Evaluating urban heat island in the critical local climate zones of an Indian city

Highlights

• Canopy layer urban heat island (UHI) evaluation using local climate zone (LCZ).

• Need for data filtration before UHI analysis is emphasised.

• Mapping LCZ subclasses is important in heterogeneous built type city.

• Winter UHI of 1.76–4.09 °C in Nagpur City.

• The procedure helps in identifying critical LCZ for intervention.

Abstract

The rise in global temperature and rapid pace of urbanisation is fuelling the need to understand cities and its climate implication. Measuring urban heat island (UHI) have significant contribution in understanding this inter-relationship. This paper aims at measuring canopy layer heat island (CLHI) in a compact city of Nagpur, India using local climate zone (LCZ) classification. It also accesses the inter-LCZ temperature difference within the city and identifies critical areas that require intervention to curb heat island. The methodology describes LCZs mapping, data collection technique using fixed station points and mobile traverse survey conducted during the month of December 2015 and February 2016 of winter season. The paper adopts rigorous data filtering technique to avoid the errors in reporting UHI of cities having compact and heterogeneous built form like Nagpur. The study covers temperature buffer analysis, sensor lag determination, forecasting, outlier analysis and Pearson – correlation technique. It also examines the thermal anomaly in different LCZ with respect to cities average for identifying critical areas. The result shows that UHI intensity within built LCZ (IUHI), in winter season for Nagpur city, ranges from 1.76 to 4.09 °C. The compact low-rise LCZs present at the urban core were found to be warmer than other major LCZ present in the inner areas of the city. The study also reports thermal variation between traditional LCZs and the LCZs with subclasses. The study concludes with identification of critical LCZs in terms of IUHI and suggests the need for intervention.

Introduction

Rapid pace of urbanisation in developing countries is leading to modification of land cover, urban fabric and urban geometry. It is also increasing the rate of urban heat absorption (Chen, Chiu, Su, Wu, & Cheng, 2017) which in turn is affecting climate of the city (Eliasson, 2000). With changes in the urban form and function, cities are modifying the overlying climate (Mills, 2007). Studies show that urban areas play a significant role in tending climate change (Kalnay & Cai, 2003). Urban heat island effect (UHI) is widely studied to examine the city’s contribution in rising temperature (e.g., Arnfield, 2003; Chow & Roth, 2006; Eliasson, 1996; Kim & Baik, 2005; Klysik & Fortuniak, 1999; Landsberg, 1981; Oke & Maxwell, 1975; Roth, 2007; Santamouris & Kolokotsa, 2016). UHI is defined as near surface air temperature difference between urban and it’s surrounding rural areas (Oke, 1982). It can be classified into three types: i) canopy layer heat island (CLHI) ii) boundary layer heat island (BLHI) and iii) surface heat island (SHI) (Voogt and Oke, 2003). CLHI deals with the air temperature between ground surface and roof height of buildings. It provides an intrinsic representation of human and energy interactions postulating the resultant urban climate. Understanding CLHI would enable urban planners and decision makers to identify the UHI critical areas and propose suitable mitigation methods.

Majority of research work investigates UHI study using the thermal difference between urban-rural class, vegetation-built density and pervious-impervious fraction classes of land cover (Stewart, 2007, Stewart, 2011). Urban interventions for mitigating UHI requires intrinsic land cover study, which considers major aspects of physical, geometrical and anthropogenic factors at the local scale. Based on these parameters, Stewart and Oke (2012) developed a land cover classification system called as Local Climate Zone (LCZ). LCZ classification has two categories of land cover—built and natural. It is further subdivided in 17 classes based on various geometrical and physical aspects (Stewart & Oke, 2012). Applicability of LCZ classification for different UHI study domains is well established by various researchers (Alexander & Mills, 2014; Alexander, Fealy, & Mills, 2016; Leconte, Bouyer, Claverie, & Pétrissans, 2015; Perera & Emmanuel, 2016; Stewart, Oke, & Krayenhoff, 2014; Savić, Milošević, Marković, Bajšanski, & Šećerov, 2016). Kochi (India), Delhi (India), Colombo (Sri Lanka) and Hong Kong are some of the tropical cities that have also evaluated UHI using LCZ framework (Perera & Emmanuel, 2016; Siu & Hart, 2013; Sharma, Hooyberghs, Lauwaet, & De Ridder, 2016; Thomas, Sherin, Ansar, & Zachariah, 2014).

This study aims at i) estimating the CLHI for the heterogeneous built city Nagpur, Maharashtra, India using the local climate zone classification and ii) identifying critical LCZs in terms of UHI for planning interventions.