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.