Microphysics effects of anthropogenic aerosols on urban heavy precipitation over the Pearl River Delta, China

Abstract:

Weather Research and Forecasting model coupled with chemistry was used to simulate cloud microphysics processes of a mesoscale urban heavy precipitation event over the Pearl River Delta, China. Two intensities of anthropogenic emissions were considered: the CTL simulation adopted the level and distribution of present-day pollution, while the CLEAN simulation adopted the aerosol condition before urbanization. The modeling reproduced the precipitation process and the results showed a more centralized distribution of precipitation and a rain rate that was 20% higher at the mature stage in CTL. A larger area of convective-type precipitation occurred in CTL (51.2%) than in CLEAN (42.3%), but less stratiform- and mixed-type precipitation occurred in CTL (7.6% and 7.3%, respectively) than in CLEAN (12.1% and 9.1%, respectively). In comparison with CLEAN, CTL produced larger quantities of cloud droplets and ice-phase particles owing to the higher number concentrations of cloud condensation nuclei and ice nuclei activated by the aerosols. Additional release of condensation/deposition heating promoted vertical upward motion and convection in clouds, which further promoted the colliding, merging, and collecting processes between large raindrops and small raindrops/droplets that ultimately generated raindrops with larger radii (600–700 μm) but smaller number concentrations. Furthermore, hydrometeors convert to precipitation more effectively in polluted conditions. In CTL, the centralized distribution and greater mass magnitude of hail and melting hail resulted in a focused distribution of rainfall. In CLEAN, the decentralized distributions and smaller mass magnitude of hail and melting hail led to a more dispersed distribution of rainfall.