Global Environmental Multiscale Model (GEM) on the latitude-longitude grid

Institution: Environment Canada, Canada

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Description:

The Global Environmental Multiscale (GEM) Model is the operational model at the Canadian Meteorological Center (CMC) and is described by Côté et al.(1998). The temporal discretization is a two-time-level Crank–Nicholson iterative semi-Lagrangian method with an advecting wind interpolated in time. Recently, the vertical staggering (Charney-Phillips grid) was introduced and is described by Girard et al. (2010). The terrain-following vertical coordinate [ln p = A(zeta) + B(zeta) ln(ps/pref); zeta = ln(pref) + ln (eta)] is adopted, where ps=Surface pressure and pref=10**5. GEM Lat-Lon is based on the global latitude–longitude grid system with a uniform Arakawa C grid.

In GEM, the dynamics and physics are time split. All prognostic variables are updated by the dynamics and then the physics. The time step is the same for dynamics and physics.

Typical horizontal resolutions, physics and dynamics time steps, and dissipation coefficients:

GEM Lat-Lon uses the latitude-longitude grid. Implicit 2D diffusion damps the winds [u,v,d(zeta)/dt,dz/dt]. A 6th order selective diffusion (Qaddouri and Lee, 2008) is adopted with a 2 Delta-x removal ratio of 4% per timestep and is applied after the physics. In the case with Simplified/Full physics, the temporal scheme is decentered along the trajectory with a off-centering parameter epsilon=.1. In the case of Idealized cyclones, a vertical sponge layer based on 2nd order implicit 2D diffusion is applied on the winds [u,v,d(zeta)/dt,dz/dt] and temperature, with vertical modulation on 3 top levels and damping coefficient C = .380000*10**6 m**2/s.

Information on the computational grid:

Horizontal: Lat-Lon Grid (Left); Vertical Staggering: Charney-Phillips (Right)

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References:

• Girard C., A.Plante, S.Gravel, A.Qaddouri, S.Chamberland, L.Spacek, V.Lee, M.Desgagné, 2010: GEM4.1: A non-hydrostatic atmospheric model (Euler equations). RPN document. http://collaboration.cmc.ec.gc.ca/science/rpn/publications/pdf/GEM4.1.pdf
• Côté,J, S.Gravel, A.Méthot, A.Patoine, M.Roch, A.Staniforth, 1998: The Operational CMC–MRB Global Environmental Multiscale (GEM) Model. Part I: Design Considerations and Formulation. Mon. Wea. Rev., 126, 1373–1395. http://journals.ametsoc.org/doi/pdf/10.1175/1520-0493%281998%29126%3C1373%3ATOCMGE%3E2.0.CO%3B2
• Qaddouri A, and V.Lee, 2008: Solution of the implicit formulation of high order diffusion for the Canadian Atmospheric GEM model. In Proceedings of the 2008 Spring Simulation Multiconference, Society for Computer Science International, April 13-16 2008, Ottawa, Ontario, Canada; 362-367. http://dl.acm.org/citation.cfm?id=1400604

 GEM-latlon will be featured as a remotely-participating model.

Additional contributions: Bernard Dugas (NetCDF), Ayrton Zadra and Paul Vaillancourt (Physics)