  The widely-adopted discretisation of the horizontal pressure gradient term
formulated by Simmons and Burridge (1981) for atmospheric models on
$\sigma$-$p$ hybrid vertical coordinate is found to incur spectral blocking for
rotational wind components at high vertical levels when used in a spectral
semi-Lagrangian model run on a linear grid. A remedy to this issue is proposed
and tested using a spectral semi-implicit semi-Lagrangian hydrostatic primitive
equations model. The proposed method removes aliasing errors at high
wavenumbers by ensuring that the rotation-free property of the pressure
gradient term on isobaric surface, a feature possessed by the continuous
system, is preserved in the discretised system, which highlights the
significance of mimetic discretisation within the context of numerical weather
prediction models.
