Aerosol-driven droplet concentrations dominate coverage and water of oceanic low-level clouds
Aerosol-driven droplet concentrations dominate coverage and water of oceanic low-level clouds
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气溶胶驱动的液滴浓度主导着海洋低层云的覆盖范围和水
DOI：
10.1126/science.aav0566
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发表时间：
2019-02-08
期刊：
影响因子：
56.9
通讯作者：
Yu, Shaocai
中科院分区：
文献类型：
--
作者：
Rosenfeld, Daniel;Zhu, Yannian;Yu, Shaocai
云效应的反思海洋上空云凝结核（CCN）气溶胶的丰度对全球温度有多大影响？Rosenfeld等人分析了CCN如何影响海洋层积云的特性，这些层积云将地球接收到的大部分太阳辐射反射回太空（参见Sato和Suzuki的透视）。CCN丰度解释了辐射冷却的大部分变异性。因此，这些云提供的辐射强迫的大小对CCN的存在比当前模式显示的要敏感得多，这表明存在其他补偿变暖效应。科学，本期p. eaav0566；另见第580页海洋层积云对云凝结核的敏感程度比以前认为的要高。人为排放的空气微粒污染可以通过增强低空云层，将更多的太阳辐射反射回太空，抵消温室气体排放引起的部分变暖。气溶胶粒子之所以有这种效果，是因为云滴必须凝结在先前存在的微小粒子上，就像露水凝结在寒冷的物体上一样；来自人为排放的更多气溶胶颗粒导致更多的小云滴。低层云增强的一个主要途径是通过减少云滴大小来抑制降雨。这使得更多的水在云中停留更长的时间，从而增加了云的覆盖率和含水量，从而将更多的太阳热量反射到太空。这种影响在海洋上最为强烈，因为海洋中有充足的水汽维持大面积的低空云层。预测全球变暖需要定量了解云量和含水量如何受到人为气溶胶的影响。对气溶胶云介导的辐射效应进行量化一直是一项重大挑战，并导致了气候预测的不确定性。很难从卫星上测量云活跃气溶胶，也很难从气象数据中分离出它们对云的影响。从卫星上获取云滴浓度和垂直风的新方法的发展代表了一项突破，使这种量化成为可能。这些方法适用于赤道至南纬40度之间的世界海洋。气溶胶和气象变量解释了95%的云辐射效应变率。结果测量到的气溶胶云介导的冷却效应比目前的估计要大得多，特别是通过气溶胶对降水的抑制作用，这使得云保留更多的水，持续时间更长，并且具有更大的分数覆盖率。这与之前的大多数观测和模拟结果相反，它们报告说，垂直整合的云水甚至可能随着气溶胶的增加而减少，尤其是在降水云中。造成这种明显差异的主要原因是，更深的云层有更多的水，更容易产生降雨，从而更有效地清除气溶胶。结果是，气溶胶较少的云有更多的水，但这与气溶胶对云的影响无关。当评估具有给定固定几何厚度的云的效果时，这种谬论被克服了。气溶胶对水含量和浅海云覆盖的大敏感性打消了另一种看法，即增加的气溶胶的影响主要是通过调整云的性质来缓冲的，这抵消了最初的气溶胶效应。例如，添加气溶胶抑制了降雨，因此云层的反应是加深到足以恢复被抑制的降雨量。但完成这一调整过程所需的时间尺度远远长于云系统的生命周期，云系统的生命周期大多在12小时以下。因此，大多数海洋浅层云没有受到气溶胶效应的缓冲，而气溶胶效应诱导的降温程度比以前认为的要大得多。结论：对于给定的几何厚度，气溶胶可以解释四分之三的低空海洋云冷却效应的变异性。云滴浓度加倍几乎使冷却加倍。这表明，对气溶胶的敏感性比以前报道的要高得多，这意味着如果将其纳入目前的气候模型，就会造成太大的降温。这一论点被用来驳斥如此大的敏感性。为了避免这种情况，一些模型中的气溶胶效应被调低了。接受这项研究揭示的大灵敏度意味着气溶胶有另一个大的正强迫，可能通过深云，这在目前的模式中没有考虑到。这揭示了额外的不确定性，必须加以考虑，并且需要在计算地球能量收支和气候预测时进行重大修订。矛盾的是，我们知识的进步增加了气溶胶云介导的辐射强迫的不确定性。但它为最终大幅减少这种不确定性铺平了道路。东北太平洋浅海云的覆盖和微滴浓度（Nd）。从船舶烟囱排放的烟雾颗粒形成云滴并升高Nd。无烟云团（Nd < ~30 cm−3）沉淀并破裂。云量随Nd的增加而增加，从而抑制降水。坚固的云层是由烟雾维持的，这些烟雾是从旧的航迹中扩散出来的，并被新的航迹所交叉。由于缺乏对海洋上云凝结核（CCN）气溶胶的可靠估计，我们通过反射太阳辐射来量化它们对云特性和冷却程度的影响的能力受到了严重限制——这是人为气候强迫的一个关键不确定性。我们介绍了一种将云特性归因于CCN并将气溶胶效应从气象效应中分离出来的方法。其应用表明，对于给定的气象学，CCN解释了云辐射冷却效应的四分之三的变率，主要是通过影响浅云量和水路径。这揭示了云辐射强迫对CCN的敏感性比以前报道的要高得多，这意味着如果将其纳入目前的气候模式，则会产生过多的冷却。这表明气溶胶增温效应的存在尚未被发现，可能是通过深云。
Reflections on cloud effects How much impact does the abundance of cloud condensation nuclei (CCN) aerosols above the oceans have on global temperatures? Rosenfeld et al. analyzed how CCN affect the properties of marine stratocumulus clouds, which reflect much of the solar radiation received by Earth back to space (see the Perspective by Sato and Suzuki). The CCN abundance explained most of the variability in the radiative cooling. Thus, the magnitude of radiative forcing provided by these clouds is much more sensitive to the presence of CCN than current models indicate, which suggests the existence of other compensating warming effects. Science, this issue p. eaav0566; see also p. 580 Marine stratocumulus clouds are more sensitive to cloud condensation nuclei than was thought. INTRODUCTION Human-made emissions of particulate air pollution can offset part of the warming induced by emissions of greenhouse gases, by enhancing low-level clouds that reflect more solar radiation back to space. The aerosol particles have this effect because cloud droplets must condense on preexisting tiny particles in the same way as dew forms on cold objects; more aerosol particles from human-made emissions lead to larger numbers of smaller cloud droplets. One major pathway for low-level cloud enhancement is through the suppression of rain by reducing cloud droplet sizes. This leaves more water in the cloud for a longer time, thus increasing the cloud cover and water content and thereby reflecting more solar heat to space. This effect is strongest over the oceans, where moisture for sustaining low-level clouds over vast areas is abundant. Predicting global warming requires a quantitative understanding of how cloud cover and water content are affected by human-made aerosols. RATIONALE Quantifying the aerosol cloud–mediated radiative effects has been a major challenge and has driven the uncertainty in climate predictions. It has been difficult to measure cloud-active aerosols from satellites and to isolate their effects on clouds from meteorological data. The development of novel methodologies to retrieve cloud droplet concentrations and vertical winds from satellites represents a breakthrough that made this quantification possible. The methodologies were applied to the world’s oceans between the equator and 40°S. Aerosol and meteorological variables explained 95% of the variability in the cloud radiative effects. RESULTS The measured aerosol cloud–mediated cooling effect was much larger than the present estimates, especially via the effect of aerosols on the suppression of precipitation, which makes the clouds retain more water, persist longer, and have a larger fractional coverage. This goes against most previous observations and simulations, which reported that vertically integrated cloud water may even decrease with additional aerosols, especially in precipitating clouds. The major reason for this apparent discrepancy is because deeper clouds have more water and produce rainfall more easily, thus scavenging the aerosols more efficiently. The outcome is that clouds with fewer aerosols have more water, but it has nothing to do with aerosol effects on clouds. This fallacy is overcome when assessing the effects for clouds with a given fixed geometrical thickness. The large aerosol sensitivity of the water content and coverage of shallow marine clouds dispels another belief that the effects of added aerosols are mostly buffered by adjustment of the cloud properties, which counteracts the initial aerosol effect. For example, adding aerosols suppresses rain, so the clouds respond by deepening just enough to restore the rain amount that was suppressed. But the time scale required for the completion of this adjustment process is substantially longer than the life cycle of the cloud systems, which is mostly under 12 hours. Therefore, most of the marine shallow clouds are not buffered for the aerosol effects, which are inducing cooling to a much greater extent than previously believed. CONCLUSION Aerosols explain three-fourths of the variability in the cooling effects of low-level marine clouds for a given geometrical thickness. Doubling the cloud droplet concentration nearly doubles the cooling. This reveals a much greater sensitivity to aerosols than previously reported, meaning too much cooling if incorporated into present climate models. This argument has been used to dismiss such large sensitivities. To avoid that, the aerosol effects in some of the models were tuned down. Accepting the large sensitivity revealed in this study implies that aerosols have another large positive forcing, possibly through the deep clouds, which is not accounted for in current models. This reveals additional uncertainty that must be accounted for and requires a major revision in calculating Earth’s energy budget and climate predictions. Paradoxically, this advancement in our knowledge increases the uncertainty in aerosol cloud–mediated radiative forcing. But it paves the way to eventual substantial reduction of this uncertainty. Coverage and droplet concentrations (Nd) of shallow marine clouds over the northeast Pacific. Smoke particles emitted from ship smokestacks form cloud droplets and elevate Nd. The smoke-free clouds (Nd < ~30 cm−3) precipitate and break up. The fraction of cloud cover increases with more Nd that suppresses precipitation. The solid cloud cover is maintained by smoke that was spread from old ship tracks, crossed by newer ones. A lack of reliable estimates of cloud condensation nuclei (CCN) aerosols over oceans has severely limited our ability to quantify their effects on cloud properties and extent of cooling by reflecting solar radiation—a key uncertainty in anthropogenic climate forcing. We introduce a methodology for ascribing cloud properties to CCN and isolating the aerosol effects from meteorological effects. Its application showed that for a given meteorology, CCN explains three-fourths of the variability in the radiative cooling effect of clouds, mainly through affecting shallow cloud cover and water path. This reveals a much greater sensitivity of cloud radiative forcing to CCN than previously reported, which means too much cooling if incorporated into present climate models. This suggests the existence of compensating aerosol warming effects yet to be discovered, possibly through deep clouds.