# Influence of Urban Green Area on Air Temperature of Surrounding Built-Up Area

## Abstract

**:**

## 1. Introduction

^{2}, a park can reduce the air temperature by up to 1.5 °C at noon time in a leeward commercial area at distance of 1 km. Yu and Hien [6] have carried out temperature and humidity measurements in two big city green areas (36 ha and 12 ha) in Singapore. A three-dimensional non-hydrostatic model (Envi-met) was applied for the simulation of Surface-Plant-Air interactions inside urban environments. Horizontal air temperature profiles in both the green area and surrounding area are calculated by the Envi-met model.

## 2. Measurements

#### 2.1. Study Site

#### 2.2. Outline of Measurements

## 3. Results

#### 3.1. Outline of Calculations

_{t}as a function of the flux Richardson number R

_{f}(Equations (8)–(11)). They do this by aggregating the buoyancy effect into the vertical eddy viscosity model coefficient C

_{µ}and the turbulent Prandtl number P

_{rt}. In this study, we used both Equation (7) and the conventional Equation (6). Equations (8)–(11) are used in calculating the vertical eddy viscosity coefficient ν

_{t}on the right side of Equation (7). Calculation conditions and the outline of calculation conditions are shown in Table 2 and Figure 4.

#### 3.2. Results

_{t}are given by Equation (6). In the non-isotropic diffusion model, the eddy viscosity coefficient ν

_{t}in the vertical direction is given by the formula of Equation (7) when considering the buoyancy effect. Distance from the green area and the heat flux component of the calculation result, at 13:00 in the mesh near the ground surface, is shown in Figure 7. In the incorporated buoyancy model, the sensible heat flux supplied from the ground surface is transported in the vertical direction due to the vertical diffusion effect, so air temperature in the mesh near the ground surface does not rise. The part of the urban area more than 50 m from the green area is dominated by the diffusion effect in the vertical direction over the advection effect.

## 4. Discussion

^{2}for daytime and 28.3 W/m

^{2}for evening. A value of 132.5 W/m

^{2}was also assumed for their intermediate value. When entering the urban area air temperature rises sharply. The smaller the wind velocity, the larger the distance influenced by the green area, and the larger the air temperature rise. As the distance from the green area increases, air temperature becomes constant. When entering the part of the urban area more than 50 m from the green area, the air temperature near the ground surface is dominated by the diffusion effect in the vertical direction rather than the advection effect from the green area.

^{2}), L is the distance from the boundary (m), α is air temperature gradient (K/m), C

_{p}is the specific heat of air (=1000 J/(kgK)), ρ is air density (=1.2 kg/m

^{3}), and U is wind velocity (m/s). Assuming α = 0.006 (K/m), it becomes Equation (13).

## 5. Conclusions

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 1.**Mobile measurement points and aerial photograph, all located in Kobe city, Japan. (

**a**) Higashi-yuen Park and business area; (

**b**) Ishiyagawa Park and residential area; (

**c**) Okurayama Park and residential area.

**Figure 3.**Distance from the green area to each mobile measurement point in urban area and air temperature rise (

**a**) at 13:00; (

**b**) at 17:00.

**Figure 5.**Calculation results and measurement results of air temperature rise at 13:00 according to the distance from the green area. (

**a**) strong wind case; (

**b**) weak wind case.

**Figure 6.**Calculation results and measurement results of air temperature rise at 17:00 according to the distance from the green area. (

**a**) strong wind case; (

**b**) weak wind case.

**Figure 7.**Distance from the green area and heat flux component of the calculation result at 13:00. (

**a**) isotropic diffusion model; (

**b**) incorporating buoyancy model.

**Figure 8.**Distance from the green area and the air temperature rise in several urban areas in the daytime.

**Figure 9.**Distance from the green area and air temperature rise in several urban areas in the evening.

**Figure 11.**Calculation results of air temperature rise according to the distance from the green area. In the cases where the sensible heat flux is (

**a**) 236.7 W/m

^{2}; (

**b**) 132.5 W/m

^{2}; (

**c**) 28.3 W/m

^{2}.

**Figure 12.**Air temperature rise by Equation (12). In the cases where the sensible heat flux is (

**a**) 236.7 W/m

^{2}; (

**b**) 132.5 W/m

^{2}; (

**c**) 28.3 W/m

^{2}.

**Figure 13.**Air temperature rise by Equation (14). In the cases where the sensible heat flux is (

**a**) 236.7 W/m

^{2}; (

**b**) 132.5 W/m

^{2}; (

**c**) 28.3 W/m

^{2}.

**Figure 14.**Air temperature rise by Equation (15). In the cases where the sensible heat flux is (

**a**) 236.7 W/m

^{2}; (

**b**) 132.5 W/m

^{2}; (

**c**) 28.3 W/m

^{2}.

Device | Method | Accuracy of Device | |
---|---|---|---|

Air temperature | Thermistor with solar radiation shield | Averaged for 5 min by sampling every 5 s | ±0.5 K |

Wind direction | Windsock | Highest frequency in 30 s | by visual inspection |

Wind velocity | Hot-wire anemometer | Averaged for 30 s by sampling every second | ±2% of indicated value |

Surface temperature | Infrared thermometer | Measured on ground and wall surface, a representative material surface at each measurement point was measured several times to obtain stable data | ±1.0 K |

13:00 | 17:00 | |
---|---|---|

Inflow air temperature with uniform vertical profile | 33 °C | 31 °C |

Inflow wind velocity at 50 m high with logarithmic vertical profile | Large: 5.6 m/s Small: 4.1 m/s | Large: 4.7 m/s Small: 4.2 m/s |

Sensible heat from ground surface | 314 W/m^{2} | 196 W/m^{2} |

Roughness parameter | 0.5 m | |

Horizontal mesh size | 50 m | |

Vertical mesh size | 3 m |

2 m/s | 3 m/s | 4 m/s | 5 m/s | 6 m/s | |
---|---|---|---|---|---|

236.7 W/m^{2} | 0.021 | 0.020 | 0.016 | 0.013 | 0.011 |

132.5 W/m^{2} | 0.019 | 0.016 | 0.012 | 0.010 | 0.008 |

28.3 W/m^{2} | 0.010 | 0.007 | 0.005 | 0.003 | 0.002 |

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**MDPI and ACS Style**

Takebayashi, H.
Influence of Urban Green Area on Air Temperature of Surrounding Built-Up Area. *Climate* **2017**, *5*, 60.
https://doi.org/10.3390/cli5030060

**AMA Style**

Takebayashi H.
Influence of Urban Green Area on Air Temperature of Surrounding Built-Up Area. *Climate*. 2017; 5(3):60.
https://doi.org/10.3390/cli5030060

**Chicago/Turabian Style**

Takebayashi, Hideki.
2017. "Influence of Urban Green Area on Air Temperature of Surrounding Built-Up Area" *Climate* 5, no. 3: 60.
https://doi.org/10.3390/cli5030060