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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 wind components are on full levels, also called "momentum levels" and the scalar fields (like temperature, humidity, hydrometeors and passive tracers) as well as the vertical velocity components are on half levels, also called "thermodynamic levels". They are alternating with the lowest atmospheric model level being a thermodynamic level and the highest being a momentum level. See also the wiki page about model levels.
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