Temporal–Spatial Patterns of Relative Humidity and the Urban Dryness Island Effect in Beijing City

Abstract

Hourly datasets obtained by automatic weather stations in Beijing, China, are developed and employed to analyze the spatial and temporal characteristics of relative humidity (RH) and urban dryness island intensity (UDII) over built-up areas. A total of 36 stations inside the sixth ring road are considered as urban sites, while six stations in suburban belts surrounding the built-up areas are taken as reference sites. Results show that the RH is obviously smaller in urban areas than in suburban areas, indicating the effect of urbanization on near-surface atmospheric moisture and RH. A further analysis of relations between RH and temperature on varied time scales shows that the variations in RH in the urban areas are not due solely to changes in temperature. The annual and seasonal mean UDII are high in central urban areas, with the strongest UDII values occurring in autumn and the weakest values occurring in spring. The diurnal UDII variations are characterized by a steadily strong UDII stage from 2000 to 0800 LT and a minimum at 1500 or 1600 LT. The rapid shifts of UDII from high (low) to low (high) occur during the periods 0800–1600 LT (1600–2000 LT). The occurrence time of the peaks varies among different seasons: the peaks appear at 0700, 2100, 2000, and 0800 LT for spring, summer, autumn, and winter, respectively. Further analysis shows that large UDII values appear in the evenings and early nights in late summer and early to midautumn and that low UDII values mainly occur in the afternoon hours of spring, winter, and late autumn.

Urban Expansion and Drying Climate in an Urban Agglomeration of East China

“…A negative (positive) UDII value for RH, E a , and Q and positive (negative) UDII value for VPD indicate that urban area is drier (wetter) than its surrounding rural area. This definition is opposite to that by Yang, Ren, and Hou (2017) who defined UDII as the rural-minus-urban difference.…”

“…Sakakibara () and Moriwaki et al () reported significant differences in water vapor levels between urban and rural areas of Japan. Some investigators also noticed that the urban environment in Beijing, China, is characterized by low RH levels (Liu et al, ; Yang et al, ). However, it remains unclear whether other parts of China (particularly the moister regions) also exhibit the UDI effect.…”

Amplified Urban Heat Islands during Heat Wave Periods

“…The RH under heat waves were lower than that under normal conditions for most hours in all three cities. This phenomenon is also called the urban dry island, which can be intensified by urban expansion as well (Hao et al, 2018;Luo & Lau, 2019;Yang, Ren, et al, 2017). The RH in Shanghai was higher than that in Beijing and Guangzhou, which could explain the highest AT in Shanghai.…”

“…A heat wave is a period of consecutive hot days (Kuglitsch et al, ) and is recognized as one of the deadliest natural disasters (Poumadère et al, ; Semenza et al, ). More frequent and severe heat waves are expected to occur in the future due to the background of continued global warming (Lewis et al, ; Meehl & Tebaldi, ) and urbanization (Yang, Ruby Leung, et al, ). Approximately 54% of the world's population already live in cities, and this number is predicted to reach 70% by 2050 (United Nations, ).…”

The Interplay Between Ozone and Urban Vegetation—BVOC Emissions, Ozone Deposition, and Tree Ecophysiology

“…The third effect on physiology that is not necessarily stress-related (if considering stress as damage that is not immediately reversible) is related to air humidity and soil moisture availability. Both are commonly lower in urban centers compared to urban woodlands and rural areas (Devakumar et al, 1999;Yang et al, 2017). Restricted water supply will decrease stomatal conductance and thereby stomatal ozone depletion and also modify BVOC emission rates as discussed above.…”