  We show that the motions of supergranules are consistent with a model in
which they are simply advected by the axisymmetric flows in the Sun's surface
shear layer. We produce a 10-day series of simulated Doppler images at a
15-minute cadence that reproduces most spatial and temporal characteristics
seen in the SOHO/MDI Doppler data. Our simulated data have a spectrum of
cellular flows with just two components -- a granule component that peaks at
spherical wavenumbers of about 4000 and a supergranule component that peaks at
wavenumbers of about 110. We include the advection of these cellular components
by the axisymmetric flows -- differential rotation and meridional flow -- whose
variations with latitude and depth (wavenumber) are consistent with
observations. We mimic the evolution of the cellular pattern by introducing
random variations to the phases of the spectral components at rates that
reproduce the levels of cross-correlation as functions of time and latitude.
Our simulated data do not include any wave-like characteristics for the
supergranules yet can reproduce the rotation characteristics previously
attributed to wave-like behavior. We find rotation rates which appear faster
than the actual rotation rates and attribute this to the projection effects. We
find that the measured meridional flow does accurately represent the actual
flow and that the observations indicate poleward flow to $65\degr-70\degr$
latitude with equatorward counter cells in the polar regions.
