3.3. Urbanization effect on the presummer precipitation over the Loess Plateau
The previous analyses suggested that the changes in large-scale atmospheric circulations are likely the main cause of the variation in extreme precipitation patterns. However, the significant discrepancies between the trends in the urban and the rural areas indicate a remarkable effect of urbanization on the variability of extreme precipitation over the LP, especially in the last 20 years. The urbanization causes many climatic problems, such as the increase of urban surface temperature, the decrease of wind speed and the increase of aerosols. However, the physical process of the urbanization effect on the extreme precipitation is quite complicated. In this section, we only preliminarily discuss the possible effects of temperature and aerosols on the urban precipitation over the LP.
The UHI effect may be one factor influencing the presummer precipitation over the LP. Except for the Xining UA with unsignificant increase of surface air temperature, all the other UAs exhibit significant UHI characteristics, with the mean trend of surface temperature in the urban areas 0.05–0.07°C·year−1 higher than that in the rural areas during 1979–2018 (Fig. 8). The previous studies showed that urbanization has different impacts on urban precipitation in wet and dry climate backgrounds (Luo and Lau 2019, Yang 2020). The warmer surface in the urban core areas induces stronger potential evapotranspiration by exponentially increasing the saturated water vapor pressure and aggravating the water vapor deficit. If the atmospheric water is scarce, the urbanization further intensifies the drying trend over the urban core areas compared with the surrounding rural areas, such as Xi’an and Taiyuan. If the atmospheric water is abundant, the heat source in the urban areas induces vertical flows and enhances convective motions, leading to more convective precipitation in the urban areas than in the rural areas, such as Luoyang and Hohhot (Yan, Chan et al. 2020).
Another major feature is the increasing aerosol emissions associated with urbanization. In this study, PM2.5 is taken as a representative to reflect the aerosol distribution in the UAs over the LP (Fig. 9). Many studies have indicated that aerosols may inhibit or promote the precipitation with different intensity (Qian Duan 2008, Ma, Huo et al. 2017, Fults, Massmann et al. 2019, Berhane and Bu 2021). For instance, aerosols have obvious inhibitory effect on the light rain in general. With sufficient water vapor, more aerosol particles will lead more condensation nuclei to be formed (aerosol microphysical effect), which further increases the extreme precipitation combined with the UHI effect. The urban precipitation characteristics in Luoyang, Hohhot and Xining UAs can be explained in this way. However, in the long run, the increase of aerosol is not conducive to the precipitation formation. Aerosols weaken the radiation reaching the ground, leading to a stable low-level atmosphere, which is unfavorable for the ascending motion and thus reduces the occurrence frequency of precipitation. When the water vapor transport weakens, the increase of aerosol makes the atmospheric stratification more stable, resulting in less precipitation frequency. Xi’an and Taiyuan UAs have experienced frequent serious air pollution incidents in recent years due to the impact of basin topography and urbanization. The significant decrease of precipitation in the two UAs should be closely related to the increase of aerosol in recent years. Therefore, the urbanization influence on precipitation is actually the result of the competition among the thermal forcing of UHI, the aerosol radiation effect and the aerosol microphysical effect.