Contrasting aerosol effects on longwave cloud forcing in South East Asia and Amazon simulated with Community Atmosphere Model version 5.3

Aerosols modify cloud microphysical and radiative properties and thus impact the shortwave (SW) and longwave cloud forcing (LWCF). This study first reports the finding of contrasting aerosol effects on LWCF in South East Asia and Amazon in the Community Atmosphere Model version 5.3 (CAM5.3), which corresponds to the sum of LW indirect and semi-direct effects investigated in Ghan et al., (2012). A series of numerical experiments is conducted to decompose the complex aerosol effects on LWCF. Our analysis indicates that the cooling (negative aerosol effects on LWCF) in Amazon is due mainly to the aerosol effects on warm clouds and the inhibition of vertical motion by the aerosol-induced radiative cooling. In contrast, the warming (positive aerosol effects on LWCF) in South East Asia is due mainly to the aerosol effect on homogeneous freezing, thus reducing the ice particle size and prolonging the existence of ice cloud. Our results emphasize that a comprehensive analysis of integrated aerosol effects on both warm and ice clouds is necessary for better understanding the aerosol effects on LWCF.

气溶胶能够改变云的微物理特性与辐射特性，进而对短波（shortwave，SW）与长波云强迫（longwave cloud forcing，LWCF）产生影响。本研究首次报道了在社区大气模式5.3版（Community Atmosphere Model version 5.3，CAM5.3）中，东南亚与亚马逊地区的气溶胶对长波云强迫的影响存在截然相反的效应，该效应对应Ghan等人（2012）研究所探讨的长波间接效应与半直接效应的总和。本研究通过一系列数值试验，对气溶胶影响长波云强迫的复杂效应进行分解。分析结果显示，亚马逊地区的降温效应（即气溶胶对长波云强迫产生负向影响）主要源于两方面：一是气溶胶对暖云（warm clouds）的作用，二是气溶胶诱导的辐射冷却对垂直运动的抑制作用。与之相反，东南亚地区的增温效应（即气溶胶对长波云强迫产生正向影响）主要源于气溶胶对均质冻结（homogeneous freezing）过程的作用，该作用可减小冰粒子粒径并延长冰云的存续时间。本研究结果强调，若要更深入地理解气溶胶对长波云强迫的影响，需对暖云与冰云的综合气溶胶效应开展全面分析。