Downscale cascades in tracer transport test cases: an intercomparison of the dynamical cores in the Community Atmosphere Model CAM5

摘要

The accurate modeling of cascades to unresolved scales is an important partof the tracer transport component of dynamical cores of weather and climatemodels. This paper aims to investigate the ability of the advection schemesin the National Center for Atmospheric Research's Community Atmosphere Modelversion 5 (CAM5) to model this cascade. In order to quantify the effects ofthe different advection schemes in CAM5, four two-dimensional tracertransport test cases are presented. Three of the tests stretch the tracerbelow the scale of coarse resolution grids to ensure the downscale cascade oftracer variance. These results are compared with a high resolution referencesolution, which is simulated on a resolution fine enough to resolve thetracer during the test. The fourth test has two separate flow cells, and isdesigned so that any tracer in the western hemisphere should not pass intothe eastern hemisphere. This is to test whether the diffusion in transportschemes, often in the form of explicit hyper-diffusion terms or implicitthrough monotonic limiters, contains unphysical mixing.An intercomparison of three of the dynamical cores of the National Center forAtmospheric Research's Community Atmosphere Model version 5 is performed. Theresults show that the finite-volume (CAM-FV) and spectral element (CAM-SE)dynamical cores model the downscale cascade of tracer variance better thanthe semi-Lagrangian transport scheme of the Eulerian spectral transform core(CAM-EUL). Each scheme tested produces unphysical mass in the easternhemisphere of the separate cells test.