Urban shading—a design option for the tropics? A study in Colombo, Sri Lanka

Abstract

Abstract:Recent urban microclimate studies in Colombo, Sri Lanka, indicate that the maximum daily temperature within street canyons decreases with increasing height to width (H/W) ratio, but higher H/W ratio negatively affects street-level wind flow. There is also evidence pointing to the cooling effect of sea breeze. The nocturnal heat island is small in contrast to daytime urban-rural differences. In this paper, we use the software ENVI-met to simulate the effect of different urban design options on air and surface temperatures, as well as on outdoor thermal comfort. The latter is expressed as the physiologically equivalent temperature (PET), an index based on air and radiant temperatures as well as wind and humidity. It is found that high albedo at street level gives the lowest air temperature during daytime, although the reduction is only about 1°C. The lowest daytime mean radiant temperatures result from high H/W ratios of streets. This has a positive effect on thermal comfort; the increase of H/W ratio from about 1 to 3 leads to a decrease in PET by about 10°C. Differences in air and surface temperatures, as well as PET, are small during the night. The results show that strategies that lead to better air temperature mitigation may not necessarily lead to better thermal comfort. However, shade enhancement through increased H/W ratios is clearly capable of significant reductions in PET, and thus, improved outdoor thermal comfort. Consequently, a critical urban design task in the humid tropics will be to guide the rapid urban growth towards efficient 'shade growth'.

Green-Roof Effects on Neighborhood Microclimate and Human Thermal Sensation

“…Therefore, there is constant tendency for numerical models such as ENVI-met to moderate extreme values and contract the diurnal range. Similar results have been reported by Johansson et al [24] and Emmanuel et al [18].…”

“…The program is designed for micro-scale simulation with a typical resolution from 0.5 to 10 m in the horizontal land surface and 10 s in time [18]. It takes into account the key modeling inputs, including site location, initial climatic parameters, soil and plant type, building structure, and thermal properties.…”

“…Human thermal comfort (HTC), which depicts human sensory perception of the thermal environment, has been widely adopted to examine the relative sustainability of urban design options from the human-health viewpoint [18,21,24]. HTC depends on some key weather parameters such as radiation, temperature, humidity and air flow.…”

Assessment of Thermal Comfort in Respect to Building Height in a High-Density City in the Tropics

“…This model calculates all important meteorological parameters such as air and surface temperature, wind speed, humidity, radiation as well as mean radiant temperature. The versatility and validity of the ENVI-met were confirmed from the previous studies (Wong et al, 2007;Emmanuel et al, 2007). For temporal stability of the numerical simulation, the time step is kept below 10s.…”

Urban Climatology ICUC6

“…Human comfort in the urban environment is a major theme of urban climate research, illustrated here by the papers of Emmanuel et al (2007), Ooka (2007) and Thorsson et al (2007). Emmanuel et al (2007) use the ENVI-met model (Bruse and Fleer, 1998) to look at the role of urban designs in Colombo, Sri Lanka and their impact on physiologically equivalent temperature.…”