Abstract
As is true in many regions, India experiences surface Urban Heat Island (UHI) effect that is well understood, but the causes of the more recently discovered Urban Cool Island (UCI) effect remain poorly constrained. This raises questions about our fundamental understanding of the drivers of rural-urban environmental gradients and hinders development of effective strategies for mitigation and adaptation to projected heat stress increases in rapidly urbanizing India. Here we show that more than 60% of Indian urban areas are observed to experience a day-time UCI. We use satellite observations and the Community Land Model (CLM) to identify the impact of irrigation and prove for the first time that UCI is caused by lack of vegetation and moisture in non-urban areas relative to cities. In contrast, urban areas in extensively irrigated landscapes generally experience the expected positive UHI effect. At night, UHI warming intensifies, occurring across a majority (90%) of India’s urban areas. The magnitude of rural-urban temperature contrasts is largely controlled by agriculture and moisture availability from irrigation, but further analysis of model results indicate an important role for atmospheric aerosols. Thus both land-use decisions and aerosols are important factors governing, modulating, and even reversing the expected urban-rural temperature gradients.
Highlights
To understand the variations in vegetation activities in urban and non-urban areas, 16-day Normalised Difference Vegetation Index (NDVI) available at 250 m spatial resolution was obtained from the MODIS Terra platform for the period 2003–2014
We considered a buffer of non-urban area for each urban area and the size of the buffer varied with the urban area
We considered the pre (February to May) and post (October-January) monsoon seasons for the analysis, and not included the monsoon (June to September) season as the LST data from the MODIS are often missing due to a high presence of cloud covers
Summary
Day and night-time difference in surface temperature between urban and non-urban areas. We find that urban areas located in the semi-arid western India show higher UHI intensity in night during the both pre and post monsoon seasons. The UHI and UCI phenomena in the Indo-Gangetic Plain and semi-arid western India, respectively indicate the role of moisture and vegetation on the surface temperature variability between the urban-core and the surrounding non-urban areas. These model simulations that highlight the role of irrigation on UHI are consistent with observations from both Terra and Aqua sensors of MODIS (Fig. 4d,e) In both day and night-time, urban areas located in irrigation dominated landscapes show the UHI phenomenon (Fig. S7), while in the absence of agricultural crops and irrigation the urban areas show UCI phenomenon. We find that the size of urban areas are not directly associated with daytime UHI/UCI intensity, the night-time UHI intensity appears to have strong associate with the size of the urban-core region (Fig. S11)
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