Abstract. General circulation models' (GCMs) estimates of the liquid water path adjustment to anthropogenic aerosol emissions differ in sign from other lines of evidence. This reduces confidence in estimates of the effective radiative forcing of the climate by aerosol–cloud interactions (ERFaci). The discrepancy is thought to stem in part from GCMs' inability to represent the turbulence–microphysics interactions in cloud-top entrainment, a mechanism that leads to a reduction in liquid water in response to an anthropogenic increase in aerosols. In the real atmosphere, enhanced cloud-top entrainment is thought to be the dominant adjustment mechanism for liquid water path, weakening the overall ERFaci. We show that the latest generation of GCMs includes models that produce a negative correlation between the present-day cloud droplet number and liquid water path, a key piece of observational evidence supporting liquid water path reduction by anthropogenic aerosols and one that earlier-generation GCMs could not reproduce. However, even in GCMs with this negative correlation, the increase in anthropogenic aerosols from preindustrial to present-day values still leads to an increase in the simulated liquid water path due to the parameterized precipitation suppression mechanism. This adds to the evidence that correlations in the present-day climate are not necessarily causal. We investigate sources of confounding to explain the noncausal correlation between liquid water path and droplet number. These results are a reminder that assessments of climate parameters based on multiple lines of evidence must carefully consider the complementary strengths of different lines when the lines disagree.

中文翻译：

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一般环流模型模拟负液态水路径-液滴数量相关性，但人为气溶胶仍然增加模拟液态水路径


摘要。大气环流模型（GCM）对人为气溶胶排放的液态水路径调整的估计与其他证据有不同的迹象。这降低了对气溶胶-云相互作用对气候的有效辐射强迫（ERFaci）估计的信心。这种差异被认为部分源于气溶胶无法代表云顶夹带中的湍流-微物理相互作用，这种机制会导致液态水减少，以响应人为气溶胶的增加。在真实大气中，增强的云顶夹带被认为是液态水路径的主要调整机制，削弱了整体 ERFaci。我们表明，最新一代的 GCM 包括在当前云滴数量和液态水路径之间产生负相关的模型，这是支持人为气溶胶减少液态水路径的关键观测证据，也是上一代 GCM 可以实现的不重现。然而，即使在具有这种负相关性的 GCM 中，由于参数化的降水抑制机制，人为气溶胶从工业化前到当今值的增加仍然导致模拟液态水路径的增加。这进一步证明当前气候的相关性不一定是因果关系。我们研究混杂因素的来源，以解释液态水路径和液滴数量之间的非因果相关性。这些结果提醒我们，当不同证据线不一致时，基于多线证据的气候参数评估必须仔细考虑不同线的互补优势。