  In incorporating the effect of atmospheric turbulence in the broadening of
spectral lines, the so-called radial-tangential macroturbulence (RTM) model has
been widely used in the field of solar-type stars, which was devised from an
intuitive appearance of granular velocity field of the Sun. Since this model
assumes that turbulent motions are restricted to only radial and tangential
directions, it has a special broadening function with notably narrow width due
to the projection effect, the validity of which has not yet been confirmed in
practice. With an aim to check whether this RTM model adequately represents the
actual solar photospheric velocity field, we carried out an extensive study on
the non-thermal velocity dispersion along the line-of-sight (V_los) by
analyzing spectral lines at various points of the solar disk based on
locally-averaged as well as high spatial-resolution spectra, and found the
following results. First, the center-to-limb run of V_los derived from
ground-based low-resolution spectra is simply monotonic with a slightly
increasing tendency, which contradicts the specific trend (an appreciable peak
at theta~45 deg) predicted from RTM. Second, the V_los values derived from a
large number of spectra based on high-resolution space observation revealed to
follow a nearly normal distribution, without any sign of peculiar distribution
expected for the RTM case. These two observational facts indicate that the
actual solar velocity field is not such simply dichotomous as assumed in RTM,
but directionally more chaotic. We thus conclude that RTM is not an adequate
model at least for solar-type stars, which would significantly overestimate the
turbulent velocity dispersion by a factor of ~2. The classical Gaussian
macroturbulence model should be more reasonable in this respect.
