Joint Seminar: A Hexagonal C-Grid Atmospheric Core Formulation for Multiscale Simulation on the Sphere

In global atmospheric solvers using icosahedral grids based on hexagons,
C-grid staggering of the horizontal velocities has been avoided because
of spuriously non-stationary geostrophic modes present in solutions
using the most obvious discretization of the Coriolis terms in the
momentum equations. The C grid is, however, the preferred grid in
mesoscale and cloud-scale models because of its very accurate
representation of horizontally divergent motions.  Within the context of
the shallow-water equations, we have developed an alternative
formulation of the Coriolis term discretization for arbitrarily
structured C grids  that, analysis reveals, recovers the stationary
geostrophic mode on hexagonal grids and also allows for exact
conservation of potential vorticity and conservation of energy to the
time integration truncation error.  We will present results from shallow
water model tests demonstrating these properties and the efficacy of
this approach.  Hexagonal C-grids have not been used or tested in
mesoscale and cloudscale models.  We will also present results from a 3D
fully compressible nonhydrostatic hexagonal C-grid cloud model
demonstrating that the hexagonal C-grid formulation is slightly more
accurate, and slightly more efficient, than existing state-of-the-art
rectangular C-grid cloud models.  Hexagonal grids provide very uniform
resolution on the sphere and have been shown to scale well on MPP
computer architectures.  Given the superior performance of the hexagonal
C-grid for cloud scale simulations, and the removal of the large-scale
problem with geostrophic modes, we believe that a C-grid hexagonal
global atmospheric solver is a viable candidate for future global
weather, climate and earth system model applications requiring accuracy
from cloud to global scales and scalability on existing and future MPP




15:15 h


Bundesstr. 53, room 022/023
Seminar Room 022/023, Ground Floor, Bundesstrasse 53, 20146 Hamburg, Hamburg


William C. Skamarock, NCAR/MMM


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