The transport of trace gases by the atmospheric circulation plays an important role in the climate system and its response to external forcing. Transport presents a challenge for Atmospheric General Circulation Models (AGCMs), as errors in both the resolved circulation and the numerical representation of transport processes can bias their abundance. In this study, two tests are proposed to assess transport by the dynamical core of an AGCM. To separate transport from chemistry, the tests focus on the age‐of‐air, an estimate of the mean transport time by the circulation. The tests assess the coupled stratosphere–troposphere system, focusing on transport by the overturning circulation and isentropic mixing in the stratosphere, or Brewer–Dobson Circulation, where transport time‐scales on the order of months to years provide a challenging test of model numerics. Four dynamical cores employing different numerical schemes (finite‐volume, pseudo‐spectral, and spectral‐element) and discretizations (cubed sphere versus latitude–longitude) are compared across a range of resolutions. The subtle momentum balance of the tropical stratosphere is sensitive to model numerics, and the first intercomparison reveals stark differences in tropical stratospheric winds, particularly at high vertical resolution: some cores develop westerly jets and others easterly jets. This leads to substantial spread in transport, biasing the age‐of‐air by up to 25% relative to its climatological mean, making it difficult to assess the impact of the numerical representation of transport processes. This uncertainty is removed by constraining the tropical winds in the second intercomparison test, in a manner akin to specifying the Quasi‐Biennial Oscillation in an AGCM. The dynamical cores exhibit qualitative agreement on the structure of atmospheric transport in the second test, with evidence of convergence as the horizontal and vertical resolution is increased in a given model. Significant quantitative differences remain, however, particularly between models employing spectral versus finite‐volume numerics, even in state‐of‐the‐art cores.

中文翻译：

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对示踪剂传输的数值影响：大气环流模型的拟议比较测试

大气循环中痕量气体的运输在气候系统及其对外部强迫的响应中起着重要作用。运输对大气总环流模型（AGCM）提出了挑战，因为解析的环流和运输过程的数值表示中的误差都会使它们的丰度产生偏差。在这项研究中，提出了两个测试以评估AGCM动力核心的运输。为了将运输与化学分开，测试着重于空气年龄，即通过循环对平均运输时间的估计。这些测试评估了平流层-对流层耦合系统，重点关注平流层中的翻转环流和等熵混合或布鲁尔-多布森环流的运移，其中，运输时间尺度从几个月到几年不等，对模型数字进行了具有挑战性的测试。在不同分辨率范围内，对采用不同数值方案（有限体积，伪光谱和光谱元素）和离散化（立方球体与纬度-经度）的四个动力核心进行了比较。热带平流层微妙的动量平衡对模型数值很敏感，第一次比对揭示了热带平流层风的明显差异，特别是在高垂直分辨率下：某些核心形成了西风急流，而另一些则形成了东风急流。这导致运输过程中的大量传播，相对于其气候平均值，使空气年龄偏离最多25％，从而难以评估运输过程的数字表示的影响。通过在第二次比对测试中约束热带风，可以消除这种不确定性，其方式类似于在AGCM中指定准两年一次振荡。在第二次测试中，动力核心在大气传输结构上显示出定性一致性，并且在给定模型中，随着水平和垂直分辨率的提高，收敛性也得到了证明。但是，即使在最先进的核中，仍存在明显的定量差异，尤其是在采用频谱与有限体积数值的模型之间。在给定模型中，随着水平和垂直分辨率的提高，具有收敛的证据。但是，即使在最先进的核中，仍存在明显的定量差异，尤其是在采用频谱与有限体积数值的模型之间。在给定模型中，随着水平和垂直分辨率的提高，具有收敛性的证据。但是，即使在最先进的核中，仍存在明显的定量差异，尤其是在采用频谱与有限体积数值的模型之间。