  Surface granulation of the Sun is primarily a consequence of thermal
transport in the outer 1 % of the radius. Its typical scale of about 1 - 2 Mm
is set by the balance between convection, free-streaming radiation, and the
strong density stratification in the surface layers.
  The physics of granulation is well understood, as demonstrated by the close
agreement between numerical simulation, theory, and observation. Superimposed
on the energetic granular structure comprising high-speed flows, are larger
scale long-lived flow systems (~ 300 m/s) called supergranules.
Supergranulation has a typical scale of 24 - 36 Mm. It is not clear if
supergranulation results from the interaction of granules or is causally linked
to deep convection or a consequence of magneto-convection. Other outstanding
questions remain: how deep are supergranules? How do they participate in global
dynamics of the Sun? Further challenges are posed by our lack of insight into
the dynamics of larger scales in the deep convection region. Recent
helioseismic constraints have suggested that convective velocity amplitudes on
large scales may be overestimated by an order of magnitude or more, implying
that Reynolds stresses associated with large-scale convection, thought to play
a significant role in the sustenance of differential rotation and meridional
circulation, might be two orders of magnitude weaker than theory and
computation predict. While basic understanding on the nature of convection on
global scales and the maintenance of global circulations is incomplete,
progress is imminent, given substantial improvements in computation, theory and
helioseismic inferences.
