Aerosol-PBL-monsoon interactions amplify semi-direct effect of biomass smoke on low cloud formation in Southeast Asia

Low clouds play a key role in the Earth-atmosphere energy balance and influence agricultural production and solar-power generation. Smoke aloft has been found to enhance marine stratocumulus over the Southeast Atlantic in austral spring through aerosol-cloud interactions, but its role in regions with strong human activities and complex monsoon circulation remains unclear. Here we show that biomass burning aerosols aloft strongly increase the low-cloud coverage over both land and ocean in subtropical southeastern Asia. The degree of this enhancement and its spatial extent are comparable to that in the Southeast Atlantic, even though the total biomass burning emissions in Southeast Asia are only one-fifth of those in Southern Africa. We find that a synergetic effect of aerosol-cloud-boundary layer interaction with the monsoon is the main reason for the strong semi-direct effect and enhanced low-cloud formation in Asia.

低云在地球-大气能量平衡中扮演关键角色，并对农业生产与太阳能发电产生显著影响。已有研究显示，南半球春季时，高空烟雾可通过气溶胶-云相互作用（aerosol-cloud interactions）增强东南大西洋海域的海洋层积云，但在人类活动密集、季风环流复杂的区域，其作用机制仍未明确。本研究证实，高空生物质燃烧气溶胶（biomass burning aerosols）可显著提升亚热带东南亚地区陆面与洋面的低云覆盖率。尽管东南亚地区的生物质燃烧排放总量仅为南部非洲的五分之一，但该增强效应的强度与空间覆盖范围均可与东南大西洋地区相匹敌。研究进一步发现，气溶胶-云-边界层相互作用（aerosol-cloud-boundary layer interaction）与季风的协同效应，是亚洲地区出现显著半直接效应并促进低云形成的核心原因。