  In the quiet regions on the solar surface, turbulent convective motions of
granulation play an important role in creating small-scale magnetic structures,
as well as in energy injection into the upper atmosphere. The turbulent nature
of granulation can be studied using spectral line profiles, especially line
broadening, which contains information on the flow field smaller than the
spatial resolution of an instrument. Moreover, the Doppler velocity gradient
along a line-of-sight (LOS) causes line broadening as well. However, the
quantitative relationship between velocity gradient and line broadening has not
been understood well. In this study, we perform bisector analyses using the
spectral profiles obtained using the Spectro-Polarimeter of the Hinode/Solar
Optical Telescope to investigate the relationship of line broadening and
bisector velocities with the granulation flows. The results indicate that line
broadening has a positive correlation with the Doppler velocity gradients along
the LOS. We found excessive line broadening in fading granules, that cannot be
explained only by the LOS velocity gradient, although the velocity gradient is
enhanced in the process of fading. If this excessive line broadening is
attributed to small-scale turbulent motions, the averaged turbulent velocity is
obtained as 0.9 km/s.
