Why do we need to model stratocumulus clouds?

Clouds reflect the Sun's energy and insulate the Earth by absorbing longwave radiation, making them a key part of our planet's energy budget. Stratocumulus clouds are shallow and usually located in the lower 2km of the atmosphere (Figure 1). They cover around a fifth of the Earth's surface, with extensive near-permanent cloud fields located to the west of all major continents (Figures 2 and 3). This large coverage over the darker land or ocean surface combined with bright, reflective cloud tops acts to increase the reflectivity of the planet (low cloud cover alone increases the Earth's albedo by ~25%, from about 0.24 to 0.30 (Hartmann et al., 1992)), whilst emitting longwave radiation with approximately the same energy as the surface. The result is a net cooling effect in most regions where these clouds are present (Wood, 2012).

Stratocumulus clouds are impacted greatly by changes in aerosol concentrations. Aerosols are gas, liquid or solid particles that are suspended in the air. Aerosols can come from natural sources, such as ocean spray or deserts, or anthropogenic sources, like smoke from vegetation burning and industrial sites. Aerosols directly affect the radiation budget through absorption and scattering of solar energy, but they also indirectly affect it through their interactions with clouds. They are key to cloud formation because they provide a surface onto which water vapour in the air can condense, at a relative humidity very close to 100%, to produce a cloud droplet. Without aerosol particles a much higher relative humidity (around 300%) would be required. As such, all cloud droplets contain aerosol particles and the properties of the cloud, such as brightness and cloud cover, are altered when more aerosol enters or leaves the cloud. These processes are known as aerosol–cloud interactions (Bellouin et al., 2020).

Considering the importance of stratocumulus clouds and their cooling effect, sustained aerosol perturbations have the potential to impact Earth's energy budget and drive climate change. The impacts of human activities on the energy budget, since the industrial period, are regularly assessed and are called radiative forcings. The effective radiative forcing accounts for rapid ‘adjustments’ to the atmosphere and was recently estimated to be between −2.65 and −0.07Wm⁻² for aerosol–cloud interactions (Bellouin et al., 2020). The range is large because it also accounts for the complete adjustments to clouds, like changes in rainfall and cloud cover. For context, the value for well-mixed greenhouse gases was estimated to be between 2.26 and 3.40Wm⁻² in the last Intergovernmental Panel on Climate Change report (Myhre et al., 2013). Further research is needed to reduce the uncertainty in aerosol–cloud interactions because it dominates the uncertainty in estimates of the total radiative forcing.

In addition to aerosol–cloud interactions, stratocumulus clouds can be altered by changes in the climate, and this could have consequences that feed back into the atmosphere. For example, the increase in CO₂ emissions warms the atmosphere and consequently the oceans. Current models suggest that warmer surface temperatures will reduce the amount of low, shallow clouds, which will decrease the planet's reflectivity and allow more solar energy to be absorbed (Ceppi et al., 2017). This feedback would act to amplify the warming from the original CO₂ emissions. Precise predictions of our future climate rely on our understanding of how stratocumulus clouds will respond to changes in climate and what climate feedbacks might occur.

中文翻译：

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用机器学习揭开层积云复杂性质的神秘面纱

为什么我们需要对层积云进行建模？

云层通过吸收长波辐射来反射太阳的能量并使地球绝缘，使它们成为我们星球能量预算的关键部分。层积云很浅，通常位于大气层下方 2 公里处（图 1）。它们大约覆盖地球表面的五分之一，在所有主要大陆的西部都有广泛的近乎永久的云场（图 2 和图 3）。这种在较暗的陆地或海洋表面上的大面积覆盖与明亮、反射性的云顶相结合，可以增加地球的反射率（仅低云量就将地球的反照率增加了约 25%，从约 0.24 增加到 0.30（Hartmann等人，  1992年))，同时发射与表面能量大致相同的长波辐射。结果是在存在这些云的大多数地区产生净冷却效果（Wood，  2012 年）。

层积云受气溶胶浓度变化的影响很大。气溶胶是悬浮在空气中的气体、液体或固体颗粒。气溶胶可以来自自然来源，如海洋喷雾或沙漠，或人为来源，如植被燃烧和工业场所产生的烟雾。气溶胶通过吸收和散射太阳能直接影响辐射收支，但它们也通过与云的相互作用间接影响它。它们是云形成的关键，因为它们提供了一个表面，空气中的水蒸气可以在相对湿度非常接近 100% 的情况下冷凝，从而产生云滴。如果没有气溶胶颗粒，则需要更高的相对湿度（约 300%）。因此，所有云滴都包含气溶胶颗粒和云的特性，当更多的气溶胶进入或离开云层时，亮度和云层覆盖率会发生变化。这些过程被称为气溶胶-云相互作用（Bellouin等人，  2020 年）。

考虑到层积云的重要性及其冷却效应，持续的气溶胶扰动有可能影响地球的能量收支并推动气候变化。自工业时代以来，人类活动对能源预算的影响会定期进行评估，称为辐射强迫。有效的辐射强迫解释了对大气的快速“调整”，最近估计，气溶胶与云之间的相互作用在-2.65至-0.07Wm ^-2之间（Bellouin等人，  2020年）。范围很大，因为它还说明了对云的完全调整，例如降雨量和云量的变化。就上下文而言，充分混合的温室气体的值估计在 2.26 和 3.40Wm ^-2之间在上一次政府间气候变化专门委员会报告中（Myhre等，  2013）。需要进一步研究以减少气溶胶-云相互作用的不确定性，因为它主导了总辐射强迫估计中的不确定性。

除了气溶胶-云的相互作用外，层积云还会因气候变化而改变，这可能会产生反馈到大气中的后果。例如，CO ₂排放量的增加使大气变暖，从而使海洋变暖。目前的模型表明，较高的地表温度将减少低层浅云的数量，这将降低行星的反射率并允许吸收更多的太阳能（Ceppi等，  2017 年）。这种反馈将放大来自原始 CO ₂排放的变暖。对我们未来气候的精确预测取决于我们对层积云将如何响应气候变化以及可能发生的气候反馈的理解。