DIURNAL CHANGES IN SURFACE URBAN HEAT ISLANDS AND THEIR RELATIONSHIPS WITH URBAN LAND COVER

Abstract

Urbanization, a prominent phenomenon since the industrial revolution, has led to significant changes in land use and land cover (LULC), resulting in the Urban Heat Island (UHI) effect. This effect, characterized by higher temperatures in urban areas compared to their rural surroundings, is influenced by the use of heat-absorbing materials, human activities, and urban geometry. The resulting high temperature affects urban residents with implications for public health, greater energy consumption, and change in local climatology. Understanding the spatial variations in land surface temperature (LST) and the thermal effects of heterogeneous urban forms is essential for developing mitigation strategies to enhance the urban thermal environment.

A modern method introduced in the past decade in urban climatology is the concept of the local climate zone (LCZ). This approach disaggregates the heterogeneity in urban settings and classifies surfaces into various built and natural land cover types, which in turn helps to interpret the patterns of LST in the local surrounding area. Previous studies have shown that different urban structures, such as residential areas, commercial zones, and industrial regions, exhibit varying surface temperature patterns. In developing countries, urban planning often results in heterogeneous urban landscapes with mixed land uses, further complicating the UHI dynamics. This study investigates the relationship between various LCZs and their LSTs in the cities of Ahmedabad and Surat in the state of Gujarat in India. The LCZ map used in this study was created from a fusion of two data sets: 1) a freely available 100m global LCZ map, and 2) a 30m land use land cover (LULC) map developed using Landsat-8 data.

A few studies have explored the diurnal variations of LST over urban areas, mainly due to the revisit cycles of polar orbiting satellite (e.g., Landsat Series, Terra, Aqua). However, in this study, we leverage LST data from The National Aeronautics and Space Administration's (NASA) latest Ecosystem Spaceborne Thermal Radiometer Experiment on Space Station (ECOSTRESS) that provides LST at 70m resolution at different diurnal periods. Then we studied how LST changes within different LCZs at various times of day to understand the thermal effects of urban landscapes.

The results reveal distinct seasonal and diurnal variations in LST. During winter, Surat exhibited consistently higher temperatures than Ahmedabad, with differences averaging around 5°C. In summer, temperatures rise significantly in both cities, peaking at 40°C in Surat and 45°C in Ahmedabad. Several LST hotspots were identified, particularly in industrial areas and densely built-up zones. For instance, an international airport in Ahmedabad showed temperatures up to 52°C on summer afternoons, significantly higher than the surrounding areas.

Distinct thermal patterns were observed across LCZs during different diurnal periods. Compact midrise areas were the hottest, while open lowrise areas were cooler during both seasons and across all diurnal cycles in both cities. Large lowrise areas were typically hotter than compact lowrise areas in the mornings and afternoons. However, large lowrise areas cooled faster, making them cooler than compact lowrise areas during the evenings and nights. In both cities, compact midrise and compact lowrise areas exhibited the highest temperatures in the evening and nighttime.

The study also analyzed the diurnal variations in LST gradients from the center to the edges of the city, revealing that during summer mornings, there was a negative correlation between LST and distance from the city center. This correlation weakened in the afternoon but strengthened again in the evenings and nights, particularly in Ahmedabad. Both cities showed stronger negative correlations in the evenings and nights, indicating a pronounced UHI effect while in the afternoon there was no correlation as the solar radiation was heating the surface uniformly.

This study emphasizes the value of using the new ECOSTRESS LST products for analyzing diurnal thermal variations with fine spatial resolution. These findings offer an understanding of the impact of various urban structures on local climate aiding city planners and developers in implementing informed heat mitigation strategies. However, the use of ECOSTRESS data comes with limitations, such as the day and night LST data not being from the same day, causing inaccuracies and uncertainties. Additionally, cloud cover can result in missing grid cells, and variations in viewing angles due to the ISS's orbit can affect the accuracy and consistency of LST measurements.