Abstract
AbstractThe 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.
Highlights
It has been a quarter of a century since Held and Suarez (1994) proposed a test to compare the dynamical cores of atmospheric general circulation models, the numerical solvers that integrate the primitive equations on the sphere
We had hoped that these idealized FR forcing could serve as our intercomparison test for tracer transport in the dynamical cores. We found that this proved too much of a test: the dynamical cores introduced in Section 3 failed to produce a comparable circulation, making it nearly impossible to assess the impact of numerics on transport
We have proposed two tests to assess the climatological representation of the stratosphere–troposphere system in primitive equation models on the sphere, the dynamical cores of atmospheric general circulation models (AGCMs)
Summary
It has been a quarter of a century since Held and Suarez (1994) proposed a test to compare the dynamical cores of atmospheric general circulation models, the numerical solvers that integrate the primitive equations on the sphere. The Held and Suarez, 1994 test, hereafter referred to as HS94, is a simple recipe for parametrizing all non-conservative processes for a dry (moisture-free) atmosphere, including diabatic processes, such as radiative transfer and convection, and frictional processes, the atmospheric boundary layer This allowed them to see if two dynamical cores with very different numerics – a pseudo-spectral versus a finite-difference scheme – would produce the same general circulation. Differences in the large-scale circulation make it difficult to focus on the role of model numerics on transport This inspired a second test, where the tropical winds in the stratosphere are specified in a manner akin to specifying the Quasi-Biennial Oscillation, which allows one to focus in on the impact of model numerics on tracer transport.
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