Air humidity characteristics of local climate zones: A three-year observational study in Nanjing: Fid-Bau Portal

Air humidity characteristics of local climate zones: A three-year observational study in Nanjing

Yang, Xiaoshan / Peng, Lilliana L.H. / Chen, Yuan / Yao, Lingye / Wang, Qingqing

Abstract Atmospheric humidity in the urban canopy layer is relevant to various fields, such as bioclimatology and public health. However, considerably less research has been conducted about urban effects on humidity than on temperature. This study aims to elucidate the spatiotemporal characteristics of humidity at the neighborhood scale. The humidity differences of 8 local climate zones (LCZs) in Nanjing, China, were analyzed based on three-year hourly observations. LCZ D (low plants) was used as the reference site. Differences in temperature (ΔT X−D), relative humidity (ΔRH X−D), and humidity ratio (ΔW X−D) were investigated. The relationships of ΔW with condensation (dew and frost) and precipitation events, meteorological parameters, and ΔT were examined. The results show that variations in ΔRH X−D exhibited a close inverse relationship with variations in ΔT X−D. Greater negative ΔRH X−D occurred during the nighttime when strong heat islands were present. Meanwhile, diurnal and seasonal patterns of ΔW X−D were clearly observed. Greater and more frequent negative ΔW X−D (moisture deficit) were observed during the daytime than during the nighttime and in warm months than in cold months, whereas the opposite trend was observed for positive ΔW X−D (moisture excess). The frequency of urban moisture excess increased during the nighttime with heavy condensation. Precipitation events significantly increased the frequency of urban moisture excess, and this effect could last until 24 h after precipitation. ΔW was influenced by meteorological parameters, but the trends were not clear. No relation was observed between ΔW and ΔT. The findings can improve understanding of urban humidity and provide valuable information for humidity-related studies.

Highlights • Hourly temperature and humidity data were collected from 8 LCZ sites for 3 years. • Differences in relative humidity and humidity ratio among the LCZ sites were analyzed. • Impact of condensation and precipitation events on humidity difference was examined. • ΔRH among LCZ classes is significant at night but insignificant during daytime. • Urban moisture deficit is more frequent and greater during daytime and warm months.