Numerical impacts on tracer transport: A proposed intercomparison test of Atmospheric General Circulation Models
Numerical impacts on tracer transport: A proposed intercomparison test of Atmospheric General Circulation Models
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DOI：
10.1002/qj.3881
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发表时间：
2020-09-07
影响因子：
8.9
通讯作者：
Lauritzen, Peter H.
中科院分区：
文献类型：
--
作者：
Gupta, Aman;Gerber, Edwin P.;Lauritzen, Peter H.
大气环流对微量气体的输送在气候系统及其对外部强迫的响应中起着重要作用。运输对大气环流模式（AGCMs）提出了挑战，因为在解决的环流和运输过程的数值表示上的误差会使它们的丰度产生偏差。在这项研究中，提出了两个测试来评估AGCM的动力核心传输。为了将运输与化学分离开来，测试的重点是空气的年龄，即循环对平均运输时间的估计。试验评估了平流层-对流层耦合系统，重点关注平流层中翻转环流和等熵混合的输送，或布鲁尔-多布森环流，其中输送的时间尺度以月到年为单位，为模式数值提供了一个具有挑战性的测试。采用不同数值格式（有限体积、伪光谱和光谱元素）和离散化（立方球与经纬度）的四种动态核心在一系列分辨率下进行了比较。热带平流层微妙的动量平衡对模式数值很敏感，第一次相互比较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：一些核心发展西风急流，而另一些则发展东风急流。这导致了运输的大量扩散，使空气年龄相对于其气候平均值的偏差高达25%，使得难以评估运输过程的数值表示的影响。这种不确定性通过在第二次相互比较试验中限制热带风来消除，其方式类似于在AGCM中指定准两年一次的振荡。在第二次试验中，动力核在大气输送结构上表现出定性的一致性，在给定模式中，随着水平和垂直分辨率的增加，有辐合的证据。然而，即使在最先进的岩心中，仍然存在显著的数量差异，特别是在采用光谱和有限体积数值的模型之间。
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.