Model Analysis of the Anthropogenic Aerosol Effect on Clouds over East Asia

A coupled meteorology and aerosol/chemistry model WRF-Chem (Weather Research and Forecast model coupled with Chemistry) was used to conduct a pair of simulations with present-day (PD) and preindustrial (PI) emissions over East Asia to examine the aerosol indirect effect on clouds. As a result of an increase in aerosols in January, the cloud droplet number increased by 650 cm-3 over the ocean and East China, 400 cm-3 over Central and Southwest China, and less than 200 cm-3 over North China. The cloud liquid water path (LWP) increased by 40-60 g m-2 over the ocean and Southeast China and 30 g m-2 over Central China; the LWP increased less than 5 g m-2 or decreased by 5 g m-2 over North China. The effective radius (Re) decreased by more than 4 mm over Southwest, Central, and Southeast China and 2 mm over North China. In July, variations in cloud properties were more uniform; the cloud droplet number increased by approximately 250-400 cm-3, the LWP increased by approximately 30-50 g m-2, and Re decreased by approximately 3 mm over most regions of China. In response to cloud property changes from PI to PD, shortwave (SW) cloud radiative forcing strengthened by 30 W m-2 over the ocean and 10 W m-2 over Southeast China, and it weakened slightly by approximately 2-10 W m-2 over Central and Southwest China in January. In July, SW cloud radiative forcing strengthened by 15 W m-2 over Southeast and North China and weakened by 10 W m-2 over Central China. The different responses of SW cloud radiative forcing in different regions was related to cloud feedbacks and natural variability.