Cloud adjustments dominate the overall negative aerosol radiative effects of biomass burning aerosols in UKESM1 climate model simulations over the south-eastern Atlantic
Cloud adjustments dominate the overall negative aerosol radiative effects of biomass burning aerosols in UKESM1 climate model simulations over the south-eastern Atlantic
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DOI：
10.5194/acp-21-17-2021
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发表时间：
2021-01-04
影响因子：
6.3
通讯作者：
Deaconu, Lucia
中科院分区：
文献类型：
--
作者：
Che, Haochi;Stier, Philip;Deaconu, Lucia
大西洋东南部(海)被地球上最大的层积云之一半永久地覆盖，在火灾季节，南部非洲大草原地区燃烧的生物量约占全球生物质燃烧排放量的三分之一。为了更好地了解这些生物质燃烧气溶胶对云的影响和海上的辐射平衡，采用了最新一代的英国地球系统模型(UKESM1)。使用CRILEY和ORACLE飞行运动的测量结果对该模型进行了评估，表明该模型具有很好的再现生物质燃烧羽流的能力。为了更详细地研究生物质燃烧气溶胶对云的影响，将其分解为辐射效应(通过吸收和散射)和微物理效应(通过云凝结核-CCN-和云微物理过程的扰动)。7-8月的平均值被用来描述火季的气溶胶、云和辐射平衡。结果表明，在海面过饱和度为0.2%时，约有65%的CCN可归因于生物质燃烧。生物质燃烧气溶胶对云和辐射的吸收作用最为显著。在大陆附近，它增加了激活方案所诊断的过饱和，而在离大陆更远的地方，它降低了过饱和的高度。结果表明，云滴数浓度与生物质燃烧气溶胶的吸收效应具有相似的响应模式。然而，微物理效应降低了过饱和度，增加了海洋上空的云滴浓度，尽管这种变化相对较小。当生物质燃烧气溶胶在层积云甲板上方时，海上的液态水路径也显着增加(主要是生物质燃烧气溶胶的吸收作用)。微物理路径导致海洋上的液态水路径略有增加。云性质的这些变化表明生物质燃烧气溶胶对该地区云的重要作用。在生物质燃烧气溶胶对辐射平衡的影响中，半直接辐射效应(由生物质燃烧气溶胶的辐射效应引起的快速调节)在海面上具有主导的冷却影响，抵消了变暖的直接辐射效应(生物质燃烧气溶胶-辐射相互作用的辐射强迫)，导致海洋总体净冷却辐射效应。然而，半直接效应的大小和符号对生物质燃烧气溶胶和云的相对位置很敏感，反映了该地区生物质燃烧烟羽和云的精确模拟的关键任务。
The south-eastern Atlantic Ocean (SEA) is semi-permanently covered by one of the most extensive stratocumulus cloud decks on the planet and experiences about one-third of the global biomass burning emissions from the southern Africa savannah region during the fire season. To get a better understanding of the impact of these biomass burning aerosols on clouds and the radiation balance over the SEA, the latest generation of the UK Earth System Model (UKESM1) is employed. Measurements from the CLARIFY and ORACLES flight campaigns are used to evaluate the model, demonstrating that the model has good skill in reproducing the biomass burning plume. To investigate the underlying mechanisms in detail, the effects of biomass burning aerosols on the clouds are decomposed into radiative effects (via absorption and scattering) and microphysical effects (via perturbation of cloud condensation nuclei - CCN - and cloud microphysical processes). July-August means are used to characterize aerosols, clouds, and the radiation balance during the fire season. Results show that around 65 % of CCN at 0.2 % supersaturation in the SEA can be attributed to biomass burning. The absorption effect of biomass burning aerosols is the most significant on clouds and radiation. Near the continent, it increases the supersaturation diagnosed by the activation scheme, while further from the continent it reduces the altitude of the supersaturation. As a result, the cloud droplet number concentration responds with a similar pattern to the absorption effect of biomass burning aerosols. The microphysical effect, however, decreases the supersaturation and increases the cloud droplet concentration over the ocean, although this change is relatively small. The liquid water path is also significantly increased over the SEA (mainly caused by the absorption effect of biomass burning aerosols) when biomass burning aerosols are above the stratocumulus cloud deck. The microphysical pathways lead to a slight increase in the liquid water path over the ocean. These changes in cloud properties indicate the significant role of biomass burning aerosols for clouds in this region. Among the effects of biomass burning aerosols on the radiation balance, the semi-direct radiative effects (rapid adjustments induced by the radiative effects of biomass burning aerosols) have a dominant cooling impact over the SEA, which offset the warming direct radiative effect (radiative forcing from biomass burning aerosol-radiation interactions) and lead to an overall net cooling radiative effect in the SEA. However, the magnitude and the sign of the semi-direct effects are sensitive to the relative location of biomass burning aerosols and clouds, reflecting the critical task of the accurate modelling of the biomass burning plume and clouds in this region.