Multiparametric model of urban park cooling island
Introduction
Increased displacement of the natural environment with urbanised areas in previous decades has led to significant changes of local microclimate conditions. One such phenomenon is the urban heat island. In the literature, different predictions of urban heat island intensity can be found, from slight (low as 0.6 °C) to very extreme (up to 12 °C) (Yagüe et al., 1991, Saitoh et al., 1996, Kłysik and Fortuniak, 1999, Rosenzweig et al., 2005, Kolokotsa et al., 2009, Memon et al., 2009). The urban heat island effect increases energy consumption for cooling and causes lower thermal comfort in the indoor as well as in outdoor urban environments. The most effective measures for mitigation of the urban heat island effect are a reduction of solar radiation absorptivity by shading, urban environment elements with higher albedo, latent heat storage, and (above all) evaporative cooling with green surfaces (Taha, 1997, Herbert et al., 1998, Santamouris et al., 2011).
Based on previous research, one can conclude that the most effective measure to mitigate urban heat islands is the integration of green surfaces and parks in the urban environment. Manglani (2004) presented an experimental analysis of the cooling effect of trees. She found that the air temperature in the tree crowns is up to 3.8 °C lower than the surrounding air temperature in the daytime, while the temperature differences are minimal during night time. She stated that the surface temperature of leaves exposed to solar radiation is up to 4.1 °C higher than the temperature of unexposed leaves inside the crown. Vegetation and trees also influence the surface temperature of the build environment. If these are shaded, for example by tree crowns, their surface temperature is up to 19 °C lower compared to unshaded surfaces, while grass layer reduced maximum surface temperature by up to 24 °C (Armson et al., 2012). Onishi et al. (2010) found that the maximum reduction of surface temperature of individual parking lots could be up to 9.260 °C in summer by planting 30% trees and 70% grass.
An extensive experimental and numerical study of the influence of vegetation in urban areas was carried out by Shashua-Bar (Shashua-Bar and Hoffman, 2000, Shashua-Bar and Hoffman, 2002, Shashua-Bar et al., 2010). Based on the experimental measurements in green and other street canyons, the authors concluded that the heat island intensity is reduced up to 3.6 °C in urban areas with the placement of trees. The green area cooling effect depends significantly on the density and size of trees. By increasing the ground vegetation coverage from 10 to 70%, the cooling effect increases from 0.5 to 3.6 °C. Due to the increased latent heat flux by evapotranspiration, the average daily relative humidity is increased by about 12%.
Alexandri and Jones (2008) evaluated the impact of green surfaces for different locations around the world based on a two-dimensional numerical model of street canyon and meteorological data. They concluded that vegetation has the most significant influence in regions with dry and hot climates. In the case of greening the entire urban environment (ground and buildings), they predict a cooling effect between 6.6 °C and 9.1 °C, while in the case of greening only the building envelope, the cooling effect is halved. A lower cooling effect of vegetation is stated by Dimoudi and Nikolopoulou (2003). For an atrium of 18 m × 18 m, surrounded by buildings, they calculated that by planting trees the air temperature could reduced up to 0.8 °C.
Chen and Wong (2006) studied the local cooling effect of parks based on measurements of temperature in the park and surrounding areas, as well as the impact of parks on the neighbourhood. For an average value of leaf area index (LAI) of 3.8, they measured 1.8–2.3 °C lower air temperatures in the park of compared with air temperatures prior entering the park. Compared to the temperatures in the typical urban environment, these were lower by up to 8.2 °C. The park impact on the urban environment was evaluated based on numerical analysis, assuming a wind speed of 1.6 m/s in the direction of residential neighbourhoods. They found that air temperatures in the neighbourhood street canyons are lower by as much as 1.6 °C due to the park's cooling effect and that the influence can be felt in a distance equal to the length of the park.
Based on this literature review, we can conclude that city parks are important contributors to mitigating the urban heat islands effect, although the mentioned studies described only individual impacts of build environment elements, but not the effect of size, density and age of trees on the cooling effect of the park. In this paper, a parametric study of city park cooling potential is presented for a typical sunny day in the hottest summer month for the selected location (L 45.8°) regarding the density and size (age) of the trees, air temperature and wind velocity in open space before the city park.
Section snippets
Modelling of the cooling effect of city parks
In numerical modelling, we assume that parks consist of three elements: a grass layer, trees, and soil under the grass layer. For each element a thermal response model was developed in form of TRNSYS simulation tool TYPE (TRNSYS 16, 2005). In this way, transient conditions in the park were considered. Extreme daily temperatures of each node of park elements, as well as mass flow rate of the water vapour source were later used as boundary conditions in a steady state numerical solution of the
Results and discussion: The park cooling island
As presented in previous chapter, PCI is defined as the extreme difference between air temperature in the park and air temperature at park inlet boundary. Since the problem is transient, the thermal response of each park element was determined with an hour-by-hour simulation using meteorological variables from the local TRY database in hottest summer month (July, Ljubljana, Slovenia) (ARSO, 2010). The local time 15:00, the ambient air temperature at 28.5 °C, solar radiation on a horizontal plane
Conclusions
This paper presents a study on the cooling island potential of city parks expressed by the value of heat cooling island. A heat cooling island is defined the as temperature difference between average air temperature in pedestrian zone along the park and the reference air temperature prior to the park. A park area of 140 m × 140 m was chosen due to the fact that such area of the park corresponds with typical urban district. The heat cooling island potential was modelled by using thermal response
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