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

Abstract

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

Dynamical core model intercomparison project: Tracer transport test cases

“…This requires a prescribed velocity component and preferably a known solution. Although there are many two-dimensional horizontal tracer test cases on the sphere, including simple solid-body rotation tests (Williamson et al, 1992), static and moving vortices (Nair and Machenhauer, 2002;Nair and Jablonowski, 2008) and deformational flows (Nair and Lauritzen, 2010;Kent et al, 2012b), very few fully three-dimensional tracer-transport tests have been offered. Examples include solid-body rotation with a sinusoidal vertical velocity (Hubbard, 2002) and the three-dimensional advection tests of Zubov et al (1999).…”

Simulations of Stratus Clouds over Eastern China in CAM5: Sources of Errors

“…9i,j). Kent et al (2012) suggested that the semi-Lagrangian transport scheme (Williamson and Rasch 1989;Williamson and Rasch 1994) in the EUL core is overall more diffusive than the flux-form semi-Lagrangian scheme (Lin et al 1994;Lin and Rood 1996) used in the FV core. This defect of the EUL core may limit its ability to simulate some sharp gradients in the real simulation.…”

“…Several studies have compared the performances between these two cores from particular aspects (e.g., Williamson 2008;Kent et al 2012). Since this study does not target evaluating the simulations but only investigating the sources of model errors, detailed comparisons are not made and only key differences are shown for our scientific concerns.…”

A standard test case suite for two-dimensional linear transport on the sphere: results from a collection of state-of-the-art schemes

“…More challenging global idealized tests have been developed since the efforts of Williamson et al (1992) such as the highly deformational (moving) vortices on the sphere (Nair and Machenhauer, 2002;Nair and Jablonowski, 2008) and the "boomerang" flows of Nair and Lauritzen (2010). Despite the high degree of deformation in the (moving) vortex test problem, in particular when simulated beyond the original specification of simulation length (Kent et al, 2012;Pudykiewicz, 2011), it has an analytical solution. The "boomerang" flows, on the other hand, do not have easily accessible analytical solutions until the end of the specified simulation time.…”

“…Similarly, LSPT2012 did not ask modelers to report on more specialized test cases that may be useful to study certain, perhaps more specialized, aspects of accuracy. For example, by running well-known deformational test cases out further in time (Pudykiewicz, 2011), one can study the downscale cascade from near grid scale to the sub-grid scale (Kent et al, 2012). Similar tests, such as many solid-body revolutions of a large constant plateau spanning many cells, can be used for "tuning" shape-preserving filters so that the peak tracer abundance does not decay linearly (if applicable) despite the initial plateau and analytic solution being very well resolved (Appendix A16).…”