A numerical and field investigation of underground temperatures under Urban Heat Island

doi:10.1016/j.buildenv.2010.12.015

Building and Environment

Volume 46, Issue 5, May 2011, Pages 1205-1210

https://doi.org/10.1016/j.buildenv.2010.12.015 Get rights and content

Abstract

This paper introduces a simplified model for underground temperature prediction in summer hot weather. The data of 14 observation sites show that the surface temperature curves are close to trapeziums, and surface temperatures are related to air temperatures. Therefore, approximated temperature trapeziums that are determined by high- and lowest air temperatures can be used to simulate the underground temperature variation. Two observation sites respectively in the urban and suburban areas were used as examples. Good agreement was obtained between simulated- and measured temperatures. Measured data indicate the average temperature under urban concrete surface is 3.70 °C greater than that of suburban bare surface. The deviation is due to the heat urban environment effect and different surfaces effect, which are about 1.68 °C and 2.02 °C, respectively. Combined with soil volumetric water content (w_v), ‘Heat’ Islands associates with ‘Dry’ Islands, which means urban soil moisture is lower than suburban soil moisture (13.9%). According to the variation of w_v and temperature deviation graphs, Urban Heat Island, ground surface types and rainfall are important factors that influence the underground soil moisture and temperatures.

Introduction

In an age of urbanization and global warming, the influence of Urban Heat Island (UHI) is more and more significant [10], [19]. Temperature variation impacts on the engineering properties of urban soils, and consequently changes the strength and stability of various engineered structures. Many previous studies show that the increase of shallow soil temperature influences soil permeability, suction of unsaturated soils, and soils shear strength etc [11], [24]. Underground temperature prediction therefore plays an important role in the research of issues related to environment, energy sources and engineering [7], [16], [22].

The causes of UHI are not the same in different climates or city features. Therefore, many models and approaches, including observation and simulation techniques, have been proposed to understand the causes of UHI formation and to mitigate the corresponding effects [9], [15], [26]. Santamouris [21] reviewed observational studies of UHI for European cities. Mirzaei [15] presented a review of the techniques used to study UHI, and discussed the abilities and limitations of each approach for the investigation of UHI mitigation and prediction. Huang et al. [9] investigated the diurnal changes of urban and rural air temperatures in four types of ground cover and Urban Heat Island of Nanjing. Their study indicates that the Urban Heat Island Intensity varies in time and on different surface types. However, surface temperatures, underground temperatures, and corresponding UHI effect are not involved.

Continuous monitoring of underground temperature is expensive. Analytical methods and computer technologies can provide cheaper, alternative ways to predict the underground temperatures. However, the underground temperature field is influenced by many factors, including solar radiation, air temperature, wind speed, rainfall, shelter, and soil properties [5], [14]. Most of these factors change irregularly and as a result, the prediction and estimation of underground temperature is complex. In previous studies, many analytical models [23], semi-analytical models [6], [27], empirical models [1], [2], numerical methods [12], [18], Fourier models [8] and neural network methods [3] have been used to solve various heat transfer problems [17]. Most of these methods require many measured parameters, some of which are hard to obtain under real field conditions.

On the basis of the relationship between air temperatures and surface temperatures, a simplified model is introduced to simulate the variation of underground soil temperatures during summer hot weather. The method has been used to estimate the underground temperatures of concrete and bare soil surfaces in two sites of Nanjing (China). The simulated temperatures are compared with the measured data to investigate the influence factors of the Urban Heat Island. Combined with the field moisture data, the influences of UHI, surface types and rainfall infiltration on the underground temperatures and moisture are analyzed.

Section snippets

Field temperature survey

Nanjing (East China, 32.04°N, 118.78°E) is the capital of Jiangsu province and situated in one of the largest economic zones of China, the Yangtze River Delta. The city’s total land area is 6598 square kilometers and the urban population is over 7 million. Seasons are distinct in Nanjing, with usually hot summers and plenty of rainfall throughout the year.

The main purpose of the field survey is to detect the diurnal variation of air temperatures and surface temperatures with concrete surface

Numerical simulation

Underground temperatures are influenced by solar radiation, air temperature, wind speed, and shelter, etc. Essentially, these factors act on the ground surface and then affect the underground temperature field. In order to simulate the underground temperature variations, we will make three assumptions: (1) the soil is homogeneous, and as a result, heat only flows in vertical direction; (2) no underground heat source; (3) rainfall infiltration is not considered. Therefore, the corresponding heat

Measured- and simulated temperatures

As shown in Fig. 3a and b, both the measured- and simulated-data indicate a high correlation between underground temperatures and air temperatures. The shallow underground temperatures fluctuate with the air temperature, and the amplitude decreases with increasing depth. The temperature amplitude is greater for the concrete surface in comparison with that of the soil surface, since both the air temperature amplitude and thermal diffusivity are much greater for concrete surfaces. Specifically,

Conclusions

Based on the field survey of air- and surface temperatures, a simplified method is introduced to simulate the variation of underground soil temperatures. According to the transient heat conduction differential equation and the relationship between air- and surface temperatures, only air temperatures at 14:00 h and diurnal lowest air temperatures are required in this model. This method was used in the numerical simulation of soil temperatures under urban concrete- and suburban soil surfaces. The

Acknowledgements

The authors would like to give special thanks for Dr. Sue Struthers to correct this paper in English writing. Financial support from key project of Natural Science Foundation of China (NSFC, NO. 40730739) and College Graduate Student Innovation Program of Jiangsu Province (CX09B_011Z) is gratefully acknowledged. Thanks to the anonymous reviewers for many valuable comments that have notably helped us improve the manuscript.

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A numerical and field investigation of underground temperatures under Urban Heat Island

Abstract

Introduction

Section snippets

Field temperature survey

Numerical simulation

Measured- and simulated temperatures

Conclusions

Acknowledgements

Building and Environment

Energy and Buildings

International Communications in Heat and Mass Transfer

Energy and Buildings

Solar Energy

Renewable Energy

Agricultural and Forest Meteorology

Building and Environment

Science of the Total Environment

Applied Clay Science

International Communications in Heat and Mass Transfer

Energy and Buildings

Building and Environment