  This paper promotes a measure to validate the hydrostatic approximation via
scaling the nontraditional Coriolis term (NCT) in the zonal momentum equation.
To demonstrate the scaling, this study simulates large-scale flow forced by a
prescribed heat source mimicking the intertropical convergence zone (ITCZ)
using a linearized forced-dissipative model. The model solves two similar
equations between which the only difference is inclusion of NCTs. The equations
are derived using the following approximations: anelastic, equatorial
beta-plane, linearized, zonally symmetric, steady, and a constant dissipation
coefficient. The large-scale flows simulated with and without NCTs are compared
in terms of the meridional-vertical circulation, the zonal wind, and the
potential temperature. Both results appear like the Hadley circulation. With
the model parameters controlled, the results without NCTs minus with NCTs are
linear biases due to omitting NCTs. The most prominent bias is a westerly wind
bias in the ITCZ heating region that emerges because omitting NCTs prevents the
associated westward acceleration when heating-induced vertical motion is
present. The zonal wind bias divided by the zonal wind with NCTs is 0.120 $\pm$
0.007 in terms of the westerly maximum and 0.0452 $\pm$ 0.0005 in terms of the
root mean square (RMS) when the prescribed ITCZ mimics the observed ITCZ in May
over the East Pacific. These normalized measures of the zonal wind bias
increase with a narrower ITCZ or an ITCZ closer to the equator because of a
weaker subtropical jet stream given the same vertical heating profile. This
difference can be traced by a nondimensional parameter scaling the ratio of the
NCT to the traditional Coriolis term. The scaling encourages restoring NCTs
into global models.

Atmospheric and Oceanic Physics