October 01, 2013Summary 2: Effect of Vegetation on Urban Heat IslandSurekha Tetali48-722 Building Performance Modeling

Green Roofs to Cool Cities ?Urban Heat Island (UHI) is a phenomenon in which the day time temperatures in urbanized regions of a city are higher than the temperatures in rural areas (Santamouris 2001). The cause of UHI effect can be attributed to urbanization with increase in built environment and population, and decrease in vegetation. In peak summers, exposed surfaces of the built environment can be 300C-400C hotter compared to the ambient air dry bulb temperature (Akbari, Pomerantz, and Taha 2001) leading to UHI effect. Various prior studies (Rizwan, Dennis, and Liu 2008, Akbari and Konopacki 2005, Akbari, Pomerantz, and Taha 2001) suggest improving albedo of material and increasing urban vegetation can mitigate the UHI effect. Urban vegetation can decrease the ambient air temperature (through evapotranspiration) and surface temperatures (by shading). In a field study conducted by (Rosenfeld et al. 1995) at Sacrameto, it is observed that trees can reduce the cooling energy use by 30-35% when placed on the south and south-west facade of the building. In Athens, on a vertical wall, shading decreased the surface temperature by 8.50C when compared to a surface directly exposed to solar radiation, and the ambient air temperature is 0.50C -3.00C cooler in the presence of trees (Papadakis, Tsamis, and Kyritsis 2001). However, in urban environments, the availability of ground area is limited and hence the potential of planting new trees is low. Given the vast area of building roofs in urban environments (Akbari, Menon, and Rosenfeld 2009), vegetation on roofs can be a potential technique in mitigating the UHI effect. Hence, in this summary, the effect of green roofs in mitigating urban heat island effect is focused.The affect of green roof on a individual building level has been studied in various studies both computationally and experimentally. Experiments by Wong et al. (Wong et al. 2003) studied the effect of intensive green roof on a building located in Singapore. The ambient temperatures measured at a height of 300mm above the roof show that air temperature over green roof is 4.20C cooler than the air temperature on hard roof. However, the ambient air temperature was similar above all the roofs, when measured at a height of 1000 mm. This uniform temperature might be because of the wind. The global temperature measured over green roof was 4.050C lower than that over the hard roof. The Mean Radiant Temperature (MRT) over the roof, calculated from the measured data showed that the green roof was cooler by a maximum of 4.50C compared to hard roof. The authors use these values to show that the long wave radiation emitted by the green roof is lower when compared to the hard roof, and hence at a urban scale, green roofs mitigate the UHI effect. Each individual green roof lowers the surface temperature of that building and the nearby air. This cooler air through advection affects the temperature of the entire city and can help in mitigating UHI. In order to quantify this mitigating affect, mesoscale atmospheric models are frequently used. Remote sensing and GIS data are used to develop high resolution land-surface data for such models.In an extensive simulation study (Rosenzweig, Solecki, and Slosberg 2006) performed mesoscale atmospheric model of NewYork City during three different days when the city was experiencing heat waves. Regional climate model MM5 (Grell, Dudhia, and Stauffer 2011) was used to simulate sensible and latent heat fluxes for the land-surface cover and the meteorological conditions. Simultaneous energy balance models for grass, trees, water and impervious mediums were run to calculate the air temperature. The model was calibrated with the measured temperatures. To study the affect of green roof on UHI, the green roof coverage was increased to 50% of the entire city. This resulted in a reduction of the peak near surface air temperature by 0.8 0F at3pmin NYC. The daily average temperature of the New York city and 6 neighboring regions studied decreased as well (range of decrease=0.4-1.1 0F).In a study conducted by Chen et al. (Chen et al. 2009), CSCRC (coupled simulations of convection, radiation, and conduction) was performed on two urban areas in Tokyo. Surface temperatures, heat flux, mean air temperatures, and mean impact index (an index developed by the authors to show the temperature rise at various points considered in the study) were studied for both the locations. The study considered heat gain from building envelope, air conditioning, and traffic. The ambient air temperature was measured at 1.5m from the road level to study the thermal environment around the pedestrians. The results shows that, in Otemanchi, which is high rise business district, the thermal conditions in the urban canyon weren't much affected by the roof material. However, the mean air temperature (MRT) was 0.2 0C lower than the MRT when the heat from traffic is considered. In case of Kayobashi, which is a medium rise business district, the MRT in the urban canyon is slightly lower (0.01 0C) when the roofs were green, and 0.37 0C lower when the heat from traffic is considered. Thus, according to this study, green roof on a low rise building might have a better impact on temperatures in urban canyon.Smith and Roebber (Smith and Roebber 2011) used Weather Research and Forecasting model coupled with a urban canopy model to simulate the local atmosphere of Chicago. The model was calibrated to the observed temperatures on a heat wave day. To study the affect of green roof, all roof tops in the urban domain were changed to green roofs. This resulted in a decrease of about 30C in temperature compared to the base run. However, the study simulated green roof indirectly by just changing the roof albedo to 0.8, thus effectively simulating a white roof. The affect of moisture, roughness, thermal inertia etc. are ignored. Including these affects can lower the temperature decrease. Green roofs add moisture to the atmosphere through transpiration. This could increase the apparent temperature as well.Sailor and Dietsch (Sailor and Dietsch 2007) use regional climate model MM5 for 20 cities across US to study the affect of green roof in mitigating the UHI using similar method as (Rosenzweig, Solecki, and Slosberg 2006). However, green roofs are simulated indirectly by increasing roofalbedo, moisture availability, roughness, and thermal inertia of a regular roof. It is observed that increasing green roof cover to 10% of the city can lower the temperature by 0.22 - 0.58 0C in different cities. The extent to which a green roof in lowers the ambient temperature at a building level depends upon several factors, for example, the Leaf Area Index (LAI) (Kumar and Kaushik 2005, Takebayashi and Moriyama 2007), climatic conditions (temperature, wind, humidity) (Jim and He 2010, Tsang and Jim 2011). As the performance of green roof changes at local level, it can change the extent to which green roof can mitigate UHI at city level as well. The different computational models in different cities with their own unique urban landscape and weather reviewed in this summary show that green roofs can mitigate UHI. Moreover, the cited references show that green roof on a low rise building might have a better impact on temperatures in urban canyon.Akbari, H., and S. Konopacki. 2005. "Calculating energy-saving potentials of heat-island reduction strategies." 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Energy and Buildings no. 22 (3):255-265. doi: 10.1016/0378-7788(95)00927-p.Rosenzweig, Cynthia, William Solecki, and Ronald Slosberg. 2006. MITIGATING NEW YORK CITYS HEAT ISLAND WITH URBAN FORESTRY, LIVING ROOFS, AND LIGHT SURFACES. Albany, NY.Sailor, David J., and Nikolaas Dietsch. 2007. "The urban heat island mitigation impact screening tool (MIST)." Environmental Modelling & Software no. 22 (10):1529-1541. doi: 10.1016/j.envsoft.2006.11.005.Santamouris, M. 2001. Energy and Climate in the Urban Built Environment. London: James and James Science Publishers.Smith, K. R., and P. J. Roebber. 2011. "Green Roof Mitigation Potential for a Proxy Future Climate Scenario in Chicago, Illinois." Journal of Applied Meteorology and Climatology no. 50 (3):507-522. doi: 10.1175/2010jamc2337.1.Takebayashi, H., and M. Moriyama. 2007. "Surface heat budget on green roof and high reflection roof for mitigation of urban heat island." 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