Effects of anthropogenic aerosols and sea salt aerosols during a summer precipitation event in the Yangtze River Delta

Abstract

Currently, we know little about the effects of anthropogenic aerosols (AAs) and sea salt aerosols (SSAs) on heavy precipitation over the densely populated coastal areas. In this view, we developed a local aerosol activation parametrization scheme which conforms to the Yangtze River Delta (YRD) characteristics using cloud condensation nucleus (CCN) with supersaturation at 0.4 (CCN0.4) and aerosol optical depth (AOD) data at 550 nm (AOD550) to improve the model performance. Meanwhile, we simulated a heavy precipitation over the YRD during early summer using the WRF-Chem model (version 4.1) with the Morrison microphysics scheme. Increased AAs and SSAs presented opposite responses in droplets concentrations and precipitation intensity. The increased AAs increased the aerosol concentrations, leading to more but smaller droplets; and the cloud droplets mixing ratio Qc was slightly increased by 17.7% due to the weakened auto-conversion from droplets to raindrops, while the rain mixing ratio Qr was decreased by about 5.7%. On the contrary, presence of SSAs increased the effective (EFC) radius of cloud droplets because of the larger particles. Along with increased AAs, due to the influences of decreased Qr and smaller droplets, probability density function (PDF) of the hourly surface rain rates shifted to the smaller values (< 0.5 mm/h), which was caused by lowered latent heat release resulting from the enhanced rain evaporative cooling and weakened condensation rate, weakening the convective intensity and updrafts. Meanwhile, decreased updrafts resulted in fewer cloud droplets being transformed to the upper air, less accretion of droplets by ice crystal and snow, and reduced ice mixing ratio Qi and snow mixing ratio Qs. The averaged surface precipitation accumulated for 24 h decreased by about 7.2%, and the intensity of radar reflectivity was decreased by about 7.0%. In contrast, presence of SSAs enhanced the auto-conversion droplets and weakened the evaporation of raindrops, thus increasing the Qr. In this case, the 24-h accumulated averaged surface rainfall was increased by about 30.1% with the larger radar reflectivity and increased area of large reflectivity values (> 20 dbz). The latent heat release and related microphysical processes were enhanced due to the changes of conversion rate, which, together with the increased Qr and larger droplets, strengthened the precipitation, increased the incidence of heavier precipitation (> 2 mm/h), and enhanced the ice-phase microphysical processes.