Abstract. Biomass burning plumes are frequently transported over the southeast Atlantic (SEA) stratocumulus deck during the southern African fire season (June–October). The plumes bring large amounts of absorbing aerosols and enhanced moisture, which can trigger a rich set of aerosol–cloud–radiation interactions with climatic consequences that are still poorly understood. We use large-eddy simulation (LES) to explore and disentangle the individual impacts of aerosols and moisture on the underlying stratocumulus clouds, the marine boundary layer (MBL) evolution, and the stratocumulus-to-cumulus transition (SCT) for three different meteorological situations over the southeast Atlantic during August 2017. For all three cases, our LES shows that the SCT is driven by increased sea surface temperatures and cloud-top entrainment as the air is advected towards the Equator. In the LES model, aerosol indirect effects, including impacts on drizzle production, have a small influence on the modeled cloud evolution and SCT, even when aerosol concentrations are lowered to background concentrations. In contrast, local semi-direct effects, i.e., aerosol absorption of solar radiation in the MBL, cause a reduction in cloud cover that can lead to a speed-up of the SCT, in particular during the daytime and during broken cloud conditions, especially in highly polluted situations. The largest impact on the radiative budget comes from aerosol impacts on cloud albedo: the plume with absorbing aerosols produces a total average 3 d of simulations. We find that the moisture accompanying the aerosol plume produces an additional cooling effect that is about as large as the total aerosol radiative effect. Overall, there is still a large uncertainty associated with the radiative and cloud evolution effects of biomass burning aerosols. A comparison between different models in a common framework, combined with constraints from in situ observations, could help to reduce the uncertainty.

中文翻译：

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比较不同烟雾条件下生物质燃烧羽流对东南大西洋低层云的模拟影响

摘要。在南部非洲火灾季节（6 月至 10 月），生物质燃烧羽流经常在东南大西洋 (SEA) 层积云甲板上输送。羽流带来大量吸收性气溶胶和增强的水分，这可能引发一系列丰富的气溶胶-云-辐射相互作用，从而产生目前仍知之甚少的气候后果。我们使用大涡模拟 (LES) 来探索和理清气溶胶和湿气对底层层积云、海洋边界层 (MBL) 演化以及三种不同气象条件下的层积云到积云转变 (SCT) 的影响。 2017 年 8 月东南大西洋上空的情况。对于所有这三种情况，我们的 LES 显示 SCT 是由海面温度升高和空气平流向赤道时的云顶夹带驱动的。在 LES 模型中，气溶胶间接效应（包括对毛毛雨产生的影响）对模拟云演化和 SCT 的影响很小，即使气溶胶浓度降低到背景浓度时也是如此。相反，局部半直接效应，即 MBL 中太阳辐射的气溶胶吸收，会导致云量减少，从而导致 SCT 加速，特别是在白天和云层破碎的情况下，尤其是在高度污染的情况下。对辐射预算的最大影响来自气溶胶对云反照率的影响：具有吸收气溶胶的羽流产生总共 3 d 的平均模拟。我们发现伴随气溶胶羽流的水分产生额外的冷却效果，其大小与总气溶胶辐射效果大致相同。总体而言，生物质燃烧气溶胶的辐射和云演化效应仍然存在很大的不确定性。在共同框架中比较不同模型，结合现场观测的约束，可能有助于减少不确定性。