Abstract
Urbanization may exert a strong influence on both near-surface and atmospheric moisture. However, studies on the effect of urbanization on atmospheric moisture using remotely sensed observations have been infrequently conducted. To fill this research gap, we used remotely sensed and station-based observations to calculate urban–rural differences in total-column, near-surface, and atmospheric moisture in the Beijing metropolis. Multiple humidity indicators were selected, including precipitation water vapor, relative humidity, water vapor pressure, and absolute humidity. Results showed that both the total-column and near-surface urban dry islands (UDIs) were detected in Beijing. A significant decreasing trend of the near-surface UDIs intensity (i.e., urban minus rural) was found, which was closely related to the expansion of built-up areas (i.e., the moisture reduction in the newly urbanized areas). However, the trend of the total-column UDIs intensity was not significant. A further investigation of the atmospheric urban–rural moisture differences showed that the UDIs effect mainly existed in the near-surface layer, while an urban wet island was found above the 950 hPa pressure level, probably due to enhanced convergence in urban areas. Generally, the UDIs intensity was the strongest in the hot, wet summer and the weakest in the dry, cold winter. However, the UDIs intensity represented by relative humidity was the largest in autumn and showed the strongest correlation with the expansion of built-up areas in Beijing. This study employed satellite observations to understand the UDIs effect and highlighted the significance of urbanization-induced moisture changes on and above the ground. Findings of this study provided new insights into how urbanization affected atmospheric moisture in the boundary layer and paved the way for process-based modeling studies.
Highlights
Introduction iationsUrbanization, the most evident expression of human modification to the Earth’s terrestrial surface, has been recognized to be a highly significant driver of local and regional climate change [1,2,3,4]
Results show that the UDIIpwv after 2008 became stronger than that during 2003–2007, when the absolute urban–rural differences in PWV were no more than 35 mm in general (Figure 3)
Compared with previous studies focused on near-surface urban dry islands (UDIs) using station-based observations, this research made a comprehensive assessment of total-column, near-surface, and atmospheric UDIs in the Beijing metropolitan region, using both remotely sensed and station-based observations
Summary
Urbanization, the most evident expression of human modification to the Earth’s terrestrial surface, has been recognized to be a highly significant driver of local and regional climate change [1,2,3,4]. Urban environments only occupy a relatively small portion of our planet’s land territory (i.e., 0.63% in 2010 [5]), they are home to about 55% of the world’s population and produce more than 70% of global energy use and associated emissions [6]. Apart from excessive anthropogenic heat emissions, the decreased vegetation cover, increased impervious surfaces, high heat-storage capacity, and impediments to atmospheric motion from urban construction modify the surface energy budget, hydrological cycling, and planetary boundary larger structure, leading to changes in temperature, air circulation, humidity, Licensee MDPI, Basel, Switzerland.
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