Contribution of local and remote anthropogenic aerosols to a record-breaking torrential rainfall event in Guangdong Province, China
Authors: Liu Zhen , Ming Yi , Zhao Chun , Lau Ngar Cheung , Guo Jianping , Bollasina Massimo , Yim Steve Hung Lam 
Published: January 2020

Abstract. A torrential rainfall case, which happened in Guangdong
Province during 14–16 December 2013, broke the historical rainfall record
in the province in terms of duration, affected area, and accumulative
precipitation. The influence of anthropogenic aerosols on this extreme
rainfall event is examined using a coupled meteorology–chemistry–aerosol
model. Up to 33.7 mm precipitation enhancement in the estuary and near the coast is mainly attributed to aerosol–cloud interactions (ACI), whereas aerosol–radiation interaction partially offsets 14 % of the precipitation increase. Our further analysis of changes in hydrometeors and latent heat
sources suggests that the ACI effects on the intensification of
precipitation can be divided into two stages: cold rain enhancement in the
former stage followed by warm rain enhancement in the latter. Responses of
precipitation to the changes in anthropogenic aerosol concentration from
local (i.e., Guangdong Province) and remote (i.e., outside Guangdong
Province) sources are also investigated through simulations with reduced
aerosol emissions from either local or remote sources. Accumulated aerosol
concentration from local sources aggregates mainly near the ground surface
and dilutes quickly after the precipitation initiated. By contrast, the
aerosols from remote emissions extend up to 8 km above ground and last much longer
before decreasing until peak rainfall begins, because aerosols are
continuously transported by the strong northerly winds. The patterns of
precipitation response to remote and local aerosol concentrations resemble
each other. However, compared with local aerosols through warm rain
enhancement, remote aerosols contribute more than twice the precipitation
increase by intensifying both cold and warm rain, occupying a predominant
role. A 10-time emission sensitivity test shows about 10 times the
PM2.5 concentration compared with the control run. Cold (warm) rain is
drastically enhanced (suppressed) in the 10× run. In response to
10× aerosol emissions, the pattern of precipitation and cloud
property changes resembles the differences between CTL and CLEAN, but with a
much greater magnitude. The precipitation average over Guangdong decreases
by 1.0 mm in the 10× run but increases by 1.4 mm in the control run
compared with the CLEAN run. We note that the precipitation increase is
concentrated within a more narrowed downstream region of the aerosol source,
whereas the precipitation decrease is more dispersed across the upstream
region. This indicates that the excessive aerosols not only suppress
rainfall, but also change the spatial distribution of precipitation,
increasing the rainfall range, thereby potentially exacerbating flood and
drought elsewhere. This study highlights the importance of considering
aerosols in meteorology to improve extreme weather forecasting. Furthermore,
aerosols from remote emissions may outweigh those from local emissions in
the convective invigoration effect.