Conversion of agricultural lands to urban uses affects regional and global climate not only through the release of greenhouse gases but also through altering land surface physical processes such as energy and water balances. Most existing studies on the meteorological impacts of urbanization focus on urban heat island effects with little attention on its impacts of atmospheric humidity, a key variable in hydrometeorology and climate science. We define the influences of urbanization on reducing atmospheric humidity and elevating vapor pressure deficit as urban dry island (UDI) effects. We conduct a case study in the Yangtze River Delta, a typical humid area in southern China that is under rapid urbanization. We examine spatiotemporal characteristics of UDI and identify potential drivers during 2001–2014. Relationships and interactions between variations of air temperature, atmospheric humidity, evapotranspiration, and leaf area index of different land cover were determined using correlation and attribution analyses at both station and regional levels. We show that atmospheric humidity decreased dramatically and vapor pressure deficit increased sharply in the urban core, resulting in enhanced UDI. In addition to global warming and localized urban heat island, UDI is closely related to the loss of vegetation cover (i.e., natural wetlands and paddies). Reduction of evapotranspiration or latent heat is another important factor contributing to UDI effects. We conclude that the role of vegetated land cover and associated ecohydrological processes in moderating UDI and maintaining a stable climate and environment should be considered in massive urban planning and global change impact assessment in southern China.
Key Points: • Proposed urban dry island (UDI) to characterize urbanization effects on reducing atmospheric humidity and elevating vapor pressure deficit • Enhanced UDI effects are explained by feedback of the ecohydrological cycle to urban land cover change • Urban heat island and UDI effects are coupled and should be collectively addressed in urban planning and climate change assessment
Hao, Lu; Huang, Xiaolin; Qin, Mengsheng; Liu, Yongqiang; Li, Wenhong; Sun, Ge. 2018. Ecohydrological processes explain urban dry island effects in a wet region, southern China. Water Resources Research. 54(9): 6757-6771. https://doi.org/10.1029/2018WR023002.