Abstract
The influence of land surface temperature (LST) makes the near-surface layer of the troposphere a key driver of urban climate. This paper assesses the temporal formation of the daytime Surface Urban Cool Island (SUCI) and night-time Surface Urban Heat Island (SUHI) effect in Erbil, Iraq, situated in a semi-arid climate region. LST retrievals from the Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua and Terra and MODIS Normalized Difference Vegetation Index (NDVI) from January 2003 to December 2014 are analysed. The relationships of LST with NDVI and the Normalized Multi-band Drought Index (NMDI) are investigated in order to assess the influence of vegetation and moisture on the observed patterns of LST and the SUCI/SUHI. The results indicate that during the daytime, in summer, autumn and winter, densely built-up areas had lower LST acting as a SUCI compared to the non-urbanised area around the city. In contrast, at night-time, Erbil experienced higher LST and demonstrated a significant SUHI effect. The relationship between LST and NDVI is affected by seasonality and is strongly inverted during spring (r2 = 0.73; p < 0.01). Contrary to previous studies of semi-arid cities, a SUCI was detected, not only in the morning, but also during the afternoon.
Highlights
Since land surface temperature moderates the lowest layer of the atmosphere, it plays an important role in the evolution of the urban climate
At night the situation is reversed and land surface temperature (LST) in the city is higher than the rural surroundings and a Surface Urban Heat Island (SUHI) is experienced
The results presented here demonstrate a different dynamic behaviour in the semi-arid climate of Erbil, where the strongest SUHI occurs at night, confirming earlier results by Cheval et al [63], and turning into a Surface Urban Cool Island (SUCI) during the day
Summary
Since land surface temperature moderates the lowest layer of the atmosphere, it plays an important role in the evolution of the urban climate. The variation between the characteristics of surface cover in urban and rural areas, in terms of three-dimensional geometry of the built environment, heat absorption, construction materials, surface albedo and vegetation abundance cause different air and surface temperatures in a city relative to the surrounding less urbanised area. The term “Surface Urban Cool Island” (SUCI) is defined as an urban area where lower surface temperatures are prevailing compared to the non-urbanised dry surroundings and is commonly found in arid and semi-arid regions [2,3]. The Urban Heat Island (UHI) effect refers to areas of higher air and surface temperatures experienced in a city compared to the rural surroundings [4,5,6]. Satellite sensors can be used to investigate the surface temperature of cities and study the growth of SUHIs and their spatial variation [7].
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