PNNL

Abstract

The urban heat island (UHI) effect – the temperature difference between urban areas and their surrounding rural areas – has unique characteristics for arid cities. Compared to cities in other climate zones, the UHI intensity in arid regions can be small, and sometimes even negative (the so-called "urban cool island (UCI)" effect). Variations in urban and rural definitions can induce uncertainty in the detection of UHI or UCI, and this is particularly important when the UHI signal is small. Here, we compare the surface UHI intensity (SUHII) estimated by a dozen different methods (with diverse urban and/or rural definitions) across 103 arid cities globally, providing a comprehensive evaluation of the uncertainty in SUHII estimates. Results show that the absolute difference in annual average SUHII (?SUHII) among methods exceeds 1 °C in over half of the arid cities during both daytime and nighttime. The overall annual mean ?SUHII for all arid cities is 1.41 °C during daytime and 1.09 °C at night. The uncertainty arising from simultaneous variations in urban and rural definitions is generally higher than that resulting from their individual changes. It is observed that, with varying definitions of urban and rural areas, nearly 50% of arid cities experience a sign reversal in daytime SUHII estimates, while approximately 10% exhibit a sign reversal in nighttime SUHII. The substantial fluctuation in LST, arising from land cover changes due to variations in urban and rural definitions, is a crucial factor contributing to the uncertainty in SUHII estimates. Overall, our results offer new insights into the ongoing debate on heat and cold islands in arid cities, emphasizing a need to standardize SUHII estimation frameworks.