Multiparametric model of urban park cooling island

https://doi.org/10.1016/j.ufug.2013.01.002Get rights and content

Abstract

This paper presents a study of mitigation of the heat island effect in the built environment with urban (city) parks. The park cooling island (PCI) effect, considering park grass cover and trees’ density and age, is determined for selected extreme summer days at various wind speeds under the optimum soil water conditions in the root zone based on an all-day quasi-stationary thermal response. PCI was determined numerically by coupling a CFD model of an urban park and quasi-steady state, two-zone thermal response boundary condition models of each park element. The boundary models are evaluated in form of multi-parameter approximation polynomials taking into account the sensible and latent heat transfer and the geometrical, optical and thermal properties of park elements. Three-dimensional CFD modelling was used for the determination of temperature, humidity and air velocity fields in an urban park with a size of 140 m × 140 m. Based on the comparison of the measured and numerically determined air temperatures in the tree crowns, we proved that the method of linking the models is adequate for temperature and flow condition modelling in the city park environment.

The results are presented in the form of local PCI as the difference between local air temperature in the pedestrian zone and the reference air temperature preceding the park. The study proved that it is possible to normalise the cooling effect using the specific dimensionless coefficient of leaf area (LAIsp), which includes an approximation of the density of trees planted in the park and their size or age. It was found out that the cooling effect of the park is up to −4.8 °C at LAIsp, equal to 3.16, which corresponds to a planting density of 45 trees per hectare, with an age of 50 years. It was also found that with the length of the park cooling effect change decreases. The optimal length of the park with a LAIsp 3 is 130 m.

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

References (32)

Cited by (83)

  • Mapping urban cool air connectivity in a megacity

    2023, Urban Climate
    Citation Excerpt :

    To mitigate UHIs, some studies have proposed the concept of urban cool islands (UCIs), which have cooler conditions than the surrounding urban matrix (Chang et al., 2007; Sun et al., 2012). Urban green spaces, such as parks (Doick et al., 2014; Lee et al., 2009; Ren et al., 2013; Vidrih and Medved, 2013), water bodies and wetlands (Katayama et al., 1990; Sun et al., 2012; Syafii et al., 2016), and cool pavements with high albedo or water infiltration capacity (Battisti et al., 2018; Nakayama and Fujita, 2010) have been identified and quantified as UCIs. The effect of an UCI can be determined by its cooling potential and spread (Bernard et al., 2018).

  • Applicability of mobile-measurement strategies to different periods: A field campaign in a precinct with a block park

    2022, Building and Environment
    Citation Excerpt :

    This study applied this index to examine the thermal effects exerted by the People's Park on its surroundings, with the specific formula as follows:PCI = Ta-out – Ta-inwhere Ta-in represents the average Ta inside the park and Ta-out represents the Ta outside the park, which is reflected outside the park, as reflected in the different distance from the park. The other indicator was the cooling efficiency, which measured the degree of the PCI with respect to the distance from the park: the temperature gradient, expected to vary along streets, would be expressed as the PCI per 100 m [44–47]. In this two-day field measurement, the mobile points were considered the main and the fixed points as the auxiliary.

View all citing articles on Scopus
View full text