Urban Heat Island Assessment for a Tropical Urban Airshed in India

Manju Mohan, Yukihiro Kikegawa, B. R. Gurjar, Shweta Bhati, Anurag Kandya, Koichi Ogawa

Centre for Atmospheric Sciences, Indian Institute of Technology-Delhi, Delhi, India.
Department of Civil Engineering, Indian Institute of Technology, Roorkee, India.
Department of Environmental Systems, Meisei University, Hino, Tokyo, Japan.

DOI: 10.4236/acs.2012.22014                

Abstract

There has been paucity of field campaigns in India in past few decades on the urban heat island intensities (UHI). Remote sensing observations provide useful information on urban heat island intensities and hotspots as supplement or proxy to in-situ surface based measurements. A case study has been undertaken to assess and compare the UHI and hotspots based on in-situ measurements and remote sensing observations as the later method can be used as a proxy in absence of in-situ measurements both spatially and temporally. Capital of India, megacity Delhi has grown by leaps and bounds during past 2 - 3 decades and strongly represents tropical climatic conditions where such studies and field campaigns are practically non-existent. Thus, a field campaign was undertaken during summer, 2008 named DELHI-I (Delhi Experiments to Learn Heat Island Intensity-I) in this megacity. Urban heat island effects were found to be most dominant in areas of dense built up infrastructure and at commercial centers. The heat island intensity (UHI) was observed to be higher in magnitude both during afternoon hours and night hours (maximum up to 8.3?C) similar to some recent studies. The three high ranking urban heat island locations in the city are within commercial and/or densely populated areas. The results of this field campaign when compared with MODIS-Terra data of land surface temperature revealed that UHI hotspots are comparable only during nighttime. During daytime, similar comparison was less satisfactory. Further, available relationship of maximum UHI with population data is applied for the current measurements and discussed in the context of maximum UHI of various other countries.

Keywords

Urban Heat Island; Land-Use Land-Cover; Micrometeorological Experiments; Remote Sensing; Anthropogenic Emissions

1. Introduction

An urban heat island is a metropolitan area which is significantly warmer than its surrounding areas [1]. The concept of urban heat island considers the air temperature difference between a city centre and the surrounding area. It is defined as “closed isotherms indicating an area of the surface that is relatively warm; most commonly associated areas of human disturbance such as towns and cities” [2]. As population centers grow in size from village to town to city, they tend to have a corresponding increase in average temperature. This consequently increases building energy demand for air-conditioning in warmer countries like that of tropical regions. This increase in energy demand could result in not only additional generation of anthropogenic heat but also further intensification of heat islands themselves [3]. Urban heat island intensity (UHI) estimations are therefore important in urban planning as well as emission reduction strategies. Studies have shown the emergence of urban heat island phenomenon in the past decade in many developing economies like Indonesia [4], Malaysia [5], Sri Lanka [6], Turkey [7], Oman [8], Chile [9], Mexico [10], Argentina [11], Nigeria [12], Ethiopia [13] and others.

Changes in land use and land cover (LULC), due to increasing population and infrastructure pressures for rapidly growing megacities, play an important role in the development of urban heat islands. A sharp rise in population was recorded in the last decade in Delhi when it reached from 9.42 million in 1991 to 13.78 million in 2001 [14]. Such rapidly increasing population in megacities is associated with somewhat similar growth rates in vehicular population, residential and commercial complexes, industries and other infrastructure resulting into significant changes in LULC and increase in anthropogenic heat emissions. All these changes augur for generating several UHI pockets and shift in dynamics of urban heat island phenomenon. Increasing temperature trends in past few decades have been observed for National Capital Region Delhi [15]. While the increasing temperatures can be a direct consequence of global warming or may be a local phenomenon as a result of LULC changes and anthropogenic heat, they necessitate study of effect of heat island phenomenon that is expected to be greater in the mega-cities.

Few studies have been undertaken in past to analyse the urban climate impact and pattern of heat islands in Delhi [16-18]. Varying heat island intensities from different authors were studied and effects on mean mixing depths were estimated by Mohan [19]. There is lack of systematic studies conducted on urban heat island phenomenon in this megacity in past few decades that includes both in-situ measurements as well as remote sensing data. Satellite data can provide useful information as a snapshot for a wider spatial domain and generally twice daily while in-situ measurements can feasibly cover a selected geographical domain with time resolution of the order of seconds to minutes throughout the day. With this background in consideration, the present study is an attempt at studying spatial temperature variation for several different types of land-use land-cover (LULC) for the assessment of urban heat island effect and comparison of such in-situ ground measurements of this field campaign with those from remote sensing tools [20]. In addition analysis of the results vis-à-vis UHI observations made for various cities of the world has also been undertaken.

2. Description of the Field Campaign

2.1. Study Area Network

The national capital territory of Delhi was chosen as the study area. The entire area of 32 km × 32 km was divided into a grid of 16 cells of 8 km × 8 km. Each grid was allotted one or more micrometeorological stations for surface temperature measurements so as to get a representation of the terrains therein. In all, 30 sites were chosen throughout the city including 3 weather stations (WS) and 27 micrometeorological stations. Figure 1 shows the measurement sites on the map of Delhi. The micrometeorological stations were chosen so as to represent a wide variety of land use/land cover categories. Broadly the stations can be classified into urban built up areas, green areas, open areas and riverside areas.

2.2. LULC Classification of the Study Area

Stewart [21] and Stewart and Oke [22] noted that there is a lack of standard and homogenous system of classification for different urban settings because of the remarkable diversity of landscapes.

Essentially, any LULC classification is based on built up area coverage, building density, green cover, and open

Figure 1. Set up of micrometeorological stations across the study area. A 12 × 12 km sub domain with greater station density is shown in lower part of the grid network. Station locations are marked by symbol with their station numbers (Base Map Source: Google Maps; NH: National Highway).

area coverage. In the present study, classification is based on these features as observed for all micrometeorological stations sites and correlated with LULC classification of the satellite imagery for the study area of Delhi [23,24]. The LULC of the study domain have been classified as:

1) Urban Built-up Areas: It covers areas with signifycant infrastructure such as buildings for residential and commercial purposes. These areas have been sub-classified into three categories based on density of the built up structures in the study domain that corroborates well with the LULC classification for Delhi based on satellite imagery [23,24].

a) Dense Canopy: These are highly built up areas (built up area > 80%) in the city with very few open spaces and narrow lanes. 1 - 2 storey residential or commercial structures, which are generally old, may be intermixed with newer higher storey (up-to 5 storey) buildings. These structures are attached at base or closeset. Built up structures in these areas are usually old and thus, when the new structures come up, they are not built in any planned manner thereby adding to the built-up clusters in these areas.

b) Medium Dense Urban Canopy 1: These are well planned colonies with 3 - 4 story buildings and some higher multistory buildings, commercial areas and shopping malls etc. Built up fraction accounts for 65% - 80% of the total area.

c) Medium Dense Urban Canopy 2: These are well planned colonies with open spaces and parks. Built up area which is about 50% - 65% is mostly covered by 2 - 3 story buildings, shopping centres, small markets and commercial areas etc.

d) Less Dense Urban Canopy: These are the canopies with residential areas intersparsed with plenty of green areas and open spaces. Built up area is between 30% - 50%.

2) Green Areas: These areas include green coverage ranging from medium dense forests to cultivated lands and parks and gardens.

a) Medium Dense Forests: Typical dense forests are non-existent in Delhi. However, the Delhi Ridge stretch is marked with dense lush vegetation and two such areas (Sanjay Van and Buddha Jayanti Park) have been chosen to represent this type of land cover in the present study.

b) Parks and Gardens: These are cultivated green areas characterized majorly by short grass and shrub vegetation represented by District Park Hauz Khas.

3) Open Areas: These areas are open lands with neither any significant built up structures nor any type of extensive natural or cultivated green coverage.

4) Riverside areas: The River Yamuna which flows through the city is expected to influence temperature in neighboring areas. Thus, two riverside areas (Sailing Club and Majnu ka Tila) were also chosen to analyze the river’s impact.

Table 1 details out all the stations classified under the above given groups.

Conflicts of Interest

The authors declare no conflicts of interest.

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