Abstract
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.
Highlights
Synoptic weather is a key factor driving air pollution events through photochemical, turbulence, wet deposition, and transport processes (Ding et al, 2009; Guo et al, 2017; Liu et al, 2001; L. Liu et al, 2019; Madronich, 1987)
More rain water is formed by accretion of cloud droplets, which indicates that intensified warm rain is the only reason for the precipitation increase with local aerosol emissions
This study finds that aerosols significantly affect local extreme weather, invigorating deep convection, via aerosol– cloud interactions (ACI) effects
Summary
Synoptic weather is a key factor driving air pollution events through photochemical, turbulence, wet deposition, and transport processes (Ding et al, 2009; Guo et al, 2017; Liu et al, 2001; L. Liu et al, 2019; Madronich, 1987). Aerosols absorb and scatter solar radiation and serve as cloud condensation nuclei and ice nuclei, which are referred to as aerosol–radiation interactions (ARI) and aerosol–cloud interactions (ACI), respectively (IPCC, 2013) Both ARI and ACI influence deep convection and precipitation (Fan et al, 2008, 2013, 2018; Koren et al, 2004; Liu et al, 2018; Rosenfeld et al, 2008). We select a torrential rainfall case in winter, which breaks the record of Guangdong Province since 1951 in terms of duration, affected area, and cumulative rainfall (Deng et al, 2015), to further understand the combined effects and relative importance of ARI and ACI for precipitation Before this heavy rainfall, the PRD region is affected by a strong haze, with PM2.5 concentration reaching 174 μg m−3.
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