  We investigate the character and role of convection in the atmosphere of a
prototypical red giant located close to the red giant branch (RGB) tip with
atmospheric parameters, Teff=3660K, log(g)=1.0, [M/H]=0.0. Differential
analysis of the atmospheric structures is performed using the 3D hydrodynamical
and 1D classical atmosphere models calculated with the CO5BOLD and LHD codes,
respectively. All models share identical atmospheric parameters, elemental
composition, opacities and equation-of-state. We find that the atmosphere of
this particular red giant consists of two rather distinct regions: the lower
atmosphere dominated by convective motions and the upper atmosphere dominated
by wave activity. Convective motions form a prominent granulation pattern with
an intensity contrast (~18%) which is larger than in the solar models (~15%).
The upper atmosphere is frequently traversed by fast shock waves, with vertical
and horizontal velocities of up to Mach ~2.5 and ~6.0, respectively. The
typical diameter of the granules amounts to ~5Gm which translates into ~400
granules covering the whole stellar surface. The turbulent pressure in the
giant model contributes up to ~35% to the total (i.e., gas plus turbulent)
pressure which shows that it cannot be neglected in stellar atmosphere and
evolutionary modeling. However, there exists no combination of the
mixing-length parameter and turbulent pressure that would allow to
satisfactorily reproduce the 3D temperature-pressure profile with 1D atmosphere
models based on a standard formulation of mixing-length theory.
