October 01, 2013 Summary 2: Effect of Vegetation on Urban Heat Island Surekha Tetali 48-722 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

October 01, 2013 Summary 2: Effect of Vegetation on Urban Heat Island Surekha Tetali 48-722 Building Performance Modeling 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 at 3pm in 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 roof albedo, 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.

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