  We have fit synthetic visibilities from 3-D (CO5BOLD + PHOENIX) and 1-D
(PHOENIX, ATLAS 12) model stellar atmospheres of Procyon (F5 IV) to
high-precision interferometric data from the VLTI Interferometer (K-band) and
from the Mark III interferometer (500 nm and 800 nm). These data sets provide a
test of theoretical wavelength dependent limb-darkening predictions. The work
of Allende Prieto et al. has shown that the temperature structure from a
spatially and temporally averaged 3-D hydrodynamical model produces
significantly less limb darkening at 500 nm relative to the temperature
structure of a 1-D MARCS model atmosphere with a standard mixing-length
approximation for convection. Our direct fits to the interferometric data
confirm this prediction. A 1-D ATLAS 12 model with ``approximate overshooting''
provides the required temperature gradient. We show, however, that 1-D models
cannot reproduce the ultraviolet spectrophotometry below 160 nm with effective
temperatures in the range constrained by the measured bolometric flux and
angular diameter. We find that a good match to the full spectral energy
distribution can be obtained with a composite model consisting of a weighted
average of twelve 1-D model atmospheres based on the surface intensity
distribution of a 3-D granulation simulation. We emphasize that 1-D models with
overshooting may realistically represent the mean temperature structure of
F-type stars like Procyon, but the same models will predict redder colors than
observed because they lack the multicomponent temperature distribution expected
for the surfaces of these stars.
