An assessment of WRF-urban schemes in simulating local meteorology for heat stress analysis in a tropical sub-Saharan African city, Lagos, Nigeria.

Megacities, such as Lagos, Nigeria, face significant challenges due to rapid urbanization and climate change, resulting in a higher intensity of the urban heat island effect, coupled with high population density, making the city fall under the category of moderate to high heat stress/risk. Despite this, very few studies have analyzed the urban impact on heat stress over the coastal city, albeit with poor resolution data. In this study, we assessed the performance of an integrated high-resolution WRF-urban scheme driven by the readily available urban canopy information of the local climate zone (LCZ) to simulate local meteorological data for analyzing the spatiotemporal pattern of heat stress over the megacity. Our results show that the WRF-BEP scheme outperformed the other evaluated urban schemes, reducing the normalized root mean squared error by 25%. Furthermore, using humidex, we found a generally high incidence of intense discomfort in highly urbanized areas and noted the significant influence of urban morphology on the pattern of heat stress, particularly at night due to the combined effect of urban warming and higher relative humidity. The most socioeconomically disadvantaged urban areas, LCZ7, were most affected, with "hot" heat stress conditions observed over 90% of the time. However, during the afternoon, we found reduced heat stress in the core urban areas which might be due to the shading effect and/or cold air advection. Our findings would be relevant in the development of the urgently needed climate/heat adaptation plans for the city and other sub-Saharan African cities.

Thermal-irradiant performance of green infrastructure typologies: Field measurement study in a subtropical climate city.

Greenery infrastructure (GI) is an important design strategy for sustainable cities and communities' development, as it brings multiple benefits including mitigating urban heat island. Based on the implementation locations, three typical GI typologies, namely green roof, green wall, and ground tree, are widely adopted in urban communities. As previous studies focused on one single GI and mainly studied their thermal features, this study aims to fill the gap by investigating three GI typologies within one site; their thermal-irradiant performance was compared for four typical summer days in a subtropical city. Firstly, stationary and transect 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. Secondly, the thermal-irradiant differences were revealed among GI typologies and temporal periods; two measuring methods were compared for their capabilities in detecting the irradiant variations near three GI typologies. Results showed that: 1) the ground tree experienced the smallest thermal-irradiant average and variation among three GI typologies; 2) the morning session (09:00-12:00) had the largest thermal-irradiant reduction and variations for three GI typologies; and 3) the six-directional method showed higher sensitivity towards the irradiant variations near three GI typologies; the globe thermometer method is not suitable for tree-shaded areas. This study provides a comprehensive understanding of proper selection of MRT measuring methods and GI implementation for thermal comfort, especially for the subtropical cities. Practically, this study shows designers and policymakers on how to implement GI typologies for climate-resilient design.

Right tree, right place (urban canyon): Tree species selection approach for optimum urban heat mitigation - development and evaluation.

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.

Quantifying the effect of rain events on outdoor thermal comfort in a high-density city, Hong Kong.

Rainfall events often cause a modification to atmospheric conditions. The impact of this phenomenon on human thermal comfort has however been less well studied. Therefore, this paper quantifies the effect of rainfall events on human thermal comfort in a hot-humid subtropical city, Hong Kong. Firstly, rainfall events were categorized based on time of occurrence, i.e., morning (on or before 11:00 LST), afternoon (12:00-15:00, LST), early evening (16:00-18:00), and all-day events. Thereafter, human thermal comfort on typical non-rainy (sunny) days and rainy days was estimated and compared by using the radiation-driven physiological equivalent temperature (PET) and non-radiation-driven temperature-humidity index (THI) and compared. Results revealed variable and stable hourly patterns of PET and THI thermal classification, respectively under different rainfall event category. The insensitivity of THI values could be due to the retained strong contribution of both input parameters (air temperature and relative humidity) on both rainy and non-rainy (sunny) days. An understanding of the mechanism of thermal changes before, during, and after rainfall events based on statistical analysis suggests a strong interplay between moisture content and air temperature as determinants of thermal comfort in the hot-humid city and not necessarily the radiation parameter. This finding suggests that while PET clearly shows the impact of rain-event; it is principally due to the strong contribution of the lowered radiant temperature in its calculation while in reality, the critical determinants of thermal comfort in such period in a hot-humid subtropical environment like Hong Kong are the moisture content and ambient temperature. Finding from the study could enhance occupational health and safety management of outdoor workplaces.

Study of traffic-related pollutant removal from street canyon with trees: dispersion and deposition perspective.

Numerical experiments involving street canyons of varying aspect ratio with traffic-induced pollutants (PM2.5) and implanted trees of varying aspect ratio, leaf area index, leaf area density distribution, trunk height, tree-covered area, and tree planting pattern under different wind conditions were conducted using a computational fluid dynamics (CFD) model, ENVI-met. Various aspects of dispersion and deposition were investigated, which include the influence of various tree configurations and wind condition on dispersion within the street canyon, pollutant mass at the free stream layer and street canyon, and comparison between mass removal by surface (leaf) deposition and mass enhancement due to the presence of trees. Results revealed that concentration level was enhanced especially within pedestrian level in street canyons with trees relative to their tree-free counterparts. Additionally, we found a dependence of the magnitude of concentration increase (within pedestrian level) and decrease (above pedestrian level) due to tree configuration and wind condition. Furthermore, we realized that only ∼0.1-3 % of PM2.5 was dispersed to the free stream layer while a larger percentage (∼97 %) remained in the canyon, regardless of its aspect ratio, prevailing wind condition, and either tree-free or with tree (of various configuration). Lastly, results indicate that pollutant removal due to deposition on leaf surfaces is potentially sufficient to counterbalance the enhancement of PM2.5 by such trees under some tree planting scenarios and wind conditions.

Evaluating the role of green infrastructures on near-road pollutant dispersion and removal: Modelling and measurement.

To enhance the quality of human life in a rapidly urbanized world plagued with high transportation, the masterful contribution of improved urban and local air quality cannot be overemphasized. In order to reduce human exposure to near-road air pollution, several approaches including the installation of roadside structural barriers especially in open street areas, such as city entrances are being applied. In the present study, the air quality around real world and idealized green infrastructures was investigated by means of numerical simulation and a short field measurement campaign. Fair agreement was found between ENVI-met modelled and measured particulate matter's concentration data around a realistic vegetation barrier indicating a fair representation of reality in the model. Several numerical experiments were conducted to investigate the influence of barrier type (vegetation/hedge and green wall) and dimensions on near-road air quality. The results show different horizontal/vertical patterns and magnitudes of upwind and downwind relative concentration (with and without a barrier) depending on wind condition, barrier type and dimension. Furthermore, an integrated dispersion-deposition approach was employed to assess the impact on air quality of near-road vegetation barrier. At last, recommendations to city and urban planners on the implementation of roadside structural barriers were made.

Simulation study of dispersion and removal of particulate matter from traffic by road-side vegetation barrier.

Well-positioned and configured vegetation barriers (VBs) have been suggested as one of the green infrastructures that could improve near-road (local) air quality. This is because of their influence on the underlying mechanisms: dispersion and mass removal (by deposition). Some studies have investigated air quality improvement by near-road vegetation barrier using the dispersion-related method while few studies have done the same using the deposition-related method. However, decision making on vegetation barrier's configuration and placement for need-based maximum benefit requires a combined assessment with both methods which are not commonly found in a single study. In the present study, we employed a computational fluid dynamics model, ENVI-met, to evaluate the air quality benefit of near-road vegetation barrier using an integrated dispersion-deposition approach. A technique based on distance between source (road) and point of peak concentration before dwindling concentration downwind begins referred to as "distance to maximum concentration (DMC)" has been proposed to determine optimum position from source and thickness of vegetation barrier for improved dispersion and deposition-based benefit, respectively. Generally, a higher volume of vegetation barrier increases the overall mass removal while it weakens dispersion of pollutant within the same domain. Hence, the benefit of roadside vegetation barrier is need-based and can be expressed as either higher mass deposition or higher mass dispersion. Finally, recommendations on applications of our findings were presented.

Multi-purpose rainwater harvesting for water resource recovery and the cooling effect.

The potential use of rainwater harvesting in conjunction with miscellaneous water supplies and a rooftop garden with rainwater harvesting facility for temperature reduction have been evaluated in this study for Hong Kong. Various water applications such as toilet flushing and areal climate controls have been systematically considered depending on the availability of seawater toilet flushing using the Geographic Information System (GIS). For water supplies, the district Area Precipitation per Demand Ratio (APDR) has been calculated to quantify the rainwater utilization potential of each administrative district in Hong Kong. Districts with freshwater toilet flushing prove to have higher potential for rainwater harvest and utilization compared to the areas with seawater toilet flushing. Furthermore, the effectiveness of using rainwater harvesting for miscellaneous water supplies in Hong Kong and Tokyo has been analyzed and compared; this revives serious consideration of diurnal and seasonal patterns of rainfall in applying such technology. In terms of the cooling effect, the implementation of a rooftop rainwater harvesting garden has been evaluated using the ENVI-met model. Our results show that a temperature drop of 1.3 °C has been observed due to the rainwater layer in the rain garden. This study provides valuable insight into the applicability of the rainwater harvesting for sustainable water management practice in a highly urbanized city.