The study shows the role of greening in reducing energy demand in buildings.
Nigerian agriculture is mainly rain-fed and basically dependent on the vagaries of weather especially rainfall. Nigeria today has about forty-four (44) weather observation stations which provide ...measurement of rainfall amount for different locations across the country. Hence, this study investigates change detection in rainfall pattern over each climatic zone of Nigeria. Data were collected for 90 years (1910–1999) period for all the weather observation stations in Nigeria, while a subdivision was made to three (3) non-overlapping climate period of 30 years i.e. 1910–1939, 1940–1969 and 1970–1999. Statistical methods were utilized to justify any change in the average monthly and annual rainfall trend using probability density function and non-parametric tests such as the Pettitt test, Wilcoxon signed-rank test and paired sample test. Results show common change points and transitions from dry to wet (upward shift) in all climatic zones. Statistical tests performed on the data show that rainfall variation over each climatic zone is significant (p<0.05) between pairs of climate periods. Suggestions were therefore made at the end of the study on the use of the contained information for socio-economic improvement and agricultural development of the zones.
Urban greenery is a natural solution to cool cities and provide comfort, clean air and significant social, health and economic benefits. This paper aims to present the latest progress on the field of ...greenery urban mitigation techniques including aspects related to the theoretical and experimental assessment of the greenery cooling potential, the impact on urban vegetation on energy, health and comfort and the acquired knowledge on the best integration of the various types of greenery in the urban frame. Also to present the recent knowledge on the impact of climate change on the cooling performance of urban vegetation and investigate and analyse possible technological solutions to face the impact of high ambient temperatures.
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