Comparing Study on Aerosol, Cloud and Precipitation in East China and USA based on MODIS and TRMM PR observations

Cloud and precipitation are the result of synthetic interaction between atmospheric thermodynamic processes and geographic environment factors. It is also commonly acknowledged that aerosol generated by human activities takes a part in the process of cloud and precipitation as well, especially in the recent 30 years. The effect of aerosol on cloud and precipitation (i.e. aerosol indirect effect) reflects the influences of human activities on weather or climate (Koren et al., 2012; Rosenfeld et al., 2012). Aerosol may make significant impacts on spatiotemporal distribution of cloud and precipitation in hydrological cycle at regional and global scales, which is a challenge in researches of climate change. The concentration of anthropogenic aerosol is closely related to the production of industry or agriculture, which is different among countries and regions. In order to understand how aerosol, cloud and precipitation vary in regions with different economic development levels, the spatial-temporal variations of the three parameters in the East mainland of both USA and China (EU and EC) together with their adjacent ocean areas are investigated based on MODIS and TRMM PR data in this study.

Results indicate that values of Aerosol Optic Depth (AOD) vary in these regions. Generally, MODIS shows the AOD in EC is nearly three times higher than that in EU. Under such distinct AOD background, the PDFs of cloud effective radius (CRe) in these regions shows clear differences for ice and mixed cloud but much smaller difference for liquid cloud. The PDFs of cloud Liquid Water Path (LWP) for the ice, mixed and liquid cloud in both regions shows large differences. Specially, strong gradient of CRe and LWP are found from EC towards the far East China Sea. It is associated with the gradient of AOD in this area. For given cloud LWP at lower levels, CRe of ice cloud and mixed cloud decrease with increasing AOD. While for higher LWP, the CRe increases with AOD. Analysis also illustrates that convective precipitation is mainly modulated by aerosol. In EC, for example, the aerosol concentration is higher while the water vapor is limited so that more aerosols-induced CCN compete with each other to obtain condensed liquid water (the second indirect effect of aerosol). Therefore, it is hard for cloud particles to grow up in such circumstances. This leads to reduction in convective rain intensity. In contrast to EC, although there is high concentration of aerosol in the adjacent sea of EC, ocean can supply enough water vapor for cloud particles to grow up. This process causes stronger intensity of convective precipitation in this area comparing to EC. For stratiform precipitation, it is mainly affected by the monsoon activities.