Urban Dry Island Effect Mitigated Urbanization Effect on Observed Warming in China

Abstract

AbstractWith urbanization occurring around weather stations, its impact on the observed air temperature has been widely recognized. However, its assessments were varied partially due to the poor understanding of its underlying mechanism. Here, we analyzed the effect of urbanization using observations obtained from ~2200 weather stations in China from 1960 to 2014. The results showed that the urbanization effect increased from 1960 to 1984 but slowed after 1995 with rapid urbanization in China, particularly in terms of the daily minimum temperature Tmin and daily mean air temperature Tmean. The urbanization effect is nearly linearly related to the urban?rural contrast of effective cloud cover (including the impact of atmospheric aerosols) derived from the observed sunshine duration. Aerosols increase atmospheric downward longwave radiation Ld through their absorption of solar radiation during the daytime, and they trap longwave radiation emitted from the surface during the nighttime. Increased anthropogenic aerosols caused the urban?rural contrast of effective cloud cover to increase from 1960 to 2014. However, the urban?rural contrast of cloud cover remained stable from 1960 to 1984 but substantially decreased due to the ?urban dry island effect? after 1995, which compensated for the impact of anthropogenic aerosols on Ld and resulted in a stable urbanization effect after 1995. The urban?rural difference in Ld increased by 0.57 W m?2 (10 yr)?1, which resulted in warming of 0.074°C (10 yr)?1 (78.2%) for Tmin, 0.037°C (10 yr)?1 (151.5%) for Tmax, and 0.056°C (10 yr)?1 (96.6%) for Tmean.