A key contribution to the latest generation of climate projections for the UK (UKCP18) was a perturbed parameter ensemble (PPE) of global coupled models based on HadGEM3-GC3.05. Together with 13 CMIP5 simulations, this PPE provides users with a dataset that samples modelling uncertainty and is ideal for use in impacts studies. Evaluations of global mean surface temperatures for this PPE have shown twenty-first century warming rates consistently at the top end of the CMIP5 range. Here we investigate one potential contributory factor to this lack of spread: that the methodology to select plausible members from a larger, related PPE of atmosphere-only experiments preferentially ruled out those predicted to have more negative climate feedbacks (i.e. lower climate sensitivities). We confirm that this is indeed the case. We show that performance in extratropical long-wave cloud forcing played a key role in this by constraining ice cloud parameters, which in turn constrained the feedback distribution (though causal links are not established). The relatively weak relationship driving this constraint is shown to arise from stronger relationships for the long-wave and short-wave cloud feedback components, which largely cancel out due to changes in tropical high clouds. Moreover, we show that the strength of these constraints is due to a structural bias in extratropical long-wave cloud forcing across the PPE. We discuss how choices made in the methodology to pick the plausible PPE members may result in an overly strong constraint when there is a structural bias and possible improvements to this methodology for the future.

对英国最新一代气候预测（UKCP18）的主要贡献是基于HadGEM3-GC3.05的全球耦合模型的扰动参数集合（PPE）。该PPE与13个CMIP5仿真一起，为用户提供了一个数据集，该数据集可对建模不确定性进行采样，非常适用于影响研究。对这种PPE的全球平均表面温度的评估表明，二十一世纪的升温速率始终处于CMIP5范围的上限。在这里，我们调查了这种缺乏传播的一个潜在的促成因素：从仅大气实验的较大的，相关的个人防护设备中选择合理的成员的方法优先排除了那些预计具有更多负面气候反馈（即较低的气候敏感性）的方法。我们确认确实如此。我们表明，通过限制冰云参数，在温带长波云强迫中的性能在其中发挥了关键作用，反过来又限制了反馈分布（尽管未建立因果关系）。示出了驱动该约束的相对较弱的关系是由于长波和短波云反馈分量的较强关系引起的，这些关系在很大程度上由于热带高云的变化而抵消。此外，我们证明了这些约束的强度是由于跨PPE的温带长波云强迫产生的结构性偏差。我们将讨论在存在结构性偏见并可能对该方法进行将来的改进时，选择合理的PPE成员时在方法上做出的选择可能会导致过强的约束。