Research on Regional Thermal Environments of in the Summer Based on an Unmanned Airship Low Altitude Thermal Infrared Remote Sensing System

Xu Yuan1, Qinglin Meng, Peng Ren*, Qiong Li Building Environment and Energy Laboratory, State Key Laboratory of Subtropical Building Science, South University of Technology, Guangzhou, China First Author1: [email protected] Corresponding Author*: [email protected]

Abstract—With urbanization, the effect of Urban Heat Island phenomenon is because the heat created by urban buildings and (UHI) becomes increasingly obvious. The Building Environment human activities gathers in urban areas to form a small scale and Energy Laboratory (BEEL) of the South China University of circulation under certain weather situations [3, 10]. In 1833, Technology (SCUT) independently developed an unmanned Lake Howard first discovered and discussed the phenomenon airship low altitude thermal infrared remote sensing system of UHI in his study of the London urban climate [11]. Since (UALTIRSS) and uses it to research urban thermal then, scholars have researched the formation mechanism and environments. In this paper, two low altitude remote sensing distribution characteristics of UHI from many aspects, such as observations are introduced in the Guangzhou Higher Education conventional observation, numerical simulation and remote Mega Center (HEMC) and Sino-Singapore Guangzhou sensing [12-19]. Knowledge City (SSGKC), representing an urban area and suburban area, respectively. Both brightness temperature With the rapid development of the Chinese economy and distributions of the observation areas with 0.8 m accuracy can be the acceleration of urbanization, the scales of cities have been seen directly from the infrared images after stitching. Also, the expanding, and the area of hard ground has been continuously methods of holistic analysis, profile analysis and local analysis are increasing. This increase makes UHI an important used qualitatively to analyze and evaluate the thermal environmental problem in the process of urban development environment of the two areas from different angles. [20]. Urban heat island intensity (UHII) is used to evaluate the Furthermore, the indexes derived from the satellite remote degree of UHI. UHII stands for the air temperature difference sensing field, such as Surface Heat Island Intensity (SHII), between the urban area and the suburbs. In Beijing, the daily Thermal Field Intensity Index (TFII) and Relative Surface Radiation Temperature (RSRT), are converted and applied to average UHII was 3.3 ℃ from 1961 to 2000 [21]. In addition, low altitude thermal infrared remote sensing (LATIRS) the annual probability of the emergence of UHI was up to quantitatively to evaluate the urban thermal environments. The 87.8% in Shanghai in 1998 [22]. UHI reduces the comfort of study’s results show that natural water (Pearl River) is best for people's lives, exacerbates air pollution, and seriously affects regulating thermal environments, while impermeable hard the life quality of residents. How to monitor quantitatively the ground is the worst. We also observed that the indexes are dynamic changes of urban thermal environments has become greatly affected by the reference area. For analyzing thermal an important issue of UHI research [23]. environments more accurately, suitable reference areas should be selected according to specific circumstances. Conventional UHI monitoring is a fixed-point or mobile trolley observation. This monitoring can accurately measure Keywords- low altitude, thermal infrared remote sensing, temperature and is suitable for quantitative research [24]. thermal environment, urban heat island, evaluation index However, this method’s synchronicity and spatial representation are poor and the observation cost is high. Remote sensing has such advantages as good time I. INTRODUCTION synchronization, wide coverage range, is intuitionistic and quantitative research. This approach can reduce costs and Recently, rapid urbanization on a global scale has led to human disturbance to local environments [25]. This method drastic changes in land use and land cover, which have a can also make up for the influence of the sparse distribution of significant impact on urban climates and have become a focus meteorological stations and well reflects the spatial distribution of attention and research [1-4]. An important feature of urban of thermal environments. The temporal and spatial changes of climate change is the changes in urban thermal environments UHI can be easily obtained through remote sensing [23]. [5]. A large number of studies show that the average urban air Considerable research on UHI has been conducted via remote temperature is generally higher than that of rural areas, which sensing. Qin Zhihao et al. deduced the single-window is known as the urban heat island (UHI) [6-9]. This algorithm for calculating land surface temperature using ground station [45]. The integrated system can enable the Landsat TM6 data and verified it [26, 27]. Zhang Yong et al. unmanned airship cruise in GPS automatic mode to complete improved the universal single - channel surface temperature the aerial task through the routes. Flight altitude was set at inversion algorithm deduced by Jimenez-Munoz and Sobrino 600m and the shooting time interval of the infrared thermal [28]. Zhang Yang et al. used TM remote sensing images to camera was set at 10 seconds. BEEL conducted two LATIRS quantitatively analyze the relationship of the vegetation index observations of the HEMC and SSGKC by the UALTIRSS in and the UHI in Wuhan [29]. LABA Ciren et al. discussed the August, 2012. temporal and spatial distribution of UHI in Lhasa [30]. Zhang Zhengdong et al. completed the inversion of planetary Because of the limitations of the equipment, two areas brightness temperature using the single-window algorithm and cannot be observed at the same time. Considering the best analyzed the spatial characteristics of UHI in Guangzhou [31]. approach, adjacent dates and similar weather conditions were With the development of high spatial resolution, high spectral chosen to conduct the observations of the two areas according resolution and high temporal resolution sensors, using thermal to the similar principle of climate. The two days were both infrared remote sensing to research UHI has become a trend. sunny and windless, and the observation times were both around noon. The results can accurately represent the There is a close relationship between urban thermal conditions of LST in summer. environments and the underlying surfaces. Studying the relationship between urban thermal conditions and the underlying surface types is of great importance to mitigate the UHI [32]. The unreasonable layout of urban underlying surfaces is one of the primary factors giving rise to UHI [33]. Yin Qiu et al. performed a comprehensive comparative analysis from the qualitative and quantitative levels, and observed that the surface brightness temperature of the city is closely related to the underlying surface type [32]. It is of great importance to undertake urban planning rationally. Based on differences of research objects, UHI is divided into atmosphere heat island and surface heat island [34, 35]. Land Surface Temperature (LST) is affected by large-scale air mass movement less than air temperature [36]. Compared with the atmosphere heat island, mostly researched using ground meteorological observation data, the surface heat island researched by remote sensing data has a higher heterogeneity in time and space. This island is more sensitive to surface features and human activities [37]. The urban heat island based on LSTs observed by thermal infrared remote sensing can be called the Urban Surface Heat Island (USHI), which shows the spatial pattern of upstream thermal infrared radiation received by the remote sensing sensor [38-41]. Guangzhou is located in the urban agglomeration center of Pearl River Delta. The intensity and scale of UHI are increasing [42]. To research the regional thermal environment of Guangzhou in summer quickly and easily, two LATIRS observations were conducted in this paper. Based on the observations, we analyzed and evaluated the thermal environment of the target area, attempting to provide a Figure 1. Composition of the UALTIRSS. reference for the future planning and design of the city. B． Research area II. RESEARCH METHOD AND AREA The area of the HEMC is about 4.85 km2, located in Xinzao A． Research method Town, Panyu District in Guangzhou, which is shown in Fig. 2. This area contains living areas, teaching areas and large public In 2009, BEEL of the SCUT researched and developed the buildings, containing concrete ground, asphalt pavement, floor first UALTIRSS to observe the urban thermal environments in tile and other hard surfaces, as well as urban greening and the China [43-45], as shown in Fig. 1. Compared with satellite Pearl River. The area of the SSGKC is approximately 6.56 km2, thermal infrared remote sensing, the image resolution of located in Jiulong Town, Huangpu District in Guangzhou, UALTIRSS within 0.8 m is more suitable for observing the which is shown in Fig. 3. In addition to the traffic trunk line, a LST on a small scale. small part of residential areas and an expanse of bare land, the rest of the area is mostly covered by farmland, shrubs, According to the aerial height and field-of-view of the infrared thermal camera, appropriate routes were set at the grassland, trees, vegetation and water. The HEMC and SSGKC underlying surfaces can reveal the overall trend and distribution can stand for an urban area and suburbs, respectively. of the thermal field. This technique can help people to understand the macro characteristics of urban thermal environments.

Figure 2. Observation area of the HEMC (2012.08). Figure 4. Infrared image of the HEMC (2012.08.20).

Figure 3. Observation area of the SSGKC (2012.08).

III. RESULTS AND ANALYSIS Figure 5. Infrared image of the SSGKC (2012.08.23). In this study, 360 and 370 infrared images were obtained during the observations of the HEMC and SSGKC, A．Holistic analysis respectively. Each image covers a small area. All of these images should be stitched together to obtain the infrared image Artificial visual interpretation is used to generally analyze of the whole observation area. After image mosaicking [44], the thermal environments from the two infrared images, where the infrared images of the HEMC and SSGKC are shown in Fig. the temperature information of the underlying surface is 4 and Fig. 5, respectively. The infrared image of urban reflected by color. An appropriate temperature scale, which can be determined according to the temperature distribution of the infrared image, is very important to make the LST expression more comprehensive and accurate. It would be better to set the temperature scale close to the range of the temperature distribution. Furthermore, to research a special area, a particular temperature scale can be set to highlight the area. The temperature scale of the HEMC is set as the interval from 20 ℃ to 80 ℃. From dark blue to bright red, temperatures increase gradually. The color of the infrared image of the HEMC is mostly from green to red. In other words, the temperature is mostly from 40 ℃ to over 80 ℃. Furthermore, the red areas, representing the high temperature values, occupy a large proportion. The temperature scales and color systems of the two images must be the same to make a comparison. Set the two of the SSGKC as the same as those of the HEMC. The color is mostly Figure 7. Temperature of the N2-S2 profile in the HEMC area. at the interval from deep blue to green, representing from 20 ℃ to over 50 ℃. Furthermore, the blue areas, representing the low temperature values, occupy a large proportion. The LST of the HEMC is significantly higher than that of the SSGKC, indicating that the USHI of Guangzhou is obvious in summer. From the two infrared images, it can be obtained that whether in urban areas or suburbs, the LST of developed areas are all clearly higher than that of undeveloped areas. In other words, the temperature of underlying surfaces after artificial construction is higher than that of natural underlying surfaces.

B．Profile analysis

Figure 8. Temperature of the E1-W1 profile in the HEMC area.

Figure 6. Temperature of the N1-S1 profile in the HEMC area.

Figure 9. Temperature of the N1-S1 profile in the SSGKC area. 4) The areas of vegetation, trees and landscape water are also "valleys" in temperature (mostly from 25 ℃to 35 ℃), indicating that greening and landscape water are also important in balancing the urban thermal field. 5) Asphalt pavement, floor tile and other such hard ground areas are “highlands” in temperature (mostly higher than 45 ℃), indicating that artificial underlying surfaces more easily cause the effect of surface heat island. 6) In general, the LST of suburbs changes gently, indicating that natural underlying surfaces are good at self- regulation in thermal fields.

C．Local analysis Several key or interesting areas can be analyzed separately Figure 10. Temperature of the N2-S2 profile in the SSGKC with local analysis to obtain several conclusions in detail. In area. this paper, we selected the areas of A1, A2, B1 and B2 as examples for local analysis. The area A1, in Fig. 3, is the village-in-city in the HEMC. In this area, the buildings are intensive, the roads are narrow and the greening is less. The area A2 is the student dormitories of in the HEMC. Compared with area A1, the greening is better, and the distance between buildings is wider. Contrasting the temperature distribution of area A1 and area A2, it can be seen that area A1 is almost covered by red and yellow, which stand for high temperatures, and has hardly any transition area. In area A2, the space between buildings is in blue, representing low temperatures. There are more temperature transition areas to regulate the thermal environment. The influence of thermal environments was better considered better in the planning and designing of area A2. Figure 11. Temperature of the E1-W1 profile in the SSGKC Area B1 and area B2 are the Science Center area. and the library and archives of Guangzhou University in the To further analyze the structure of thermal fields, profile HEMC, respectively. Both areas are “highlands” in temperature, analysis is a good method. Considering the distribution caused by large buildings and large floor tile squares. characteristics of the LST and the types of underlying surfaces, The analysis indicates that when the building intervals are six section lines in the N-S direction and the W-E direction larger and greening is better, the average value of LST is lower. were drawn in total, which are shown in Fig. 4 and Fig. 5. The Transition areas can well regulate the thermal fields. temperatures of the section positions are shown from Fig. 6 to Furthermore, for evacuating people, a large public building Fig. 11. often contains a large square, and effective measures are From these figures it can be obtained that the thermal fields lacking to regulate outdoor thermal environments. Therefore, of Guangzhou have the following characteristics: these places easily become the “highlands” in LST and more attentions needs to be paid to the designing and improvement 1) Urban underlying surface temperature (mostly from of thermal environments. 30 ℃to 75 ℃) is significantly higher than that of the suburbs (mostly from 20 ℃ to 45 ℃). IV. EVALUATION INDEX 2) There are "plateaus" and "steep walls" in urban land surface temperature, as well as uneven grooves. This A．Surface heat island intensity (SHII) characteristic indicates that the LST of urban areas greatly SHII can be defined as the difference between urban land varies due to the different types of underlying surfaces. surface temperature and suburban land surface temperature, 3) The temperature curve of the Pearl River area is the reflecting the extent to which the LST of urban areas is higher "valley" in temperature (approximately 25 ℃). Furthermore, than that of suburbs. Formula (1) is the expression of the SHII. the scope is wide and the temperature is the lowest in the urban area. This finding reflects that natural water has a low temperature characteristic and is of great importance in (1) [46] balancing the urban thermal field. The meanings of these symbols are as follows: is the The meanings of these symbols are as follows: is the SHII corresponding to point in the infrared image, is the RSRT of point in the urban area, is the surface temperature surface temperature of point , is the number of effective of point , and is the average temperature of the urban area pixels in suburban farmland area, and is the surface and suburban farmland area. According to Sun [47], the temperature of suburban farmland. farmland area, obtained by equidistant extension from the city center to the outer, should be equal to the urban area. For selecting the suburbs, three requirements should be satisfied: first, the area should contain farmland; second, the Because the observation area of LATIRS is limited and the structure and soil properties of farmland should be stable; and distribution is relatively dispersed, it is difficult to obtain the third, the farmland should be flat, containing at least 5*5 pixels farmland as the same area as the city by equidistant extension in the infrared image [46]. from the city center to the outer. To apply the index proposed for satellite remote sensing to the LATIRS, we adopted a fuzzy The LATIRS has high precision such that a piece of approach to make a conversion. The observation area of the farmland may contain hundreds pixels under normal HEMC is taken as the benchmark to calculate the average circumstances in the infrared image. Temperature distributions temperature of the urban area. Almost the same number of of the same kind of underlying surfaces change little in the pixels are selected in the SSGKC area to calculate the average adjacent areas. Considering the sensitivity of LATIRS to temperature of the suburbs. At the same flight altitude, the temperature, the SHII of an area with many pixels is more same number of pixels represents the same area. The average accurate and representative than that of a single point. One area surface temperature of area can be used for , then II can be can be analyzed as a whole to obtain the average value of the seen as the RSRT of area . SHII corresponding to this area. The average surface temperature of area I can be used for , V. CONCLUSIONS AND OUTLOOK then can be the SHII corresponding to area . One area can be In this paper, two LATIRS observations were conducted analyzed as a whole to obtain the average value of the SHII using the UALTIRSS and a qualitative analysis method was corresponding to this area. established to analyze the regional thermal environment of Guangzhou in summer from a holistic aspect, profile aspect B．Thermal field intensity index (TFII) and local aspect. The analysis indicates the following: The TFII is defined as the normalization of surface thermal 1. In Guangzhou, the LST of the urban area is obviously fields. It can make a clear judgment on the relative high higher than that of the suburbs, reflecting the obvious effect of temperatures and low temperatures. The formula of TFII is as USHI. follows: 2. Because of the differences in underlying surfaces, the LST intensely fluctuates in the urban areas of Guangzhou. In contrast, the LST of the suburbs has a smooth change, (2) [46] reflecting the good self-regulation of natural underlying surfaces. The meanings of these symbols are as follows: is the TFII corresponding to point in the infrared image; is the 3. For the research areas, large public buildings have less surface temperature of point ; is the effective minimum effective measures to regulate the thermal environment in the surface temperature; and is the effective maximum surrounding areas, and more attention needs to be paid to the surface temperature. thermal environment. The symbols are subsequently converted to be more The analysis method is feasible and can be used suitable for the LATIRS as follows: is the TFII systematically and qualitatively to analyze the thermal corresponding to area in the infrared image, is the average environments from infrared images. surface temperature of area , is the average surface Furthermore, Thermal Field Intensity Index (TFII) and temperature of the area that has the lowest temperature, and Relative Surface Radiation Temperature (RSRT), are converted is the average surface temperature of the area that has the and applied to low altitude thermal infrared remote sensing highest temperature. Then can be obtained as the TFII (LATIRS) quantitatively to evaluate the urban thermal corresponding to area , environments. The SHII can directly reflect the harsh degree of the thermal environment of an area compared to the suburbs. The TFII can directly reflect the temperature level of an area in a whole infrared image. The RSRT can reflect the temperature ． C Relative surface radiation temperature (RSRT) level of an area compared to the average temperatures of urban Sun Sa-mei et al. [49] introduced the concept of RSRT to areas and suburbs. express the strength of UHI. The formula of RSRT is as follows: ACKNOWLEDGMENT This work was financially supported by by the National Natural Science Foundation of China (Grants No. 51778237, (3) 51590912), the China Postdoctoral Science Foundation funded project (2014M562178) (2014M560662), Natural Science Foundation of Guangdong Province of China Applied Meteorological Science, vol. 14, pp. 61-68, 2003. (2015A030306035), Hong Kong Scholars Program [16] H. Fan and D. J. 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