Neighborhood Characteristic & Climatic Zones in Norwest City
Characterizing the physical nature of cities assist in understanding local climates & quantifying the impact of urban design on temperature
One of the key methods of classifying urban neighbourhoods based on their contribution to local climates is through mapping Local Climate Zones (LCZ). LCZ maps represent a systematic and universal classification scheme for categorizing urban areas, such that we quantify not only urban-rural differences in microclimate but also intra-urban variabilities based on urban design.
The LCZ scheme comprises 17 zones based mainly on properties of surface structure (building and tree height & density) and surface cover (pervious vs impervious ratio). Collectively, these parameters drive the microclimate and thermal environments of urban neighborhoods. For instance, pervious surface covers (i.e., porous or permeable surfaces that allow water to percolate through to the area underneath) retain water in urban areas which leads to more cooling potential during warm periods.
Overall, the spatial variability in urban temperature is influenced by the local climate zone.
The Local Climate Zone map in Sydney shows the diverse characteristics of urban neighbourhoods and high spatial variability in urban surface and structures. Compact high-rise areas are only seen in older inner-city suburbs that are often water-bound and in the proximity of the central business district. The remaining built-up areas in eastern and western suburbs are covered with open low- to mid-rise built environments.
There are thirteen LCZs in Sydney with three dominating built categories: sparsely built, open low-rise, and compact low-rise neighbourhoods. Being surrounded by mountains to the west and national parks to the north and south, dense trees is further seen as a dominant natural land cover in and surrounding Greater Sydney Region.
Comparing the LCZ map (left) with Land Surface Temperature (LST, right) demonstrates that built types generally have warmer LSTs than natural land covers. Additionally, hotspots of LST coincides with several large low-rise or heavy-industry urban areas, where a high percentage of impervious materials (and lack of vegetation and water bodies) are seen.
Local Climate Zone map of Norwest City
To better understand the impact of urban design and structure in Norwest City, the local climate zone map of the area can be obtained at 100m resolution (shown below). Open and compact low-rise zones are seen as dominant built forms in Norwest and Bella Vista, while the percentage of large low-rise and heavy industry is slightly larger compared to the Greater Sydney Region. The most dominant natural covers, on the other hand, are scattered trees and low plants with the largest clusters seen in Bella Vista Farm and Castle Hill Country Club.
Local Climate Zone map of Norwest City and Bella Vista, NSW (100m resolution).
Surface Cover: Built Environment Vs Natural Covers
Vegetation cover (including trees and low plants) in Norwest and Bella Vista, NSW (100m resolution).
In addition to LCZ maps, the climatic profile of the area can be characterized by analysing the surface cover, noting the balance between impervious built materials (hard surfaces such as roads and buildings that don't allow water to seep into the ground) and natural land covers (such as vegetation, water, and soil).
The total vegetation cover (which includes the fraction of trees and low vegetation in each grid cell) in Norwest City ranges from 5-70%, with an average of 38% in the entire area. Norwest City is particularly successful in implementing low vegetation in the built areas (shown in the graph below) The average tree fraction (identified by the % of the area covered by trees above 2m), however, is below 15%. The ongoing initiatives to increase tree coverage (particularly in compact built areas) in Norwest City is therefore critical for improving outdoor thermal comfort and decreasing heat stress in the area.
A comparison of total vegetation in Norwest City with Parramatta and Sydney CBD further indicates a higher percentage of vegetation in Norwest City which is not only focused on parks and open spaces, but is clearly seen in the urban areas. These initiatives subsequently impact urban temperature and human thermal comfort.
Building density in Norwest and Bella Vista, NSW (100m resolution).
On the other hand, mean building density and impervious fraction (including roads and other built forms) in Norwest and Bella Vista are at 35% and 45%, respectively, providing ample natural land covers that can ameliorate extreme thermal environment and avoid surface water runoff of rainwater and irrigation.
Urban Morphology: Height of urban and natural elements
Mean building height (m) in Norwest and Bella Vista, NSW (100m resolution).
In addition to surface covers in urban areas, the 3D urban and natural structures are critical in determining the thermal environment and influencing outdoor thermal comfort.
The building height is one of the critical factors that dominate urban ventilation, shading, as well as radiation trapping in built environments. Currently, the mean building height in Norwest and Bella Vista area is only 7-8m, indicating that mid- to high-rise urban developments in the area are balanced with low-rise (up to two storeys) built-up areas.
Mean tree height (m) in Norwest and Bella Vista, NSW (100m resolution).
The tree height, on the other hand, ranges from 2 to 15m, with an average height of 6m in the area. The presence of mature trees at the same or higher height of buildings is critical in increasing evapotranspiration and reducing air temperature, while providing desirable thermal comfort conditions outdoors.
Urban Climate Modeling: Impact of Urban Design on Urban Temperature and Heat Stress at Norwest City
Modelling urban climate at high resolution assists us in determining how urban design factors and local climate zones result in spatial variability in urban air temperature as well as heat stress (shown in the map using the heat stress indicator, Universal Thermal Climate Index). The graph below shows that areas with close proximity to each other can, in fact, exhibit distinct characteristics in their thermal environment. We further observe that areas with higher vegetation cover, lower urban density, or higher building height have a lower daily maximum of heat stress.
Spatial map of heat stress index (as shown by the UTCI map) at 2pm of an average summer day.
Categorizing air temperature and heat stress index based on the local climate zone classified in the area, we observe a clear pattern between different LCZs. Compared to natural land covers, the mean air temperature in the built-up LCZs can be 2 degrees higher. Comparing different built zones, it is clear that large low-rise or heavy-industry urban areas exhibit the largest air temperature, with approximately 1C higher temperature observed compared to sparsely built neighbourhoods.
Box plots indicating the variability in air temperature in Norwest and Bella Vista based on their local climate zone.
Comparing the heat stress index, the spatial variability is more evident in different climate zones. The difference between built-up areas and natural land covers can be as high as 8C. More importantly, compact as well as large low-rise or heavy-industry urban areas experience moderate heat stress on an average summer day while sparsely built and open low-rise areas are consistently in No Thermal Stress zones.
Box plots indicating the variability in heat stress index (UTCI) in Norwest and Bella Vista based on their local climate zone.
