Correlation between Land surface temperature and spectral indices through assessment of UHI magnitude in Coimbatore, India

Abstract Rapid urbanization and human activities have dramatically altered urban microclimates, particularly in tropical regions vulnerable to the increasing Urban Heat Island (UHI) phenomenon. This study is needed due to the growing evidence that urban expansion worsens UHI effects, which impact environmental sustainability and urban resilience. In recent decades, urbanization in Tier-2 cities in India has intensified the UHI effect. Coimbatore's growth has exacerbated these challenges, threatening ecological balance and urban quality of life. Therefore, this study systematically examines the relationship between urbanization and UHI intensity through an analysis of Land Use Land Cover (LULC) changes, Land Surface Temperature (LST) trends, and spectral indices (Normalized Difference Vegetation Index (NDVI), Normalized Difference Built-up Index (NDBI), and Normalized Difference Moisture Index (NDMI)) over 28 years from 1994 to 2022. Supervised classification for LULC mapping, integration of spectral indices, and scatter plot correlation analyses are employed to identify the relationships between LULC transformations and UHI dynamics, highlighting the Urban Hotspots (UHS). The results reveal an increase of 92.7 km² in built-up areas, alongside a decline of 36.7% in vegetation cover. Over the study period, LST rose from 29.18°C in 1994 to 40.42°C in 2022, indicating an annual average temperature increase of 0.28°C. The spectral indices reinforce these findings, illustrating decreasing vegetation (NDVI), expanding urban surfaces (NDBI), and reduced moisture levels (NDMI), which significantly correlate with the intensification of UHI effects. By utilizing Urban Thermal Field Variation Index (UTFVI) and UHI index, the study identifies a consistent upward trend in UHI intensity, especially in densely built-up and barren regions. UHS analysis identifies the northeastern and northwestern parts of the city as areas of rapid urbanization and thermal stress. The study's findings serve as a crucial resource for policymakers and urban planners in tackling the growing challenges of urban thermal stress in rapidly developing tropical cities.