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In the horizontal, the Arakawa-C grid is used. In the vertical, the Charney-Phillips grid - see graphic below.
The horizontal winds are on the full levels, also called "momentum levels" and the scalar fields (like temperature, humidity and other tracers, as well as vertical velocity) are on the half levels, also called "thermodynamic levels". The are alternating with the lowest model level being a thermodynamic level, the highest a momentum level. See also wiki about model levels.
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