Effects of anthropogenic and sea salt aerosols on a heavy rainfall event during the early-summer rainy season over coastal Southern China

Effects of anthropogenic and sea salt aerosols (AA and SSA, respectively) on heavy rainfall in the densely populated monsoon coastal areas remain poorly known. To address this issue, a representative heavy rainfall event over coastal Southern China is simulated using the Weather Research and Forecasting model coupled to Chemistry (WRF-Chem). Results show that the initial responses of clouds to AA and SSA are opposite due to their different size distributions: increasing the AA emission by tenfold leads to larger mixing ratio and number concentration of cloud droplets (Q(c) and N-c) with reduced size; on the contrary, presence of SSA results in less but larger cloud droplets as activation of smaller aerosols is inhibited on the order of 10(5) to 10(6) m(-3) due to increased competition for the supersaturated water vapor between SSA and other smaller particles as cloud condensation nuclei. In the polluted environment, rain number concentration (N-r) is reduced to about a half of that in the clean environment due to weakened auto-conversion from cloud droplets to raindrops. The reduced N-r combined with enhanced accretion of cloud droplets by raindrops result in larger mean size and faster mean fall speed of raindrops, therefore probability distribution of surface hourly rain rate shifts toward values > about 2 mm h(-1). The 24-h rainfall accumulation increases up to about 80 mm at the centers, although the region-averaged amount increases only 5.32%. Meanwhile, increasing the AA emission decreases total mixing ratio of ice-phase hydro meteors while the previously reported convective intensification is not evident. When the SSA emission is included in the polluted environment, the significantly inhibited activation of submicron aerosols leads to weakening of the following liquid-, mixed-, and ice-phase microphysical processes and the convective intensity to some extents. The probability distribution of hourly rain rates shifts toward values < about 5 mm h(-1), while the 24-h rainfall accumulation hardly changes significantly with a decrease of 4.19% in the regionally averaged amount. The overall small impacts of aerosols, SSA in particular, are related to the strong synoptic forcing governing the heavy rainfall production in this case.

Automated summary

The effects of anthropogenic and sea salt aerosols on heavy rainfall in coastal areas were investigated using a model. The results showed different initial responses of clouds to these aerosols, with increased anthropogenic aerosols causing larger cloud droplets while sea salt aerosols resulted in fewer but larger cloud droplets. In polluted environments, rain number concentration was reduced and raindrops became larger and faster, leading to a shift in the distribution of surface hourly rain rate towards higher values. The accumulation of 24-hour rainfall increased in the centers but only slightly on average. The inclusion of sea salt aerosols further weakened the microphysical processes and convective intensity, resulting in a shift towards lower hourly rain rates and a slight decrease in the average 24-hour rainfall accumulation. The overall impacts of aerosols, especially sea salt aerosols, were small due to the strong synoptic forcing controlling the heavy rainfall production in this case.