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Abstract

In models like CAM which use terrain following coordinates, there can
be large numerical artifacts in regions of rough topography related to
how one numerically computes horizontal gradients. The "pressure
gradient" term, in terrain following coordinates decomposes into two
large terms which ideally would cancel, but instead when small
relative errors in each term fail to exactly cancel, the result is
large errors creating spurious sources of divergence leading to
anomalously large vertical velocities over steep terrain.

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